Merged PR 584959: Update csc to 3.7.0 and fix nullability warnings

This is the second PR that is required to make the final PR for adding .net5 support simpler.

The PR updates the compiler to a 3.7 version (this version does not support C# 9 yet), and fixes a bunch of nullability warnings that appear after the upgrade to 3.7 and some of them will be visible only when targeting .net 5.

This is another low risk PR.

Public/Src/Cache/ContentStore/AppTest/AppTests.cs

@@ -132,7 +132,9 @@ namespace BuildXL.Cache.ContentStore.App.Test
                 if (!serviceProcess.HasExited)
                 {
                     serviceProcess.Kill();
+#pragma warning disable AsyncFixer02 // WaitForExitAsync should be used instead
                     serviceProcess.WaitForExit();
+#pragma warning restore AsyncFixer02
                 }
             }
         }
@@ -165,7 +167,9 @@ namespace BuildXL.Cache.ContentStore.App.Test
 
             var result = await Task.Run(() =>
             {
+#pragma warning disable AsyncFixer02 // WaitForExitAsync should be used instead
                 process.WaitForExit();
+#pragma warning restore AsyncFixer02
                 return process.ExitCode;
             });
 

Public/Src/Cache/ContentStore/Distributed/ContentLocations/ContentHashWithSizeAndLocations.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.ContractsLight;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using BuildXL.Cache.ContentStore.Distributed.NuCache;
 using BuildXL.Cache.ContentStore.Hashing;
@@ -72,7 +73,7 @@ namespace BuildXL.Cache.ContentStore.Distributed
         }
 
         /// <inheritdoc />
-        public bool Equals(ContentHashWithSizeAndLocations other)
+        public bool Equals([AllowNull]ContentHashWithSizeAndLocations other)
         {
             if (other is null)
             {
@@ -84,7 +85,7 @@ namespace BuildXL.Cache.ContentStore.Distributed
                 return true;
             }
 
-            return Locations.SequenceEqual(other.Locations) && ContentHash.Equals(other.ContentHash) && Size == other.Size;
+            return Locations!.SequenceEqual(other.Locations!) && ContentHash.Equals(other.ContentHash) && Size == other.Size;
         }
 
         /// <inheritdoc />

Public/Src/Cache/ContentStore/Distributed/NuCache/BinManager.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.ContractsLight;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Linq;
 using BuildXL.Cache.ContentStore.Hashing;
@@ -617,8 +618,10 @@ namespace BuildXL.Cache.ContentStore.Distributed.NuCache
 
         private class Comparer : IComparer<MachineWithBinAssignments>
         {
-            public int Compare(MachineWithBinAssignments x, MachineWithBinAssignments y)
+            public int Compare([AllowNull]MachineWithBinAssignments x, [AllowNull]MachineWithBinAssignments y)
             {
+                Contract.Requires(x != null);
+                Contract.Requires(y != null);
                 return x.BinsAssignedTo.Count != y.BinsAssignedTo.Count
                     ? x.BinsAssignedTo.Count.CompareTo(y.BinsAssignedTo.Count)
                     : x.MachineId.Index.CompareTo(y.MachineId.Index);

Public/Src/Cache/ContentStore/Distributed/NuCache/ClusterStateInternal.cs

@@ -165,7 +165,7 @@ namespace BuildXL.Cache.ContentStore.Distributed.NuCache
                 return new Result<MachineLocation[]>(locationsResult);
             }
 
-            return locationsResult.Value
+            return locationsResult.Value!
                 .Where(machineId => !_inactiveMachinesSet[machineId] && !_closedMachinesSet[machineId])
                 .Select(id => _locationByIdMap[id.Index])
                 .ToArray();

Public/Src/Cache/ContentStore/Distributed/NuCache/DistributedCentralStorage.cs

@@ -321,8 +321,8 @@ namespace BuildXL.Cache.ContentStore.Distributed.NuCache
             var result = await _locationStore.GetBulkAsync(context, new[] { hash, startedCopyHash }).ThrowIfFailure();
             var info = result.ContentHashesInfo[0];
 
-            var startedCopyLocations = result.ContentHashesInfo[1].Locations;
-            var finishedCopyLocations = info.Locations;
+            var startedCopyLocations = result.ContentHashesInfo[1].Locations!;
+            var finishedCopyLocations = info.Locations!;
             var pendingCopies = startedCopyLocations.Except(finishedCopyLocations).Count();
 
             return (new ContentHashWithSizeAndLocations(info.ContentHash, info.Size, TranslateLocations(info.Locations!)), pendingCopies);

Public/Src/Cache/ContentStore/Distributed/Sessions/ReadOnlyDistributedContentSession.cs

@@ -1311,7 +1311,7 @@ namespace BuildXL.Cache.ContentStore.Distributed.Sessions
                         // A machine in the build may be a designated location for the hash,
                         // but we won't pushing to the same machine twice, because 'replicatedLocations' argument
                         // has a local machine that we're about to push for inside the ring copy.
-                        var candidates = designatedLocationsResult.Value
+                        var candidates = designatedLocationsResult.Value!
                             .Except(replicatedLocations).ToArray();
 
                         if (candidates.Length > 0)

Public/Src/Cache/ContentStore/Hashing/ChunkInfo.cs

@@ -3,6 +3,7 @@
 
 using System;
 using BuildXL.Cache.ContentStore.Interfaces.Utils;
+using System.Diagnostics.CodeAnalysis;
 
 #pragma warning disable CS3001 // CLS
 #pragma warning disable CS3003
@@ -46,7 +47,7 @@ namespace BuildXL.Cache.ContentStore.Hashing
         }
 
         /// <inheritdoc/>
-        public bool Equals(ChunkInfo other)
+        public bool Equals([AllowNull]ChunkInfo other)
         {
             return Offset == other.Offset && Size == other.Size && ByteArrayComparer.ArraysEqual(Hash, other.Hash);
         }

Public/Src/Cache/ContentStore/Hashing/ContentHash.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Diagnostics.ContractsLight;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using BuildXL.Cache.ContentStore.Interfaces.Utils;
 using BuildXL.Cache.ContentStore.UtilitiesCore;
@@ -212,7 +213,7 @@ namespace BuildXL.Cache.ContentStore.Hashing
         }
 
         /// <inheritdoc />
-        public bool Equals(ContentHash other)
+        public bool Equals([AllowNull]ContentHash other)
         {
             return _bytes.Equals(other._bytes) && ((int)_hashType).Equals((int)other._hashType);
         }
@@ -231,7 +232,7 @@ namespace BuildXL.Cache.ContentStore.Hashing
         }
 
         /// <inheritdoc />
-        public int CompareTo(ContentHash other)
+        public int CompareTo([MaybeNull]ContentHash other)
         {
             var compare = _bytes.CompareTo(other._bytes);
             return compare != 0 ? compare : ((int)_hashType).CompareTo((int)other._hashType);

Public/Src/Cache/ContentStore/Hashing/ContentHasher.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Diagnostics;
 using System.Diagnostics.ContractsLight;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Security.Cryptography;
 using System.Threading;
@@ -120,7 +121,7 @@ namespace BuildXL.Cache.ContentStore.Hashing
                     } while (bytesJustRead > 0);
 
                     hasher.TransformFinalBlock(buffer, 0, 0);
-                    var hashBytes = hasher.Hash;
+                    var hashBytes = hasher.Hash!;
 
                     // Retrieve the DedupNode before losing the hasher token.
                     switch (Info.HashType)
@@ -420,7 +421,7 @@ namespace BuildXL.Cache.ContentStore.Hashing
             }
 
             /// <inheritdoc />
-            public override IAsyncResult BeginRead(byte[] buffer, int offset, int count, AsyncCallback callback, object? state)
+            public override IAsyncResult BeginRead(byte[] buffer, int offset, int count, [AllowNull]AsyncCallback callback, object? state)
             {
                 throw new NotImplementedException();
             }
@@ -446,7 +447,7 @@ namespace BuildXL.Cache.ContentStore.Hashing
                     var handle = Buffer.GetBuffer();
                     handle.Value.CopyFrom(buffer, offset, count);
 
-                    return _hashingBufferBlock.SendAsync(handle);
+                    return _hashingBufferBlock!.SendAsync(handle);
                 }
                 else
                 {
@@ -488,7 +489,7 @@ namespace BuildXL.Cache.ContentStore.Hashing
             }
 
             /// <inheritdoc />
-            public override IAsyncResult BeginWrite(byte[] buffer, int offset, int count, AsyncCallback callback, object? state)
+            public override IAsyncResult BeginWrite(byte[] buffer, int offset, int count, [AllowNull]AsyncCallback callback, object? state)
             {
                 throw new NotImplementedException();
             }
@@ -571,7 +572,7 @@ namespace BuildXL.Cache.ContentStore.Hashing
                         {
                             _ownerStream.ThrowIfDisposed();
                             Contract.Assert(Finalized);
-                            return _hasherToken.Hasher.Hash;
+                            return _hasherToken.Hasher.Hash!;
                         }
                     }
                 }

Public/Src/Cache/ContentStore/Hashing/DedupChunkHashAlgorithm.cs

@@ -17,7 +17,7 @@ namespace BuildXL.Cache.ContentStore.Hashing
         private readonly SHA512 _hasher = new SHA512CryptoServiceProvider();
 
         /// <inheritdoc />
-        public override byte[] Hash => TruncateTo256Bits(base.Hash);
+        public override byte[] Hash => TruncateTo256Bits(base.Hash!);
 
         /// <inheritdoc />
         public override int HashSize => 8 * DedupSingleChunkHashInfo.Length;
@@ -43,7 +43,7 @@ namespace BuildXL.Cache.ContentStore.Hashing
         protected override byte[] HashFinal()
         {
             _hasher.TransformFinalBlock(EmptyArray, 0, 0);
-            return TruncateTo256Bits(_hasher.Hash);
+            return TruncateTo256Bits(_hasher.Hash!);
         }
 
         internal byte[] HashFinalInternal()

Public/Src/Cache/ContentStore/Hashing/DedupNode.cs

@@ -5,6 +5,7 @@ using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Diagnostics.ContractsLight;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Linq;
 using System.Text;
@@ -394,7 +395,7 @@ namespace BuildXL.Cache.ContentStore.Hashing
             {
                 foreach (DedupNode node in EnumerateInnerNodesDepthFirst())
                 {
-                    foreach (DedupNode chunk in node.ChildNodes.Where(n => n.Type == NodeType.ChunkLeaf))
+                    foreach (DedupNode chunk in node.ChildNodes!.Where(n => n.Type == NodeType.ChunkLeaf))
                     {
                         yield return chunk;
                     }
@@ -427,7 +428,7 @@ namespace BuildXL.Cache.ContentStore.Hashing
         }
 
         /// <inheritdoc/>
-        public bool Equals(DedupNode other)
+        public bool Equals([AllowNull]DedupNode other)
         {
             return ByteArrayComparer.ArraysEqual(Hash, other.Hash);
         }

Public/Src/Cache/ContentStore/Hashing/FixedBytes.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Diagnostics.ContractsLight;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using BuildXL.Cache.ContentStore.Interfaces.Utils;
 using BuildXL.Cache.ContentStore.UtilitiesCore;
@@ -139,7 +140,7 @@ namespace BuildXL.Cache.ContentStore.Hashing
         }
 
         /// <inheritdoc />
-        public bool Equals(FixedBytes other)
+        public bool Equals([AllowNull]FixedBytes other)
         {
             byte* o = other._bytes;
             fixed (byte* p = _bytes)
@@ -173,7 +174,7 @@ namespace BuildXL.Cache.ContentStore.Hashing
         }
 
         /// <inheritdoc />
-        public int CompareTo(FixedBytes other)
+        public int CompareTo([MaybeNull]FixedBytes other)
         {
             byte* o = other._bytes;
             fixed (byte* p = _bytes)

Public/Src/Cache/ContentStore/Hashing/MurmurHash3.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Diagnostics.ContractsLight;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using BuildXL.Cache.ContentStore.Interfaces.Utils;
 
@@ -296,7 +297,7 @@ namespace BuildXL.Cache.ContentStore.Hashing
         }
 
         /// <inheritdoc />
-        public bool Equals(MurmurHash3 other)
+        public bool Equals([AllowNull]MurmurHash3 other)
         {
             return other.High == High && other.Low == Low;
         }

Public/Src/Cache/ContentStore/Hashing/MurmurHash3_32.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Diagnostics.ContractsLight;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using BuildXL.Cache.ContentStore.Interfaces.Utils;
 
@@ -168,7 +169,7 @@ namespace BuildXL.Cache.ContentStore.Hashing
         }
 
         /// <inheritdoc />
-        public bool Equals(MurmurHash3_32 other)
+        public bool Equals([AllowNull]MurmurHash3_32 other)
         {
             return other.Hash == Hash;
         }

Public/Src/Cache/ContentStore/Interfaces/Extensions/StreamExtensions.cs

@@ -36,7 +36,8 @@ namespace BuildXL.Cache.ContentStore.Interfaces.Extensions
             var settings = new DataContractJsonSerializerSettings {UseSimpleDictionaryFormat = true};
             var serializer = new DataContractJsonSerializer(typeof(T), settings);
             stream.Position = 0;
-            return (T)serializer.ReadObject(stream);
+            // TODO: suppressing this for now, but the clients need to respect that the result of this method may be null.
+            return (T)serializer.ReadObject(stream)!;
         }
 
         /// <summary>

Public/Src/Cache/ContentStore/Interfaces/FileSystem/AbsolutePath.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.ContractsLight;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.IO;
 using System.Runtime.CompilerServices;
@@ -302,7 +303,7 @@ namespace BuildXL.Cache.ContentStore.Interfaces.FileSystem
         public string GetPathWithoutLongPathPrefix() => RemoveLongPathPrefixIfNeeded(Path);
 
         /// <inheritdoc />
-        public bool Equals(AbsolutePath other)
+        public bool Equals([AllowNull]AbsolutePath other)
         {
             return base.Equals(other);
         }

Public/Src/Cache/ContentStore/Interfaces/FileSystem/RelativePath.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Diagnostics.ContractsLight;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.Linq;
 
@@ -71,7 +72,7 @@ namespace BuildXL.Cache.ContentStore.Interfaces.FileSystem
         }
 
         /// <inheritdoc />
-        public bool Equals(RelativePath other)
+        public bool Equals([AllowNull]RelativePath other)
         {
             return base.Equals(other);
         }

Public/Src/Cache/ContentStore/Interfaces/Results/CopyFileResult.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System;
+using System.Diagnostics.CodeAnalysis;
 
 namespace BuildXL.Cache.ContentStore.Interfaces.Results
 {
@@ -111,7 +112,7 @@ namespace BuildXL.Cache.ContentStore.Interfaces.Results
         public static implicit operator bool(CopyFileResult result) => result.Succeeded;
 
         /// <inheritdoc />
-        public bool Equals(CopyFileResult other)
+        public bool Equals([AllowNull]CopyFileResult other)
         {
             return EqualsBase(other) && other != null && Code == other.Code;
         }

Public/Src/Cache/ContentStore/Interfaces/Results/CreateSessionResult.cs

@@ -58,7 +58,7 @@ namespace BuildXL.Cache.ContentStore.Interfaces.Results
         public readonly T Session;
 
         /// <inheritdoc />
-        public bool Equals(CreateSessionResult<T> other)
+        public bool Equals([AllowNull]CreateSessionResult<T> other)
         {
             if (other is null || !base.Equals(other))
             {

Public/Src/Cache/ContentStore/Interfaces/Results/ErrorResult.cs

@@ -50,7 +50,7 @@ namespace BuildXL.Cache.ContentStore.Interfaces.Results
         public override bool Succeeded => false;
 
         /// <inheritdoc />
-        public bool Equals(ErrorResult other)
+        public bool Equals([AllowNull]ErrorResult other)
         {
             return EqualsBase(other) && other != null;
         }

Public/Src/Cache/ContentStore/Interfaces/Results/FileExistenceResult.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System;
+using System.Diagnostics.CodeAnalysis;
 
 namespace BuildXL.Cache.ContentStore.Interfaces.Results
 {
@@ -97,7 +98,7 @@ namespace BuildXL.Cache.ContentStore.Interfaces.Results
         public bool Exists => Code == ResultCode.FileExists;
 
         /// <inheritdoc />
-        public bool Equals(FileExistenceResult other)
+        public bool Equals([AllowNull]FileExistenceResult? other)
         {
             return EqualsBase(other) && other != null && Code == other.Code;
         }

Public/Src/Cache/ContentStore/Interfaces/Results/ObjectResult.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Diagnostics.ContractsLight;
+using System.Diagnostics.CodeAnalysis;
 
 namespace BuildXL.Cache.ContentStore.Interfaces.Results
 {
@@ -75,7 +76,7 @@ namespace BuildXL.Cache.ContentStore.Interfaces.Results
         public T? Data { get; }
 
         /// <inheritdoc />
-        public bool Equals(ObjectResult<T> other)
+        public bool Equals([AllowNull]ObjectResult<T> other)
         {
             if (other is null || !base.Equals(other))
             {

Public/Src/Cache/ContentStore/Interfaces/Results/PinResult.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Diagnostics.ContractsLight;
+using System.Diagnostics.CodeAnalysis;
 
 namespace BuildXL.Cache.ContentStore.Interfaces.Results
 {
@@ -121,7 +122,7 @@ namespace BuildXL.Cache.ContentStore.Interfaces.Results
         public override bool Succeeded => Code == ResultCode.Success;
 
         /// <inheritdoc />
-        public bool Equals(PinResult other)
+        public bool Equals([AllowNull]PinResult other)
         {
             return EqualsBase(other) && other != null && Code == other.Code;
         }

Public/Src/Cache/ContentStore/Interfaces/Results/PlaceFileResult.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Diagnostics.ContractsLight;
+using System.Diagnostics.CodeAnalysis;
 
 namespace BuildXL.Cache.ContentStore.Interfaces.Results
 {
@@ -139,7 +140,7 @@ namespace BuildXL.Cache.ContentStore.Interfaces.Results
         }
 
         /// <inheritdoc />
-        public bool Equals(PlaceFileResult other)
+        public bool Equals([AllowNull]PlaceFileResult other)
         {
             return EqualsBase(other) && other != null && Code == other.Code;
         }

Public/Src/Cache/ContentStore/Interfaces/Results/PutResult.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Diagnostics.ContractsLight;
 using BuildXL.Cache.ContentStore.Hashing;
+using System.Diagnostics.CodeAnalysis;
 
 namespace BuildXL.Cache.ContentStore.Interfaces.Results
 {
@@ -93,7 +94,7 @@ namespace BuildXL.Cache.ContentStore.Interfaces.Results
         public readonly bool ContentAlreadyExistsInCache;
 
         /// <inheritdoc />
-        public bool Equals(PutResult other)
+        public bool Equals([AllowNull]PutResult other)
         {
             return
                 base.Equals(other)

Public/Src/Cache/ContentStore/Interfaces/Results/ResultBase.cs

@@ -177,7 +177,7 @@ namespace BuildXL.Cache.ContentStore.Interfaces.Results
         /// <summary>
         /// Returns true if 'this' and 'other' are not succeeded and both have the same error text.
         /// </summary>
-        protected bool ErrorEquals(ResultBase other)
+        protected bool ErrorEquals(ResultBase? other)
         {
             if (other is null)
             {

Public/Src/Cache/ContentStore/Interfaces/Results/ResultOfT.cs

@@ -78,6 +78,14 @@ namespace BuildXL.Cache.ContentStore.Interfaces.Results
             _result = default;
         }
 
+        /// <inheritdoc />
+        // The following attribute allows the compiler to understand that if Succeeded is true, then Value is not null.
+        // This is not technically correct, because the caller can specify 'isNullAllowed: true' in the constructor,
+        // but this is a good enough behavior for us, because in vast majority of cases, isNullAllowed is false.
+        // [MemberNotNullWhen(true, nameof(Value))]
+        // WIP ST: this is not working with the 3.7 compiler!!
+        public override bool Succeeded => base.Succeeded;
+
         /// <summary>
         /// Gets the resulting value.
         /// </summary>
@@ -105,9 +113,9 @@ namespace BuildXL.Cache.ContentStore.Interfaces.Results
         }
 
         /// <inheritdoc />
-        public bool Equals(Result<T> other)
+        public bool Equals([AllowNull]Result<T> other)
         {
-            return Succeeded && other.Succeeded ? Value!.Equals(other.Value) : ErrorEquals(other);
+            return Succeeded && other?.Succeeded == true ? Value!.Equals(other.Value) : ErrorEquals(other);
         }
 
         /// <inheritdoc />

Public/Src/Cache/ContentStore/Interfaces/Sessions/ContentHashWithPath.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System;
+using System.Diagnostics.CodeAnalysis;
 using BuildXL.Cache.ContentStore.Hashing;
 using BuildXL.Cache.ContentStore.Interfaces.FileSystem;
 using BuildXL.Cache.ContentStore.Interfaces.Utils;
@@ -33,7 +34,7 @@ namespace BuildXL.Cache.ContentStore.Interfaces.Sessions
         }
 
         /// <inheritdoc />
-        public bool Equals(ContentHashWithPath other)
+        public bool Equals([AllowNull]ContentHashWithPath other)
         {
             return Hash.Equals(other.Hash) && Path.Equals(other.Path);
         }

Public/Src/Cache/ContentStore/Library/Service/Grpc/GrpcContentServer.cs

@@ -594,6 +594,7 @@ namespace BuildXL.Cache.ContentStore.Service.Grpc
                             return Unit.Void;
                         }
 
+                        Contract.Assert(store != null);
                         result = await store.HandlePushFileAsync(cacheContext, hash, disposableFile.Path, token);
                     }
 
@@ -1044,7 +1045,7 @@ namespace BuildXL.Cache.ContentStore.Service.Grpc
                 _contentHash = contentHash;
             }
 
-            public static PushCopyLimiter Create(Context context, ConcurrencyLimiter<ContentHash> limiter, ContentHash hash, IPushFileHandler store)
+            public static PushCopyLimiter Create(Context context, ConcurrencyLimiter<ContentHash> limiter, ContentHash hash, IPushFileHandler? store)
             {
                 var (added, overTheLimit) = limiter.TryAdd(hash, respectTheLimit: true);
 

Public/Src/Cache/ContentStore/Library/Service/Grpc/GrpcCopyClientKey.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System;
+using System.Diagnostics.CodeAnalysis;
 
 namespace BuildXL.Cache.ContentStore.Service.Grpc
 {
@@ -24,7 +25,7 @@ namespace BuildXL.Cache.ContentStore.Service.Grpc
         }
 
         /// <nodoc />
-        public bool Equals(GrpcCopyClientKey other)
+        public bool Equals([AllowNull]GrpcCopyClientKey other)
         {
             return string.Equals(Host, other.Host, StringComparison.InvariantCultureIgnoreCase)
                 && GrpcPort == other.GrpcPort;

Public/Src/Cache/ContentStore/Library/Stores/FileSystemContentStoreInternalChecker.cs

@@ -498,7 +498,7 @@ namespace BuildXL.Cache.ContentStore.Stores
             }
 
             /// <inheritdoc />
-            public bool Equals(SelfCheckState other) => string.Equals(Epoch, other.Epoch) && LastReconcileTime == other.LastReconcileTime && LastPosition.Equals(other.LastPosition);
+            public bool Equals([AllowNull]SelfCheckState other) => string.Equals(Epoch, other.Epoch) && LastReconcileTime == other.LastReconcileTime && LastPosition.Equals(other.LastPosition);
 
             /// <inheritdoc />
             public override bool Equals(object? obj)

Public/Src/Cache/ContentStore/Library/Stores/PayloadFromDisk.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using BuildXL.Cache.ContentStore.Hashing;
 
 namespace BuildXL.Cache.ContentStore.Stores
@@ -37,6 +38,6 @@ namespace BuildXL.Cache.ContentStore.Stores
     public class ByHashPayloadFromDiskComparer<T> : IComparer<PayloadFromDisk<T>>
     {
         /// <inheritdoc />
-        public int Compare(PayloadFromDisk<T> left, PayloadFromDisk<T> right) => left.Hash.CompareTo(right.Hash);
+        public int Compare([AllowNull]PayloadFromDisk<T> left, [AllowNull]PayloadFromDisk<T> right) => left!.Hash.CompareTo(right!.Hash);
     }
 }

Public/Src/Cache/ContentStore/Vsts/BlobReadOnlyContentSession.cs

@@ -417,7 +417,7 @@ namespace BuildXL.Cache.ContentStore.Vsts
                             var success = DownloadUriCache.Instance.TryGetDownloadUri(contentHash, out var preauthUri);
                             uri = success ? preauthUri.NotNullUri : new Uri("http://empty.com");
 
-                            Directory.CreateDirectory(Directory.GetParent(path).FullName);
+                            Directory.CreateDirectory(Directory.GetParent(path)!.FullName);
 
                             // TODO: Investigate using ManagedParallelBlobDownloader instead (bug 1365340)
                             await ParallelHttpDownload.Download(
@@ -463,7 +463,7 @@ namespace BuildXL.Cache.ContentStore.Vsts
                     }
                     catch (StorageException storageEx) when (storageEx.InnerException is WebException webEx)
                     {
-                        if (((HttpWebResponse)webEx.Response).StatusCode == HttpStatusCode.NotFound)
+                        if (((HttpWebResponse?)webEx.Response)?.StatusCode == HttpStatusCode.NotFound)
                         {
                             return null;
                         }

Public/Src/Cache/DistributedCache.Host/Configuration/BuildXL.Cache.Host.Configuration.dsc

@@ -22,6 +22,7 @@ namespace Configuration {
         skipDocumentationGeneration: true,
 
         allowUnsafeBlocks: false,
+        nullable: true,
         
         internalsVisibleTo: [
             "BuildXL.Cache.Host.Test"

Public/Src/Cache/DistributedCache.Host/Configuration/DeploymentConfiguration.cs

@@ -5,6 +5,8 @@ using System;
 using System.Collections.Generic;
 using BuildXL.Cache.ContentStore.Interfaces.Secrets;
 
+#nullable disable
+
 namespace BuildXL.Cache.Host.Configuration
 {
     /// <summary>

Public/Src/Cache/DistributedCache.Host/Configuration/DeploymentManifest.cs

@@ -3,6 +3,8 @@
 
 using System.Collections.Generic;
 
+#nullable disable
+
 namespace BuildXL.Cache.Host.Configuration
 {
     public class DeploymentManifest

Public/Src/Cache/DistributedCache.Host/Configuration/DeploymentParameters.cs

@@ -4,6 +4,8 @@
 using System;
 using System.Collections.Generic;
 
+#nullable disable
+
 namespace BuildXL.Cache.Host.Configuration
 {
     public class HostParameters

Public/Src/Cache/DistributedCache.Host/Configuration/DeploymentServiceConfiguration.cs

@@ -15,6 +15,6 @@ namespace BuildXL.Cache.Host.Configuration
         /// <summary>
         /// Configure the logging behavior for the service
         /// </summary>
-        public LoggingSettings LoggingSettings { get; set; } = null;
+        public LoggingSettings? LoggingSettings { get; set; } = null;
     }
 }

Public/Src/Cache/DistributedCache.Host/Configuration/DistributedCacheServiceConfiguration.cs

@@ -3,7 +3,7 @@
 
 using System.Runtime.Serialization;
 using BuildXL.Cache.ContentStore.Interfaces.Logging;
-
+#nullable disable
 namespace BuildXL.Cache.Host.Configuration
 {
     [DataContract]

Public/Src/Cache/DistributedCache.Host/Configuration/DistributedContentSettings.cs

@@ -9,7 +9,7 @@ using BuildXL.Cache.ContentStore.Grpc;
 using BuildXL.Cache.ContentStore.Interfaces.Distributed;
 using BuildXL.Cache.ContentStore.Interfaces.Logging;
 using BuildXL.Cache.ContentStore.Interfaces.Utils;
-
+#nullable disable
 namespace BuildXL.Cache.Host.Configuration
 {
     /// <summary>

Public/Src/Cache/DistributedCache.Host/Configuration/DistributedContentSettingsValidator.cs

@@ -29,7 +29,7 @@ namespace BuildXL.Cache.Host.Configuration
                 {
                     if (result != ValidationResult.Success)
                     {
-                        errorList.Add(result.ErrorMessage);
+                        errorList.Add(result.ErrorMessage!);
                     }
                 }
             }

Public/Src/Cache/DistributedCache.Host/Configuration/LauncherApplicationSettings.cs

@@ -15,6 +15,6 @@ namespace BuildXL.Cache.Host.Configuration
         /// <summary>
         /// Configure the logging behavior for the service
         /// </summary>
-        public LoggingSettings LoggingSettings { get; set; } = null;
+        public LoggingSettings? LoggingSettings { get; set; } = null;
     }
 }

Public/Src/Cache/DistributedCache.Host/Configuration/LauncherManifest.cs

@@ -5,6 +5,8 @@ using System.Collections.Generic;
 using System.ComponentModel;
 using System.Runtime.Serialization;
 
+#nullable disable
+
 namespace BuildXL.Cache.Host.Configuration
 {
     /// <summary>

Public/Src/Cache/DistributedCache.Host/Configuration/LauncherSettings.cs

@@ -5,6 +5,8 @@ using System;
 using System.Collections.Generic;
 using BuildXL.Cache.ContentStore.Interfaces.FileSystem;
 
+#nullable disable
+
 namespace BuildXL.Cache.Host.Configuration
 {
     /// <summary>

Public/Src/Cache/DistributedCache.Host/Configuration/LocalCasServiceSettings.cs

@@ -3,6 +3,7 @@
 
 using System.Runtime.Serialization;
 using BuildXL.Cache.ContentStore.Grpc;
+#nullable disable
 
 namespace BuildXL.Cache.Host.Configuration
 {

Public/Src/Cache/DistributedCache.Host/Configuration/LocalCasSettings.cs

@@ -9,6 +9,8 @@ using System.Runtime.Serialization;
 using BuildXL.Cache.ContentStore.Interfaces.FileSystem;
 using BuildXL.Cache.ContentStore.Interfaces.Utils;
 
+#nullable disable
+
 namespace BuildXL.Cache.Host.Configuration
 {
     [DataContract]

Public/Src/Cache/DistributedCache.Host/Configuration/NamedCacheSettings.cs

@@ -5,6 +5,8 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Runtime.Serialization;
 
+#nullable disable
+
 namespace BuildXL.Cache.Host.Configuration
 {
     /// <summary>

Public/Src/Cache/DistributedCache.Host/Configuration/ValidationAttributes.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel.DataAnnotations;
+using System.Diagnostics.CodeAnalysis;
 
 #nullable enable
 
@@ -21,11 +22,11 @@ namespace BuildXL.Cache.Host.Configuration
                 typeof(uint), typeof(float)
             };
 
-            protected override ValidationResult IsValid(object value, ValidationContext validationContext)
+            protected override ValidationResult IsValid([AllowNull]object value, ValidationContext validationContext)
             {
                 if (value == null)
                 {
-                    return ValidationResult.Success;
+                    return Success;
                 }
 
                 // Make sure that we can handle nullable numerical types
@@ -60,7 +61,7 @@ namespace BuildXL.Cache.Host.Configuration
             {
                 return (value > _min || (_minInclusive && value == _min)) &&
                        (value < _max || (_maxInclusive && value == _max))
-                    ? ValidationResult.Success
+                    ? Success
                     : new ValidationResult($"{validationContext.DisplayName} should be in range {(_minInclusive ? "[" : "(")}{_min},{_max}{(_maxInclusive ? "]" : ")")} but its value is {value}");
             }
         }
@@ -76,23 +77,25 @@ namespace BuildXL.Cache.Host.Configuration
                 _allowNull = allowNull;
             }
 
-            protected override ValidationResult IsValid(object value, ValidationContext validationContext)
+            protected override ValidationResult IsValid([AllowNull]object value, ValidationContext validationContext)
             {
                 if (_allowNull && value is null)
                 {
-                    return ValidationResult.Success;
+                    return Success;
                 }
 
                 if (value is string s)
                 {
                     if (Enum.IsDefined(_enumType, s))
                     {
-                        return ValidationResult.Success;
+                        return Success;
                     }
                 }
 
                 return new ValidationResult($"{validationContext.DisplayName} has value '{value}', which is not a valid value for enum {_enumType.FullName}");
             }
         }
+
+        private static ValidationResult Success => ValidationResult.Success!;
     }
 }

Public/Src/Cache/Monitor/Library/Notifications/LogNotifier.cs

@@ -35,7 +35,7 @@ namespace BuildXL.Cache.Monitor.Library.Notifications
         {
             public override TimeSpan Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
             {
-                return TimeSpan.Parse(reader.GetString());
+                return TimeSpan.Parse(reader.GetString()!);
             }
 
             public override void Write(Utf8JsonWriter writer, TimeSpan value, JsonSerializerOptions options)

Public/Src/Cache/Monitor/Library/Rules/Kusto/MachineReimagesRule.cs

@@ -89,7 +89,7 @@ namespace BuildXL.Cache.Monitor.Library.Rules.Kusto
                         var examples = string.Empty;
                         if (!string.IsNullOrEmpty(result.Machines))
                         {
-                            var machines = JsonSerializer.Deserialize<List<string>>(result.Machines);
+                            var machines = JsonSerializer.Deserialize<List<string>>(result.Machines)!;
                             examples = ". Examples: " + string.Join(", ", machines.Take(_configuration.MaximumExamplesOnAlert).Select(m => $"`{m}`"));
                         }
 

Public/Src/Cache/Monitor/Library/Scheduling/RuleScheduler.cs

@@ -129,14 +129,14 @@ namespace BuildXL.Cache.Monitor.App.Scheduling
             _logger = logger;
             _clock = clock;
 
-            _concurrencyBuckets = configuration.MaximumConcurrency.ToDictionary(kvp => kvp.Key, kvp => {
+            _concurrencyBuckets = configuration.MaximumConcurrency?.ToDictionary(kvp => kvp.Key, kvp => {
                 if (kvp.Value <= 0)
                 {
                     return null;
                 }
 
                 return new SemaphoreSlim(kvp.Value);
-            });
+            }) ?? new Dictionary<string, SemaphoreSlim?>();
 
             _notifier = notifier;
         }

Public/Src/Cache/Roxis/Common/ByteString.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Text;
 
@@ -61,7 +62,7 @@ namespace BuildXL.Cache.Roxis.Common
             return obj is ByteString byteString && Equals(byteString);
         }
 
-        public bool Equals(ByteString other)
+        public bool Equals([AllowNull]ByteString other)
         {
             return Value.SequenceEqual(other.Value);
         }

Public/Src/Engine/UnitTests/Engine/Test.BuildXL.Engine.dsc

@@ -24,7 +24,7 @@ namespace Engine {
     ];
 
     // Update the value of this variable if you change the version of Microsoft.Net.Compilers in config.dsc.
-    const microsoftNetCompilerSpec = f`${Context.getMount("FrontEnd").path}/Nuget/specs/Microsoft.Net.Compilers/3.5.0/module.config.bm`;
+    const microsoftNetCompilerSpec = f`${Context.getMount("FrontEnd").path}/Nuget/specs/Microsoft.Net.Compilers/3.7.0/module.config.bm`;
 
     @@public
     export const categoriesToRunInParallel = [

Public/Src/Pips/Dll/Filter/PipFilter.cs

@@ -10,8 +10,6 @@ using BuildXL.Tracing;
 using BuildXL.Utilities;
 using BuildXL.Utilities.Collections;
 
-#pragma warning disable 1591 // disabling warning about missing API documentation; TODO: Remove this line and write documentation!
-
 namespace BuildXL.Pips.Filter
 {
     /// <summary>
@@ -436,6 +434,7 @@ namespace BuildXL.Pips.Filter
             return GetDerivedSpecificHashCode();
         }
 
+        /// <nodoc />
         protected static ReadOnlyHashSet<T> ParallelProcessAllOutputs<T>(
             IPipFilterContext context,
             Action<PipId, HashSet<T>> action,

Public/Src/Utilities/Collections/ArrayView.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.ContractsLight;
+using System.Diagnostics.CodeAnalysis;
 
 namespace BuildXL.Utilities.Collections
 {
@@ -104,7 +105,7 @@ namespace BuildXL.Utilities.Collections
         }
 
         /// <inheritdoc />
-        public bool Equals(ArrayView<T> other)
+        public bool Equals([AllowNull]ArrayView<T> other)
         {
             return m_array == other.m_array &&
                 m_start == other.m_start &&

Public/Src/Utilities/Collections/CollectionUtilities.cs

@@ -457,6 +457,7 @@ namespace BuildXL.Utilities.Collections
 
             bool result = dictionary.TryGetValue(key, out resultingValue);
             Contract.Assert(result);
+            Contract.Assert(resultingValue != null); // Not needed for .net5, but required for .netcore3.1
 
             return resultingValue;
         }

Public/Src/Utilities/Collections/ItemResources.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System;
+using System.Diagnostics.CodeAnalysis;
 
 namespace BuildXL.Utilities.Collections
 {
@@ -63,7 +64,7 @@ namespace BuildXL.Utilities.Collections
         }
 
         /// <inheritdoc />
-        public bool Equals(ItemResources other)
+        public bool Equals([AllowNull]ItemResources other)
         {
             return SemaphoreIncrements == other.SemaphoreIncrements;
         }

Public/Src/Utilities/Collections/MultiValueDictionary.cs

@@ -126,7 +126,8 @@ namespace BuildXL.Utilities.Collections
         }
 
         /// <inheritdoc />
-        public bool TryGetValue(TKey key, [NotNullWhen(true)][MaybeNull]out IReadOnlyList<TValue> multiValues)
+#pragma warning disable CS8767 // Nullability fof reference types in type of parameter 'multiValues' doesn't match implicitly implemented member
+        public bool TryGetValue([NotNull]TKey key, [MaybeNullWhen(false)]out IReadOnlyList<TValue> multiValues)
         {
             if (m_backingDictionary.TryGetValue(key, out List<TValue>? mutableMultiValues))
             {

Public/Src/Utilities/Collections/ReadOnlyArray.cs

@@ -5,6 +5,7 @@ using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Diagnostics.ContractsLight;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 
 namespace BuildXL.Utilities.Collections
@@ -112,7 +113,7 @@ namespace BuildXL.Utilities.Collections
         }
 
         /// <inheritdoc />
-        public bool Equals(ReadOnlyArray<T> other)
+        public bool Equals([AllowNull]ReadOnlyArray<T> other)
         {
             return m_array == other.m_array;
         }

Public/Src/Utilities/Collections/SortedReadOnlyArray.cs

@@ -5,6 +5,7 @@ using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Diagnostics.ContractsLight;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 
 namespace BuildXL.Utilities.Collections
@@ -220,7 +221,7 @@ namespace BuildXL.Utilities.Collections
         }
 
         /// <inheritdoc />
-        public bool Equals(SortedReadOnlyArray<TValue, TComparer> other)
+        public bool Equals([AllowNull]SortedReadOnlyArray<TValue, TComparer> other)
         {
             return m_array == other.m_array;
         }

Public/Src/Utilities/Collections/StructTuple.cs

@@ -126,7 +126,7 @@ namespace BuildXL.Utilities
         public T Value => m_value;
 
         /// <inheritdoc />
-        public bool Equals(EquatableClass<T> other)
+        public bool Equals([AllowNull]EquatableClass<T> other)
         {
             return EqualityComparer<T>.Default.Equals(m_value, other.m_value);
         }
@@ -219,7 +219,7 @@ namespace BuildXL.Utilities
         public T Value => m_value;
 
         /// <inheritdoc />
-        public bool Equals(EquatableEnum<T> other)
+        public bool Equals([AllowNull]EquatableEnum<T> other)
         {
             // Though enums aren't IEquatable for some reason, there's a nonboxing specialization provided by EqualityComparer.Default
             return EqualityComparer<T>.Default.Equals(m_value, other.m_value);
@@ -302,7 +302,7 @@ namespace BuildXL.Utilities
         }
 
         /// <inheritdoc />
-        public bool Equals(StructTuple<TItem1, TItem2> other)
+        public bool Equals([AllowNull]StructTuple<TItem1, TItem2> other)
         {
             return EqualityComparer<TItem1>.Default.Equals(Item1, other.Item1) &&
                    EqualityComparer<TItem2>.Default.Equals(Item2, other.Item2);

Public/Src/Utilities/Instrumentation/Common/Diagnostic.cs

@@ -62,7 +62,7 @@ namespace BuildXL.Utilities.Instrumentation.Common
         }
 
         /// <inheritdoc />
-        public bool Equals(Diagnostic other)
+        public bool Equals([AllowNull]Diagnostic other)
         {
             return ErrorCode == other.ErrorCode && Level == other.Level && string.Equals(Message, other.Message);
         }

Public/Src/Utilities/Instrumentation/Common/Location.cs

@@ -44,7 +44,7 @@ namespace BuildXL.Utilities.Instrumentation.Common
         }
 
         /// <inheritdoc />
-        public bool Equals(Location other) => string.Equals(File, other.File) && Line == other.Line && Position == other.Position;
+        public bool Equals([AllowNull]Location other) => string.Equals(File, other.File) && Line == other.Line && Position == other.Position;
 
         /// <inheritdoc />
         public override bool Equals(object? obj)

cg/nuget/cgmanifest.json

@@ -1347,7 +1347,7 @@
         "Type": "NuGet",
         "NuGet": {
           "Name": "Microsoft.NETCore.Compilers",
-          "Version": "3.5.0"
+          "Version": "3.7.0"
         }
       }
     },
@@ -1437,7 +1437,7 @@
         "Type": "NuGet",
         "NuGet": {
           "Name": "Microsoft.Net.Compilers",
-          "Version": "3.5.0"
+          "Version": "3.7.0"
         }
       }
     },

config.dsc

@@ -85,8 +85,8 @@ config({
                 { id: "System.Diagnostics.DiagnosticSource", version: "4.0.0-beta-23516", alias: "System.Diagnostics.DiagnosticsSource.ForEventHub"},
 
                 // Roslyn
-                { id: "Microsoft.Net.Compilers", version: "3.5.0" }, // Update Public/Src/Engine/UnitTests/Engine/Test.BuildXL.Engine.dsc if you change the version of Microsoft.Net.Compilers.
-                { id: "Microsoft.NETCore.Compilers", version: "3.5.0" },
+                { id: "Microsoft.Net.Compilers", version: "3.7.0" }, // Update Public/Src/Engine/UnitTests/Engine/Test.BuildXL.Engine.dsc if you change the version of Microsoft.Net.Compilers.
+                { id: "Microsoft.NETCore.Compilers", version: "3.7.0" },
                 { id: "Microsoft.CodeAnalysis.Common", version: "3.5.0" },
                 { id: "Microsoft.CodeAnalysis.CSharp", version: "3.5.0" },
                 { id: "Microsoft.CodeAnalysis.VisualBasic", version: "3.5.0" },