Merge pull request #45151 from ryzngard/issue/telemetry_determinism

Fix possible problem with TelemetryId

src/EditorFeatures/CSharpTest/Extensions/TelemetryExtensionTests.cs

+// 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.
+
+using System;
+using Microsoft.CodeAnalysis.Editor.Shared.Extensions;
+using Xunit;
+
+namespace Microsoft.CodeAnalysis.Editor.CSharp.UnitTests.Extensions
+{
+    public class TelemetryExtensionTests
+    {
+        [Fact]
+        public void TestConstantTelemetryId()
+        {
+            var expected = Guid.Parse("00000000-0000-0000-c4c5-914100000000");
+            var actual = typeof(TelemetryExtensionTests).GetTelemetryId();
+            var actualBytes = actual.ToByteArray();
+
+            // The first 4 bytes are using platform dependent hashcode and 
+            // are not deterministic. This is a known limitation and corrected 
+            // with the last 8 bytes of the GUID
+            for (int i = 0; i < 4; i++)
+            {
+                actualBytes[i] = 0;
+            }
+
+            // If the assertion fails then telemetry ids could be changing
+            // making them hard to track. It's important to not regress 
+            // the ability to track telemetry across versions of Roslyn.
+            Assert.Equal(new Guid(actualBytes), expected);
+        }
+    }
+}

src/EditorFeatures/Core/Shared/Extensions/TelemetryExtensions.cs

@@ -12,14 +12,31 @@ namespace Microsoft.CodeAnalysis.Editor.Shared.Extensions
     internal static class TelemetryExtensions
     {
         public static Guid GetTelemetryId(this Type type, short scope = 0)
-            => new Guid(type.GetTelemetryPrefix(), scope, 0, 0, 0, 0, 0, 0, 0, 0, 0);
-
-        public static int GetTelemetryPrefix(this Type type)
         {
             type = GetTypeForTelemetry(type);
 
             // AssemblyQualifiedName will change across version numbers, FullName won't
-            return type.FullName.GetHashCode();
+
+            // GetHashCode on string is not stable. From documentation: 
+            // The hash code itself is not guaranteed to be stable. 
+            // Hash codes for identical strings can differ across .NET implementations, across .NET versions, 
+            // and across .NET platforms (such as 32-bit and 64-bit) for a single version of .NET. In some cases, 
+            // they can even differ by application domain. 
+            // This implies that two subsequent runs of the same program may return different hash codes.
+            //
+            // As such, we keep the original prefix that was being used for legacy purposes, but 
+            // use a stable hashing algorithm (FNV) that doesn't depend on platform 
+            // or .NET implementation. We can map the prefix across legacy versions, but 
+            // as we support more platforms and variations of builds the suffix will be constant
+            // and usable
+            var prefix = type.FullName.GetHashCode();
+            var suffix = Roslyn.Utilities.Hash.GetFNVHashCode(type.FullName);
+
+            // Suffix is the remaining 8 bytes, and the hash code only makes up 4. Pad 
+            // the remainder with an empty byte array
+            var suffixBytes = BitConverter.GetBytes(suffix).Concat(new byte[4]).ToArray();
+
+            return new Guid(prefix, scope, 0, suffixBytes);
         }
 
         public static Type GetTypeForTelemetry(this Type type)