dotnet/wpf · error · NotSupportedException

Stream does not support Read

Error message

Stream does not support Read

What it means

The WriterStream in XamlStream is write-only: its Read override unconditionally throws NotSupportedException. Any attempt to read bytes from this stream fails because it only feeds data from the BAML writer to the reader.

Solutions

1. Read from the corresponding reader side of the XamlStream instead of the writer stream.
2. Write to a MemoryStream if you later need to read the bytes back, and feed that to the API.
3. Remove read operations from code paths that only own the writer stream.

Example fix

// before
int b = writerStream.ReadByte(); // throws
// after
writerStream.Write(data, 0, data.Length);
// read from the XamlStream reader side instead:
int b = xamlStreamReader.Read();

if (!stream.CanRead) throw new InvalidOperationException("This stream is write-only");

bool canRead = stream is { CanRead: true };

try { stream.Read(buffer, 0, count); }
catch (NotSupportedException) { /* use the reader side of the XamlStream instead */ }

When it happens

Trigger: Calling Read/ReadByte (directly or via helpers like StreamReader, CopyTo, ToArray-style reads) on the XamlStream writer stream.

Common situations: Generic stream utilities (CopyTo, reading back what was written, hashing content) applied to the write half of the internal XAML parser pipe.

Related errors

• ArgumentOutOfRangeException(nameof(offset))
• SR.ParserWriterNoSeekEnd
• SR.ParserWriterUnknownOrigin
• Stream does not support Position setter
• can’t seek on baseStream

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/c2152ea37c0780d9. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/XamlStream.cs:662

        /// </summary>
        public override long Position
        {
            get
            {
                return -1;
            }
            set
            {
                throw new NotSupportedException();
            }
        }

        /// <summary>
        /// Override of Stream.Read
        /// </summary>
        public override int Read(byte[] buffer, int offset, int count)
        {
            throw new NotSupportedException();
        }

        /// <summary>
        /// Override of Stream.ReadByte
        /// </summary>
        public override int ReadByte()
        {
            throw new NotSupportedException();
        }

        /// <summary>
        /// Override of Stream.Seek
        /// </summary>
        public override long Seek(long offset, SeekOrigin loc)
        {
            return StreamManager.WriterSeek(offset,loc);
        }

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