Revert "chore: remove unused attributes"

This reverts commit c87e23bd84.

.vsts-ci.yml

@@ -12,8 +12,7 @@ jobs:
     BuildConfiguration: Release
     ANDROID_NDK_HOME: C:\Microsoft\AndroidNDK64\android-ndk-r16b
     ANDROID_NDK_PATH: C:\Microsoft\AndroidNDK64\android-ndk-r16b
-    AndroidNdkDirectory: C:\Microsoft\AndroidNDK64\android-ndk-r16b
-    DOTNET_CLI_TELEMETRY_OPTOUT: true
+    AndroidNdkDirectory: C:\Microsoft\AndroidNDK64\android-ndk-r16b 
 
   steps:
   
@@ -86,7 +85,6 @@ jobs:
 
   variables:
     NUGET_PACKAGES: $(build.sourcesdirectory)/.nuget
-    DOTNET_CLI_TELEMETRY_OPTOUT: true
 
   steps:
   - checkout: self

CommunityToolkit.Common/Attributes/NotNullIfNotNullAttribute.cs

@@ -0,0 +1,29 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+#if !NETSTANDARD2_1_OR_GREATER
+
+namespace System.Diagnostics.CodeAnalysis
+{
+    /// <summary>
+    /// Specifies that the output will be non-null if the named parameter is non-null.
+    /// </summary>
+    /// <remarks>Internal copy from the BCL attribute.</remarks>
+    [AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, AllowMultiple = true, Inherited = false)]
+    internal sealed class NotNullIfNotNullAttribute : Attribute
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="NotNullIfNotNullAttribute"/> class.
+        /// </summary>
+        /// <param name="parameterName">The associated parameter name. The output will be non-null if the argument to the parameter specified is non-null.</param>
+        public NotNullIfNotNullAttribute(string parameterName) => ParameterName = parameterName;
+
+        /// <summary>
+        /// Gets the associated parameter name.
+        /// </summary>
+        public string ParameterName { get; }
+    }
+}
+
+#endif

CommunityToolkit.Common/Attributes/NotNullWhenAttribute.cs

@@ -0,0 +1,29 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+#if !NETSTANDARD2_1_OR_GREATER
+
+namespace System.Diagnostics.CodeAnalysis
+{
+    /// <summary>
+    /// Specifies that when a method returns <see cref="ReturnValue"/>, the parameter will not be null even if the corresponding type allows it.
+    /// </summary>
+    /// <remarks>Internal copy from the BCL attribute.</remarks>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+    internal sealed class NotNullWhenAttribute : Attribute
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="NotNullWhenAttribute"/> class.
+        /// </summary>
+        /// <param name="returnValue">The return value condition. If the method returns this value, the associated parameter will not be null.</param>
+        public NotNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+        /// <summary>
+        /// Gets a value indicating whether the annotated variable is not <see langword="null"/>.
+        /// </summary>
+        public bool ReturnValue { get; }
+    }
+}
+
+#endif