What causes "SR.FileModeUnsupported" in dotnet/wpf?

Use FileMode.OpenOrCreate or FileMode.Open instead of Append.

dotnet/wpf · error · ArgumentException

SR.FileModeUnsupported

Error message

SR.FileModeUnsupported

What it means

StorageRoot.Open rejects FileMode.Append because the compound-file (Structured Storage) API has no append semantics for root storages. The method converts a FileMode into native STGM flags and throws ArgumentException for modes it cannot represent.

Solutions

Use FileMode.OpenOrCreate or FileMode.Open instead of Append.
To add content, open the package with Open/OpenOrCreate and add new parts; compound files do not support append.
Guard FileMode values before calling Package.Open.

Example fix

// before
Package.Open(path, FileMode.Append, FileAccess.Write);
// after
Package.Open(path, FileMode.OpenOrCreate, FileAccess.ReadWrite);

Defensive patterns

Strategy: validation

Validate before calling

if (mode == FileMode.Append)
    throw new ArgumentException("Append is not supported for compound file packages");

Type guard

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

Try / catch

try { pkg = Package.Open(path, mode, access); }
catch (ArgumentException ex) when (ex.Message.Contains("FileMode")) { /* fall back to OpenOrCreate */ }

Prevention

Never use FileMode.Append with Package.Open / StorageRoot.Open.
Centralize FileMode-to-package mapping in one validated helper.
Remember compound files have no append semantics.

When it happens

Trigger: Calling StorageRoot.Open (or Package.Open on a compound file) with FileMode.Append.

Common situations: Code written generically over FileMode values that handles Append; appending to an existing OPC package by mistake instead of OpenOrCreate.

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

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/4cd121584fb96698. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsBase/System/IO/Packaging/CompoundFile/StorageRoot.cs:245

        int sectorSize )
    {
        int  grfMode = 0;
        int  returnValue = 0;

        // Simple path validation
        ContainerUtilities.CheckStringAgainstNullAndEmpty( path, "Path" );

        Guid IID_IStorage = new Guid(0x0000000B,0x0000,0x0000,0xC0,0x00,
                                     0x00,0x00,0x00,0x00,0x00,0x46);

        IStorage newRootStorage;

        ////////////////////////////////////////////////////////////////////
        // Generate STGM from FileMode
        switch(mode)
        {
            case FileMode.Append:
                throw new ArgumentException(
                    SR.FileModeUnsupported);
            case FileMode.Create:
                grfMode |= SafeNativeCompoundFileConstants.STGM_CREATE;
                break;
            case FileMode.CreateNew:
                {
                    FileInfo existTest = new FileInfo(path);
                    if( existTest.Exists )
                    {
                        throw new IOException(
                            SR.FileAlreadyExists);
                    }
                }
                goto case FileMode.Create;
            case FileMode.Open:
                break;
            case FileMode.OpenOrCreate:
                {

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