dotnet/wpf · error · IOException

SR.StreamAlreadyExist

Error message

SR.StreamAlreadyExist

What it means

IOException thrown by StreamInfo.Create when FileMode.CreateNew is requested but a stream with this name already exists in the parent storage (StreamInfo detected a cached underlying stream, core.safeIStream != null). CreateNew semantics require the target not to exist, so the library fails rather than overwriting. This mirrors the classic FileExists failure for the compound-file world.

Solutions

1. Check existence first (e.g. try Open with FileMode.Open, or track created streams) and skip creation if present.
2. Use FileMode.Create instead if overwriting the existing stream is acceptable.
3. Delete the existing stream (StreamInfo.Delete) before CreateNew.
4. Ensure Create is only called once per StreamInfo instance.

Example fix

// before
streamInfo.Create(content, FileAccess.Write, FileMode.CreateNew); // throws if exists
// after
try { streamInfo.Create(content, FileAccess.Write, FileMode.CreateNew); }
catch (IOException) { /* already exists: open instead */ }
// or simply overwrite:
streamInfo.Create(content, FileAccess.Write, FileMode.Create);

bool exists = false;
try { streamInfo.GetStream(FileMode.Open, FileAccess.Read); exists = true; } catch { }

try { streamInfo.Create(content, access, FileMode.CreateNew); }
catch (IOException) { /* stream exists: open, skip, or delete then recreate */ }

When it happens

Trigger: Calling streamInfo.Create(..., FileMode.CreateNew) on a StreamInfo that already wraps an existing/open stream; calling Create twice on the same StreamInfo without deleting the stream.

Common situations: Re-running initialization code that creates fixed-name package streams; duplicate resource names (e.g. two parts with the same URI in a package); idempotency logic missing around stream creation.

Understand the failure class

Background: "already exists" / EEXIST / FileAlreadyExistsException: what the 'file already exists' error means and how to fix it — this error's family across 37 libraries.

Related errors

• Cannot create a stream in a read-only package.
• Cannot create data stream.
• Cannot open data stream.
• Cannot perform this function on a stream that does not…
• FileMode value is not valid.

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/d1ae8f88895077c7. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsBase/System/IO/Packaging/CompoundFile/StreamInfo.cs:499

                    core.exposedStream = null;

                    if( null != core.safeIStream )
                    {
                        // Release reference
                        ((IDisposable) core.safeIStream).Dispose();
                        core.safeIStream = null;
                    }

                    // Cleanup done, create new stream in its place.
                    grfMode |= SafeNativeCompoundFileConstants.STGM_CREATE;
                    createdSafeIStream = CreateStreamOnParentIStorage(
                            core.streamName, 
                        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.

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