Use OSPath.PathComparer in StaticWebAssetEndpoint.ToAssetFileDictionary

src/StaticWebAssetsSdk/Tasks/Data/StaticWebAssetEndpoint.cs

@@ -7,6 +7,7 @@
 using System.Collections.Concurrent;
 using System.Diagnostics;
 using System.Text.Json.Serialization;
+using Microsoft.AspNetCore.StaticWebAssets.Tasks.Utils;
 using Microsoft.Build.Framework;
 
 namespace Microsoft.AspNetCore.StaticWebAssets.Tasks;
@@ -175,7 +176,7 @@ internal void MarkProperiesAsModified()
 
     internal static IDictionary<string, List<StaticWebAssetEndpoint>> ToAssetFileDictionary(ITaskItem[] candidateEndpoints)
     {
-        var result = new Dictionary<string, List<StaticWebAssetEndpoint>>(candidateEndpoints.Length / 2);
+        var result = new Dictionary<string, List<StaticWebAssetEndpoint>>(candidateEndpoints.Length / 2, OSPath.PathComparer);
 
         foreach (var candidate in candidateEndpoints)
         {

test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/StaticWebAssetEndpointToAssetFileDictionaryTest.cs

@@ -0,0 +1,49 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#nullable disable
+
+using Microsoft.Build.Framework;
+using Microsoft.Build.Utilities;
+using Microsoft.NET.TestFramework;
+
+namespace Microsoft.AspNetCore.StaticWebAssets.Tasks;
+
+public class StaticWebAssetEndpointToAssetFileDictionaryTest
+{
+    // ToAssetFileDictionary keys are absolute filesystem paths (endpoint.AssetFile, typically
+    // populated by upstream targets via MSBuild's %(FullPath) modifier). On Windows the
+    // filesystem is case-insensitive, so two endpoints whose AssetFile values differ only in
+    // casing must collide into a single dictionary entry — otherwise downstream lookups such
+    // as ApplyCompressionNegotiation's `endpointsByAsset.TryGetValue(compressedAsset.Identity, ...)`
+    // can silently miss when MSBuild round-trips path casing differently between producers
+    // (e.g. ResolveProjectReferences vs DefineStaticWebAssets).
+    //
+    // This test is Windows-only because OSPath.PathComparer is OrdinalIgnoreCase only on
+    // Windows; on Linux/macOS it is StringComparer.Ordinal (matching the case-sensitive
+    // filesystem), which is observationally identical to the default Dictionary comparer.
+    [PlatformSpecificFact(TestPlatforms.Windows)]
+    public void GroupsEndpoints_ByPath_CaseInsensitively_OnWindows()
+    {
+        var upperCased = MakeEndpointItem("C:\\Repo\\WWWRoot\\site.css", "/_content/lib/site.css");
+        var lowerCased = MakeEndpointItem("c:\\repo\\wwwroot\\site.css", "/_content/lib/site.css.gz");
+
+        var result = StaticWebAssetEndpoint.ToAssetFileDictionary([upperCased, lowerCased]);
+
+        result.Should().HaveCount(1,
+            "two endpoints whose AssetFile values differ only in casing point to the same " +
+            "file on Windows and must share a single dictionary entry");
+        var only = result.Values.Single();
+        only.Should().HaveCount(2,
+            "both endpoints must be grouped under the case-insensitive key");
+    }
+
+    private static ITaskItem MakeEndpointItem(string assetFile, string route) =>
+        new TaskItem(route, new Dictionary<string, string>
+        {
+            [nameof(StaticWebAssetEndpoint.AssetFile)] = assetFile,
+            [nameof(StaticWebAssetEndpoint.Selectors)] = "",
+            [nameof(StaticWebAssetEndpoint.ResponseHeaders)] = "",
+            [nameof(StaticWebAssetEndpoint.EndpointProperties)] = "",
+        });
+}