dotnet/wpf · error · ArgumentException

FileMode value is not valid.

Error message

FileMode value is not valid.

What it means

ArgumentException thrown by StreamInfo.Create for FileModes that make no sense when creating a new stream: Append, Open, OpenOrCreate, Truncate, and the default catch-all for invalid enum values. Unlike GetStream, Create only accepts FileMode.Create and FileMode.CreateNew; everything else is rejected with SR.FileModeInvalid. The message text is 'FileMode value is not valid.'

Solutions

1. Pass only FileMode.Create or FileMode.CreateNew to StreamInfo.Create.
2. Route open-existing scenarios to StreamInfo.GetStream (or streamInfo.Stream) instead of Create.
3. Validate/whitelist the FileMode before calling Create.
4. Fix config/serialization sources that supply arbitrary FileMode values.

Example fix

// before
streamInfo.Create(content, FileAccess.Write, FileMode.OpenOrCreate);
// after
FileMode mode = /* ... */;
if (mode != FileMode.Create && mode != FileMode.CreateNew)
    throw new ArgumentException("StreamInfo.Create only accepts Create or CreateNew.");
streamInfo.Create(content, FileAccess.Write, mode);

if (mode != FileMode.Create && mode != FileMode.CreateNew) throw new ArgumentException("StreamInfo.Create accepts only FileMode.Create or FileMode.CreateNew.");

bool IsValidCreateMode(FileMode m) => m == FileMode.Create || m == FileMode.CreateNew;

try { streamInfo.Create(content, access, mode); }
catch (ArgumentException) { streamInfo.Create(content, access, FileMode.Create); }

When it happens

Trigger: Calling streamInfo.Create with FileMode.Open/OpenOrCreate/Append/Truncate, or with an out-of-range value cast to FileMode; reusing a FileMode chosen for GetStream in a Create call.

Common situations: Shared helper methods that take a FileMode parameter and pass it to both GetStream and Create; config-driven FileModes; copy-paste between the two APIs.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

• SR.FileModeInvalid
• Cannot have leading path delimiter.
• CompoundFile path must be non-empty.
• SR.CompoundFilePathNullEmpty
• SR.DataSpaceLabelInvalidEmpty

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/df477fe064e320b3. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsBase/System/IO/Packaging/CompoundFile/StreamInfo.cs:512

                        grfMode );    
                    break;
                case FileMode.CreateNew:
                    // If we've created a CFStream, this fails because stream is already there.
                    if( null != core.safeIStream )
                        throw new IOException(
                            SR.StreamAlreadyExist);

                    // Need to call Create API with NULL create flags
                    createdSafeIStream = CreateStreamOnParentIStorage(
                            core.streamName, 
                        grfMode );
                    break;
                case FileMode.Append:   // None of these are valid in a Create
                case FileMode.Open:
                case FileMode.OpenOrCreate:
                case FileMode.Truncate:
                default:
                    throw new ArgumentException(
                        SR.FileModeInvalid);
            }

            core.safeIStream = createdSafeIStream;
            // At this point we passed all previous checks and got the underlying IStream.
            //  Set our data space label to the given label, and the stream to the retrieved stream.
            core.dataSpaceLabel = dataSpace;
            if( null != dataSpace )
            {
                dataSpaceManager.CreateDataSpaceMapping( 
                    new CompoundFileStreamReference( parentStorage.FullNameInternal, core.streamName ), 
                    core.dataSpaceLabel );
            }

            Stream returnStream = 
                BuildStreamOnUnderlyingIStream( core.safeIStream, openFileAccess, this );

            _needToGetTransformInfo = false;    // We created stream with the given dataspace setting

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