[Bug] DllNotFoundException for msalruntime.dll when application path contains non-ASCII characters on English OS

[natsuki-okuda]

Library version used

4.74.1

.NET version

The issue is confirmed on both .NET 8.0 and .NET Framework 4.8.

Scenario

PublicClient - desktop app

Is this a new or an existing app?

This is a new app or experiment

Issue description and reproduction steps

Description:
When running a Windows Forms application that uses MSAL.NET for WAM authentication on an English version of Windows 10 Enterprise (Build 18363, en-US), a DllNotFoundException occurs for msalruntime.dll if the application's execution path contains non-ASCII (e.g., Japanese) characters. The authentication process works correctly if the path consists only of ASCII characters. The issue is reproducible on both .NET 8.0 and .NET Framework 4.8.

Reproduction Steps:

1. Set up a development environment on an English version of Windows 10 Enterprise (Build 18363).
2. Create a simple Windows Forms application (on .NET 8.0 or .NET Framework 4.8) that uses MSAL.NET to acquire a token interactively with WAM.
3. Place the compiled application executable and its dependencies (including msalruntime.dll) into a folder with a path containing Japanese characters.
   • Failing Path Example: C:\テスト\MyWinFormsApp.exe
4. Run the application and attempt to authenticate.
5. Observe that the application throws a DllNotFoundException.
6. Move the entire application folder to a path containing only ASCII characters.
   • Working Path Example: C:\Test\MyWinFormsApp.exe
7. Run the application again and attempt to authenticate.
8. Observe that the authentication process completes successfully.

[InnerExceptions]
ErrorMessage: The type initializer for 'Microsoft.Identity.Client.NativeInterop.API' threw an exception.
StackTrace": at Microsoft.Identity.Client.NativeInterop.Module.AddRef(String handleName)
at Microsoft.Identity.Client.NativeInterop.Core..ctor()
at Microsoft.Identity.Client.Platforms.Features.RuntimeBroker.RuntimeBroker.<>c.<.cctor>b__26_0()

ErrorMessage: "Unable to load msalruntime dlls : 0",
StackTrace: at Microsoft.Identity.Client.NativeInterop.WindowsPlatformUtils.LoadLibrary(String path)
at Microsoft.Identity.Client.NativeInterop.API..cctor()

Relevant code snippets

// Standard interactive token acquisition code for a public client application.
// The issue is not in the C# code itself, but in the environment where it runs.

var app = PublicClientApplicationBuilder.Create(clientId)
    .WithAuthority(authority)
    .WithRedirectUri("http://localhost")
    .Build();

AuthenticationResult result;
try
{
    var accounts = await app.GetAccountsAsync();
    result = await app.AcquireTokenSilent(scopes, accounts.FirstOrDefault())
                    .ExecuteAsync();
}
catch (MsalUiRequiredException)
{
    // This is where the exception is thrown in the failing scenario.
    // For WinForms, 'this' can be passed if called from the Form class.
    result = await app.AcquireTokenInteractive(scopes)
                    .WithParentActivityOrWindow(this) 
                    .ExecuteAsync();
}

Expected behavior

The application should be able to load msalruntime.dll and perform WAM authentication successfully, regardless of whether the application's file path contains non-ASCII characters.

Identity provider

Microsoft Entra ID (Work and School accounts and Personal Microsoft accounts)

Regression

No, this is a new issue to me

Solution and workarounds

Workaround:
Ensure that the entire file path for the application executable and all its dependencies consists only of ASCII characters. Avoid using Japanese, accented, or other multi-byte characters in folder names.