Fix DllImport library names for case-sensitive filesystems

[Copilot]

On case-sensitive filesystems (Windows with WSL, developer mode, or per-directory case sensitivity), DllImport fails to load native libraries due to relying on .NET's automatic platform-specific extension/prefix addition, which can produce incorrect casing.

Changes

• NativeMethods.shared.cs: Changed desktop platform library names from "onnxruntime" to "onnxruntime.dll" and "ortextensions" to "ortextensions.dll"
• Explicitly specifying extensions ensures consistent behavior across case-sensitive and case-insensitive filesystems
• Android/iOS platform-specific names unchanged

Impact

Windows: No changes required - libraries already named onnxruntime.dll

Linux/macOS: Native packaging may need updates to provide onnxruntime.dll in runtime folders (either as actual filename or symlink to libonnxruntime.so/libonnxruntime.dylib)

// Before (relied on automatic extension addition)
internal const string DllName = "onnxruntime";

// After (explicit extension for consistency)
internal const string DllName = "onnxruntime.dll";

Fixes #23509

Original prompt

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This section details on the original issue you should resolve

<issue_title>Does not work on case-sensitive filesystems</issue_title>
<issue_description>### Describe the issue

Library does not work on case-sensitive filesystems. We get:

Unhandled exception. System.TypeInitializationException: The type initializer for 'Microsoft.ML.OnnxRuntime.NativeMethods' threw an exception.
 ---> System.EntryPointNotFoundException: Unable to find an entry point named 'OrtGetApiBase' in DLL 'onnxruntime'.
   at Microsoft.ML.OnnxRuntime.NativeMethods.OrtGetApiBase()
   at Microsoft.ML.OnnxRuntime.NativeMethods..cctor()
   --- End of inner exception stack trace ---
   at Microsoft.ML.OnnxRuntime.SessionOptions..ctor()
   at Microsoft.ML.OnnxRuntime.InferenceSession..ctor(String modelPath)
   at Program.<Main>$(String[] args) in Z:\temp\onnxtest\Program.cs:line 1

Probably due to a mistyped filename somewhere.

To reproduce

Create new C# project, use this Program.cs:

new Microsoft.ML.OnnxRuntime.InferenceSession("");

Urgency

No response

Platform

Windows

OS Version

Microsoft Windows [Version 10.0.19045.6332]

ONNX Runtime Installation

Released Package

ONNX Runtime Version or Commit ID

1.22.1

ONNX Runtime API

Other / Unknown

Architecture

X64

Execution Provider

Other / Unknown

Execution Provider Library Version

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

@snnn I checked the code, still do not have much clue. We didn't specify the extension name. Maybe it was added by the .net runtime.

https://github.com/microsoft/onnxruntime/blob/main/csharp/src/Microsoft.ML.OnnxRuntime/NativeMethods.shared.cs#L831</comment_new>
<comment_new>@snnn

Maybe we should just specify explicit extensions then in DllImport

That might be the easiest fix. Would like to submit a PR?</comment_new>
<comment_new>@snnn
Don't close it.</comment_new>

• Fixes Does not work on case-sensitive filesystems #26129

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