Making resolve request handler callable in cohosting

- Making the resolve completion use document for cohosting
- Serializing TextDocument property into Data member of completion items so that Roslyn will forward the request to us
- Basic sanity test (shows we are getting called now)

src/Razor/src/Microsoft.VisualStudio.LanguageServices.Razor/LanguageClient/Cohost/CohostDocumentCompletionEndpoint.cs

@@ -25,6 +25,7 @@ using Microsoft.VisualStudio.Razor.Snippets;
 using Response = Microsoft.CodeAnalysis.Razor.Remote.RemoteResponse<Microsoft.VisualStudio.LanguageServer.Protocol.VSInternalCompletionList?>;
 using RoslynCompletionParams = Roslyn.LanguageServer.Protocol.CompletionParams;
 using RoslynLspExtensions = Roslyn.LanguageServer.Protocol.RoslynLspExtensions;
+using RoslynTextDocumentIdentifier = Roslyn.LanguageServer.Protocol.TextDocumentIdentifier;
 
 namespace Microsoft.VisualStudio.Razor.LanguageClient.Cohost;
 
@@ -64,7 +65,7 @@ internal sealed class CohostDocumentCompletionEndpoint(
                 Method = Methods.TextDocumentCompletionName,
                 RegisterOptions = new CompletionRegistrationOptions()
                 {
-                    ResolveProvider = true, // TODO - change to true when Resolve is implemented
+                    ResolveProvider = false, // TODO - change to true when Resolve is implemented
                     TriggerCharacters = CompletionTriggerAndCommitCharacters.AllTriggerCharacters,
                     AllCommitCharacters = CompletionTriggerAndCommitCharacters.AllCommitCharacters
                 }
@@ -88,8 +89,11 @@ internal sealed class CohostDocumentCompletionEndpoint(
             return null;
         }
 
+        // Save as it may be modified if we forward request to HTML language server
+        var originalTextDocumentIdentifier = request.TextDocument;
+
         // Return immediately if this is auto-shown completion but auto-shown completion is disallowed in settings
-        var clientSettings =  _clientSettingsManager.GetClientSettings();
+        var clientSettings = _clientSettingsManager.GetClientSettings();
         var autoShownCompletion = completionContext.TriggerKind != CompletionTriggerKind.Invoked;
         if (autoShownCompletion && !clientSettings.ClientCompletionSettings.AutoShowCompletion)
         {
@@ -187,6 +191,11 @@ internal sealed class CohostDocumentCompletionEndpoint(
                 completionContext.TriggerCharacter);
         }
 
+        if (combinedCompletionList != null)
+        {
+            AddResolutionParams(combinedCompletionList, originalTextDocumentIdentifier);
+        }
+
         return combinedCompletionList;
     }
 
@@ -273,6 +282,15 @@ internal sealed class CohostDocumentCompletionEndpoint(
         return completionList;
     }
 
+    private static void AddResolutionParams(VSInternalCompletionList completionList, RoslynTextDocumentIdentifier textDocumentIdentifier)
+    {
+        foreach (var item in completionList.Items)
+        {
+            var resolutionParams = CohostDocumentCompletionResolveParams.Create(textDocumentIdentifier);
+            item.Data = JsonSerializer.SerializeToElement(resolutionParams);
+        }
+    }
+
     internal TestAccessor GetTestAccessor() => new(this);
 
     internal readonly struct TestAccessor(CohostDocumentCompletionEndpoint instance)

src/Razor/src/Microsoft.VisualStudio.LanguageServices.Razor/LanguageClient/Cohost/CohostDocumentCompletionResolveEndpoint.cs

@@ -17,7 +17,7 @@ namespace Microsoft.VisualStudio.Razor.LanguageClient.Cohost;
 [Export(typeof(IDynamicRegistrationProvider))]
 [ExportCohostStatelessLspService(typeof(CohostDocumentCompletionResolveEndpoint))]
 #pragma warning restore RS0030 // Do not use banned APIs
-internal sealed class CohostDocumentCompletionResolveEndpoint : AbstractRazorCohostRequestHandler<RoslynVSInternalCompletionItem, RoslynVSInternalCompletionItem>, IDynamicRegistrationProvider
+internal sealed class CohostDocumentCompletionResolveEndpoint : AbstractRazorCohostDocumentRequestHandler<RoslynVSInternalCompletionItem, RoslynVSInternalCompletionItem>, IDynamicRegistrationProvider
 {
     protected override bool MutatesSolutionState => false;
 
@@ -29,18 +29,45 @@ internal sealed class CohostDocumentCompletionResolveEndpoint : AbstractRazorCoh
         {
             return [new Registration()
             {
-                Method = Methods.TextDocumentCompletionResolveName
+                Method = Methods.TextDocumentCompletionResolveName,
+                RegisterOptions = new CompletionRegistrationOptions()
+                {
+                    ResolveProvider = true
+                }
             }];
         }
 
         return [];
     }
 
-    protected override Task<RoslynVSInternalCompletionItem> HandleRequestAsync(RoslynVSInternalCompletionItem request, RazorCohostRequestContext context, CancellationToken cancellationToken)
-        => HandleRequestAsync(request);
-
-    private Task<RoslynVSInternalCompletionItem> HandleRequestAsync(RoslynVSInternalCompletionItem request)
+    protected override RazorTextDocumentIdentifier? GetRazorTextDocumentIdentifier(RoslynVSInternalCompletionItem request)
     {
+        var completionResolveParams = CohostDocumentCompletionResolveParams.GetCohostDocumentCompletionResolveParams(request);
+        return Roslyn.LanguageServer.Protocol.RoslynLspExtensions.ToRazorTextDocumentIdentifier(completionResolveParams.TextDocument);
+    }
+
+    protected override Task<RoslynVSInternalCompletionItem> HandleRequestAsync(RoslynVSInternalCompletionItem request, RazorCohostRequestContext context, CancellationToken cancellationToken)
+        => HandleRequestAsync(request, cancellationToken);
+
+    private Task<RoslynVSInternalCompletionItem> HandleRequestAsync(RoslynVSInternalCompletionItem request, CancellationToken cancellationToken)
+    {
+        if (cancellationToken.IsCancellationRequested)
+        {
+            return Task.FromResult(request);
+        }
+
+        // TODO: actual request processing code
+
         return Task.FromResult(request);
     }
+
+    internal TestAccessor GetTestAccessor() => new(this);
+
+    internal readonly struct TestAccessor(CohostDocumentCompletionResolveEndpoint instance)
+    {
+        public Task<RoslynVSInternalCompletionItem> HandleRequestAsync(
+            RoslynVSInternalCompletionItem request,
+            CancellationToken cancellationToken)
+                => instance.HandleRequestAsync(request, cancellationToken);
+    }
 }

src/Razor/src/Microsoft.VisualStudio.LanguageServices.Razor/LanguageClient/Cohost/CohostDocumentCompletionResolveParams.cs

@@ -0,0 +1,52 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the MIT license. See License.txt in the project root for license information.
+
+using System.Text.Json;
+using System;
+using System.Text.Json.Serialization;
+using Roslyn.LanguageServer.Protocol;
+
+namespace Microsoft.VisualStudio.Razor.LanguageClient.Cohost;
+
+// Data that's getting sent with each completion item so that we can provide document ID
+// to Roslyn language server which will use the URI to figure out that language of the request
+// and forward the request to us. It gets serialized as Data member of the completion item.
+// Without it, Roslyn won't forward the completion resolve request to us.
+internal sealed class CohostDocumentCompletionResolveParams
+{
+    // NOTE: Capital T here is required to match Roslyn's DocumentResolveData structure, so that the Roslyn
+    //       language server can correctly route requests to us in cohosting. In future when we normalize
+    //       on to Roslyn types, we should inherit from that class so we don't have to remember to do this.
+    [JsonPropertyName("TextDocument")]
+    public required VSTextDocumentIdentifier TextDocument { get; set; }
+
+    public static CohostDocumentCompletionResolveParams Create(TextDocumentIdentifier textDocumentIdentifier)
+    {
+        var vsTextDocumentIdentifier = textDocumentIdentifier is VSTextDocumentIdentifier vsTextDocumentIdentifierValue
+        ? vsTextDocumentIdentifierValue
+            : new VSTextDocumentIdentifier() { Uri = textDocumentIdentifier.Uri };
+
+        var resolutionParams = new CohostDocumentCompletionResolveParams()
+        {
+            TextDocument = vsTextDocumentIdentifier
+        };
+
+        return resolutionParams;
+    }
+
+    public static CohostDocumentCompletionResolveParams GetCohostDocumentCompletionResolveParams(VSInternalCompletionItem request)
+    {
+        if (request.Data is not JsonElement paramsObj)
+        {
+            throw new InvalidOperationException($"Invalid Completion Resolve Request Received");
+        }
+
+        var resolutionParams = paramsObj.Deserialize<CohostDocumentCompletionResolveParams>();
+        if (resolutionParams is null)
+        {
+            throw new InvalidOperationException($"request.Data should be convertible to {nameof(CohostDocumentCompletionResolveParams)}");
+        }
+
+        return resolutionParams;
+    }
+}

src/Razor/test/Microsoft.VisualStudio.LanguageServices.Razor.Test/Cohost/CohostDocumentCompletionResolveEndpointTest.cs

@@ -0,0 +1,58 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the MIT license. See License.txt in the project root for license information.
+
+using System.Text.Json;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Razor.Test.Common;
+using Microsoft.CodeAnalysis.ExternalAccess.Razor;
+using Xunit;
+using Xunit.Abstractions;
+using RoslynTextDocumentIdentifier = Roslyn.LanguageServer.Protocol.TextDocumentIdentifier;
+using RoslynVSInternalCompletionItem = Roslyn.LanguageServer.Protocol.VSInternalCompletionItem;
+
+namespace Microsoft.VisualStudio.Razor.LanguageClient.Cohost;
+
+public class CohostDocumentCompletionResolveEndpointTest(ITestOutputHelper testOutputHelper) : CohostEndpointTestBase(testOutputHelper)
+{
+    [Fact]
+    public async Task ResolveReturnsSelf()
+    {
+        await VerifyCompletionItemResolveAsync(
+            input: """
+                This is a Razor document.
+
+                <div st$$></div>
+
+                The end.
+                """,
+             initialItemLabel: "TestItem1",
+             expectedItemLabel: "TestItem1");
+    }
+
+    private async Task VerifyCompletionItemResolveAsync(
+        TestCode input,
+        string initialItemLabel,
+        string expectedItemLabel)
+    {
+        var document = await CreateProjectAndRazorDocumentAsync(input.Text);
+
+        var endpoint = new CohostDocumentCompletionResolveEndpoint();
+
+        var textDocumentIdentifier = new RoslynTextDocumentIdentifier()
+        {
+            Uri = document.CreateUri()
+        };
+
+        var resolutionParams = CohostDocumentCompletionResolveParams.Create(textDocumentIdentifier);
+
+        var request = new RoslynVSInternalCompletionItem()
+        {
+            Data = JsonSerializer.SerializeToElement(resolutionParams),
+            Label = initialItemLabel
+        };
+
+        var result = await endpoint.GetTestAccessor().HandleRequestAsync(request, DisposalToken);
+
+        Assert.Equal(result.Label, expectedItemLabel);
+    }
+}