dotnet/wpf · error · NotSupportedException

SR.SetLengthNotSupported

Error message

SR.SetLengthNotSupported

What it means

CFStream.SetLength throws NotSupportedException with SR.SetLengthNotSupported when the stream was opened without write access. Changing a compound-file stream's size requires write mode; on a read-only stream the operation is unsupported, matching Stream contract. The check happens after the disposed check and before length validation.

Solutions

1. Open the compound file with write access (FileAccess.ReadWrite) so CanWrite is true
2. Guard with if (stream.CanWrite) before calling SetLength
3. If the stream is meant to be read-only, avoid resizing; copy data to a writable MemoryStream instead
4. Catch NotSupportedException and fall back to a writable copy

Example fix

// before
readOnlyCfStream.SetLength(newSize);
// after
if (!readOnlyCfStream.CanWrite) {
    var ms = new MemoryStream();
    readOnlyCfStream.CopyTo(ms);
    ms.SetLength(newSize);
}

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

static bool CanResize(Stream s) => s.CanWrite;

try { stream.SetLength(newLength); } catch (NotSupportedException) { /* copy to writable stream and retry */ }

When it happens

Trigger: Calling SetLength on a CFStream obtained from a compound file opened read-only (e.g. FileInfo.OpenRead, FileAccess.Read, or a packaging stream opened with read access).

Common situations: Opening an .docx/.xlsx OLE compound file read-only and then trying to resize an embedded stream; code paths that assume the stream is writable because they were tested against FileStream in read-write mode.

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

• NotSupportedException
• SR.SetPositionNotSupported
• Stream does not support Write
• ArgumentOutOfRangeException(nameof(offset))
• can’t seek on baseStream

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/6bfdfb79c53cbe38. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsBase/MS/Internal/IO/Packaging/CompoundFile/CFStream.cs:192

                throw new System.ComponentModel.InvalidEnumArgumentException("origin", (int) origin, typeof(SeekOrigin));
        }

        _safeIStream.Seek( offset, translatedSeekOrigin, out seekPos );

        return seekPos;
    }

    /// <summary>
    /// See .NET Framework SDK under System.IO.Stream
    /// </summary>
    /// <param name="newLength">New length</param>
    public override void SetLength( long newLength )
    {
        CheckDisposedStatus();

        if (!CanWrite)
        {
            throw new NotSupportedException(SR.SetLengthNotSupported);
        }

        if( 0 > newLength )
        {
            throw new ArgumentOutOfRangeException(nameof(newLength),
                SR.StreamLengthNegative);
        }
        
        _safeIStream.SetSize( newLength );

        // updating the stream pointer if the stream has been truncated.
        if (newLength < this.Position)
            this.Position = newLength;
    }

    /// <summary>
    /// See .NET Framework SDK under System.IO.Stream
    /// </summary>

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