dotnet/wpf · error · NotSupportedException

Stream does not support Write

Error message

Stream does not support Write

What it means

ReaderStream overrides Stream.Write(byte[], int, int) to throw NotSupportedException because it is strictly a read-side view over the ReadWriteStreamManager; CanWrite returns false and all writes are rejected.

Solutions

1. Write only to the writer-side XamlStream; treat ReaderStream as strictly read-only.
2. Check stream.CanWrite before selecting it as a write destination.
3. If both read and write are needed on the same data, use a MemoryStream or temporary file and pass writable views to each side.
4. Catch NotSupportedException around write attempts on streams of unknown writability.

Example fix

// before
readerStream.Write(buffer, 0, buffer.Length); // NotSupportedException
// after
if (readerStream.CanWrite)
{
    readerStream.Write(buffer, 0, buffer.Length);
}
else
{
    throw new InvalidOperationException("ReaderStream is read-only; write via the writer stream.");
}

if (!stream.CanWrite) throw new InvalidOperationException("Cannot write to a read-only ReaderStream.");

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

try { stream.Write(buffer, 0, count); }
catch (NotSupportedException) { /* route writes to the writer-side stream */ }

When it happens

Trigger: Calling Write (or WriteByte, or APIs like StreamWriter/BinaryWriter/CopyTo destinations that write) on the reader-side ReaderStream. Source: XamlStream.cs:857-860.

Common situations: Passing the reader stream as a destination to CopyTo, StreamWriter, serializers, or image encoders; utility code that writes a header back into a stream it is also reading.

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

• SR.SetLengthNotSupported
• Image_OriginalStreamReadOnly
• NotSupportedException
• NotSupportedException
• NotSupportedException

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/a3aa7f7a1e563570. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/XamlStream.cs:859

        {
            return StreamManager.ReaderSeek(offset,loc);
        }

        /// <summary>
        /// Override of Stream.SetLength
        /// </summary>
        public override void SetLength(long value)
        {
            throw new NotSupportedException();
        }


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

        #endregion Overrides

        #region Methods

        /// <summary>
        /// Called by the reader to indicate all bytes up to and
        /// including the position are no longer needed
        /// After making this call the reader cannot go back and
        /// read this data
        /// </summary>
        /// <param name="position"></param>
        internal void ReaderDoneWithFileUpToPosition(long position)
        {
            StreamManager.ReaderDoneWithFileUpToPosition(position);
        }

View on GitHub (pinned to 81131a70a4)