dotnet/wpf · error · ArgumentException

SR.FileModeUnsupported

Error message

SR.FileModeUnsupported

What it means

ArgumentException thrown by StreamInfo.GetStream when FileMode.Append is requested. Append mode has no meaning for OLE structured-storage streams inside a compound file, so the library rejects it up front. The caller must pick a FileMode supported by the compound file model (Create, CreateNew, Open, OpenOrCreate).

Solutions

1. Replace FileMode.Append with FileMode.OpenOrCreate and seek to the end of the stream if append semantics are needed.
2. Use FileMode.Open with FileAccess.Write, then seek to stream length before writing.
3. Restructure the call site so append-style writes use a standard FileStream outside the compound file if applicable.

Example fix

// before
Stream s = streamInfo.GetStream(FileMode.Append, FileAccess.Write);
// after
Stream s = streamInfo.GetStream(FileMode.OpenOrCreate, FileAccess.Write);
s.Seek(0, SeekOrigin.End); // append semantics

if (mode == FileMode.Append) throw new ArgumentException("FileMode.Append is not supported by CompoundFile StreamInfo.GetStream; use OpenOrCreate and seek to end.");

bool IsSupportedGetStreamMode(FileMode m) => m == FileMode.Create || m == FileMode.Open || m == FileMode.OpenOrCreate;

try { s = streamInfo.GetStream(mode, access); }
catch (ArgumentException) { s = streamInfo.GetStream(FileMode.OpenOrCreate, access); s.Seek(0, SeekOrigin.End); }

When it happens

Trigger: Calling streamInfo.GetStream(FileMode.Append, ...) or GetStream(FileMode.Append, access, sharing) on a CompoundFile StreamInfo.

Common situations: Porting code written for System.IO.File/FileStream (where Append is valid) to the Packaging/CompoundFile API; writing generic file-helper code that passes FileMode.Append.

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.FileModeInvalid
• Data space transform stack includes undefined transform…
• FileMode value is not valid.
• Transform identifier type is not supported.
• Transform label name is already in use.

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/ac32e39907a7ef20. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsBase/System/IO/Packaging/CompoundFile/StreamInfo.cs:264

            if (parentStorage.Root.OpenAccess == FileAccess.ReadWrite)
            {
                // Generate the access flags from the access parameter
                access = FileAccess.ReadWrite;
            }

            // Generate the access flags from the access parameter
            SafeNativeCompoundFileMethods.UpdateModeFlagFromFileAccess( access, ref grfMode );

            // Only SHARE_EXCLUSIVE for now, FileShare issue TBD
            grfMode |= SafeNativeCompoundFileConstants.STGM_SHARE_EXCLUSIVE;

            CheckAccessMode(grfMode);

            // Act based on FileMode
            switch(mode)
            {
                case FileMode.Append:
                    throw new ArgumentException(
                        SR.FileModeUnsupported);
                case FileMode.Create:
                    // Check to make sure root container is not read-only, and that
                    //  we're not pointlessly trying to create a read-only stream.
                    CreateTimeReadOnlyCheck(openFileAccess);
                    
                    // Close down any existing streams floating out there
                    if (null != core.exposedStream)
                    {
                        ((Stream)(core.exposedStream)).Close();
                    }
                    core.exposedStream = null;
                    
                    if( null != core.safeIStream )
                    {
                        // Close out existing stream
                        ((IDisposable) core.safeIStream).Dispose();
                        core.safeIStream = null;

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