Write the received file from the stream for handle based FileStreams

src/Verify.Tests/Comparer/HandleStreamTests.Mismatch.verified.bin

@@ -0,0 +1 @@
+TheVerifiedValue

src/Verify.Tests/Comparer/HandleStreamTests.cs

@@ -0,0 +1,24 @@
+// A FileStream built from a handle may have no usable Name: .NET Framework reports
+// "[Unknown]", and modern .NET falls back to that when the path cannot be resolved
+// from the handle. The New path already copes, so only a mismatch exercised it.
+public class HandleStreamTests
+{
+    [Fact]
+    public async Task Mismatch()
+    {
+        using var directory = new TempDirectory();
+        var path = directory.BuildPath("source.txt");
+        File.WriteAllText(path, "TheReceivedValue");
+
+        using var source = File.OpenRead(path);
+        using var stream = new FileStream(source.SafeFileHandle, FileAccess.Read);
+
+        var settings = new VerifySettings();
+        settings.DisableDiff();
+
+        await Assert.ThrowsAsync<VerifyException>(() => Verify(stream, "bin", settings));
+
+        var received = CurrentFile.Relative($"HandleStreamTests.Mismatch.{Namer.RuntimeAndVersion}.received.bin");
+        Assert.Equal("TheReceivedValue", File.ReadAllText(received));
+    }
+}

src/Verify/Compare/FileComparer.cs

@@ -47,7 +47,10 @@ static async Task<EqualityResult> InnerCompare(FilePair file, Stream receivedStr
                 return new(Equality.Equal, compareResult.Message, null, null);
             }
 
-            IoHelpers.CopyFile(fileStream.Name, file.ReceivedPath);
+            // Not CopyFile(fileStream.Name): a FileStream built from a handle has no
+            // usable Name. WriteStream keeps the copy-by-path fast path and falls back
+            // to the handle, which is what the New and empty paths above already do.
+            await IoHelpers.WriteStream(file.ReceivedPath, fileStream);
             return new(Equality.NotEqual, compareResult.Message, null, null);
         }
 

src/todo.md

@@ -76,7 +76,7 @@ All six resolved 2026-08-16 (five fixed here; the inline item resolved as by-des
 - [ ] **Negative sub-hour offsets render unsigned.**
   `Verify/Serialization/DateFormatter_DateTimeOffset.cs:74-82` — `TimeSpan.FromMinutes(-30)` renders `0-30` (positive twin is `+0-30`). No real timezone in that range; constructed offsets only.
 
-- [ ] **Mismatch crash for handle-based `FileStream` received streams.**
+- [x] **Mismatch crash for handle-based `FileStream` received streams.**
   `Verify/Compare/FileComparer.cs:50` — NotEqual fast path copies by `fileStream.Name` with no fallback; handle-based streams have `Name == "[Unknown]"`. First (New) run succeeds via the guarded `IoHelpers.WriteStream` path; later mismatches throw the generic "Failed to compare files".
 
 - [ ] **`PrefixUnique` set is case-sensitive on case-insensitive filesystems.**