dotnet/wpf · error · NotSupportedException

NotSupportedException

Error message

NotSupportedException

What it means

LazyFileStream in ResourcesGenerator is a read-only stream backed by a resource file opened with FileAccess.Read. SetLength is not a meaningful operation on a read-only file, so the override unconditionally throws NotSupportedException.

Solutions

1. Do not call SetLength on this stream; use CanWrite/CanSeek checks before invoking write APIs.
2. Copy the resource stream into a writable MemoryStream or FileStream and pass that to the write-oriented API.
3. Wrap the stream in a ReadOnlyStream adapter that reports CanWrite=false so consumers never attempt writes.

Example fix

// before
writerStream.SetLength(0);
// after
using (var ms = new MemoryStream())
{
    resourceStream.CopyTo(ms);
    ms.SetLength(0); // operate on writable stream
}

if (!stream.CanWrite)
    throw new InvalidOperationException("Stream is read-only; SetLength is not supported");

bool IsWritable(Stream s) => s != null && s.CanWrite;

try { stream.SetLength(0); }
catch (NotSupportedException) { /* copy to writable stream instead */ }

When it happens

Trigger: Any call to SetLength(long) on the LazyFileStream instance returned by ResourcesGenerator (e.g. code treating the resource stream as writable, or a component like a serializer that calls SetLength before writing).

Common situations: Passing the read-only resource stream to APIs that expect a writable Stream (e.g. writers that truncate via SetLength), instead of supplying a MemoryStream or file-backed writable stream.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• 0x80041705
• 0x80041705
• Animation_NoTextChildren
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• Can only call SelectAll when SelectionMode is Multiple or…

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/6d58b63956268578. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationBuildTasks/Microsoft/Build/Tasks/Windows/ResourcesGenerator.cs:93

            {
                get { return SourceStream.Position; }
                set { SourceStream.Position = value; }
            }

            public override int Read(byte[] buffer, int offset, int count)
            {
                return SourceStream.Read(buffer, offset, count);
            }

            public override long Seek(long offset, SeekOrigin origin)
            {
                return SourceStream.Seek(offset, origin);
            }

            public override void SetLength(long value)
            {
                // This is backed by a readonly file.
                throw new NotSupportedException();
            }

            public override void Write(byte[] buffer, int offset, int count)
            {
                // This is backed by a readonly file.
                throw new NotSupportedException();
            }

            protected override void Dispose(bool disposing)
            {
                if (disposing)
                {
                    _sourceStream?.Dispose();
                    _sourceStream = null;
                }
            }
        }
        

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