Avoid calling GetTempFileName in BitmapDownload

[apoorvdarshan]

Summary

• Replace the Win32 GetTempFileName P/Invoke in BitmapDownload.BeginDownload with Path.GetRandomFileName() + CREATE_NEW in a retry loop, avoiding IOException when the folder contains 65k+ temp files
• Add CREATE_NEW native constant and remove the now-unused GetTempFileName P/Invoke wrapper
• Follows the same pattern already used by FileHelper.CreateAndOpenTemporaryFile in the codebase

Fixes #259

Test plan

• Verify bitmap download from URI still works (temp file created with DELETE_ON_CLOSE semantics)
• Verify no regressions in BitmapDecoder async download path
• Confirm removed GetTempFileName P/Invoke has no other callers in production code

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR addresses issue #259 by replacing the Win32 GetTempFileName P/Invoke with Path.GetRandomFileName() + CREATE_NEW in a retry loop. The change prevents IOException when the temporary folder contains 65k+ files, which is a known limitation of the Win32 GetTempFileName API.

Changes:

• Removed the GetTempFileName P/Invoke wrapper from UnsafeNativeMethodsOther.cs (no longer used)
• Added CREATE_NEW constant to NativeMethodsOther.cs for proper file creation semantics
• Replaced GetTempFileName usage in BitmapDownload.BeginDownload with a retry loop using Path.GetRandomFileName() and CREATE_NEW disposition

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/Microsoft.DotNet.Wpf/src/Shared/MS/Win32/UnsafeNativeMethodsOther.cs	Removed unused GetTempFileName P/Invoke wrapper and related code
src/Microsoft.DotNet.Wpf/src/Shared/MS/Win32/NativeMethodsOther.cs	Added CREATE_NEW constant for Win32 CreateFile disposition
src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/Imaging/BitmapDownload.cs	Replaced GetTempFileName with retry loop using Path.GetRandomFileName() and CREATE_NEW; removed unused imports

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[h3xds1nz]

A better solution would be to directly use Path.GetTempFileName whose implementation on .NET 8+ already solved this problem.

[apoorvdarshan]

Thanks for the suggestion @h3xds1nz! You're right — Path.GetTempFileName on .NET 8+ already handles this internally with random filenames and retry logic. I've simplified the PR to just replace the Win32 P/Invoke with Path.GetTempFileName() directly.

[h3xds1nz]

@apoorvdarshan It can still throw an exception, so for correctness, it should be under the exception handler; though it's very unlikely anyone's gonna be that lucky in their lifetime (but exceptional cases, hey).

Other issue is that this implementation now changes the observable behaviour as the files will be now stored in %temp%, so that should be noted.

[apoorvdarshan]

@h3xds1nz Good catch, moved it inside the try block. And yeah the location changes but since it's DELETE_ON_CLOSE it shouldn't really matter in practice.

[lindexi]

Reference: #696