dotnet/wpf · error · NotSupportedException

Write-only mode is not supported.

Error message

Write-only mode is not supported.

What it means

UpdateModeFlagFromFileAccess converts a FileAccess into an OLE STGM mode. Write-only access (FileAccess.Write alone) is deliberately unsupported because write-only compound-file scenarios are tricky and rarely used, so a NotSupportedException (SR.WriteOnlyUnsupported) is thrown.

Solutions

1. Open with FileAccess.ReadWrite instead of FileAccess.Write
2. If write-only semantics are desired, emulate by opening ReadWrite and not reading
3. Refactor to avoid reusing the write-only FileAccess value for packaging APIs

Example fix

// before
container = new CompoundFileContainer(stream, FileAccess.Write, ...);
// after
container = new CompoundFileContainer(stream, FileAccess.ReadWrite, ...);

if (access == FileAccess.Write) access = FileAccess.ReadWrite; // write-only unsupported

static bool IsSupportedAccess(FileAccess a) => a != FileAccess.Write;

try { OpenContainer(stream, access); } catch (NotSupportedException) { access = FileAccess.ReadWrite; OpenContainer(stream, access); }

When it happens

Trigger: Opening a compound file (e.g. via CompoundFileContainer or related packaging APIs) with FileAccess.Write but not FileAccess.Read.

Common situations: Code that opens files for writing-only elsewhere and reuses FileAccess.Write when opening a WPF package container.

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.

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AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/b4a255c43524a616. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsBase/MS/Internal/IO/Packaging/CompoundFile/NativeCompoundFileAPIs.cs:31

using CultureInfo = System.Globalization.CultureInfo;

namespace MS.Internal.IO.Packaging.CompoundFile
{
    internal static class SafeNativeCompoundFileMethods
    {
        /// <summary>
        /// Utility function to update a grfMode value based on FileAccess.
        /// 6/12/2002: Fixes bug #4938, 4960, 5096, 4858
        /// </summary>
        /// <param name="access">FileAccess we're translating</param>
        /// <param name="grfMode">Mode flag parameter to modify</param>
        internal static void UpdateModeFlagFromFileAccess( FileAccess access, ref int grfMode )
        {
            // Supporting write-only scenarios container-wide gets tricky and it 
            //  is rarely used.  Don't support it for now because of poor 
            //  cost/benefit ratio.
            if( FileAccess.Write == access )
                throw new NotSupportedException(
                    SR.WriteOnlyUnsupported);
            
            // Generate STGM from FileAccess
            // STGM_READ is 0x00, so it's "by default"
            if( (  FileAccess.ReadWrite                == (access &  FileAccess.ReadWrite) )  ||
                ( (FileAccess.Read | FileAccess.Write) == (access & (FileAccess.Read | FileAccess.Write))) )
            {
                grfMode |= SafeNativeCompoundFileConstants.STGM_READWRITE;
            }
            else if( FileAccess.Write == (access & FileAccess.Write) )
            {
                grfMode |= SafeNativeCompoundFileConstants.STGM_WRITE;
            }
            else if( FileAccess.Read != (access & FileAccess.Read))
            {
                throw new ArgumentException(
                    SR.FileAccessInvalid);
            }

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