Custom SqlClientAuthenticationProvider being overwritten by default one

[Charles-Gagnon]

Describe the bug

When a custom provider is registered in MDS 5.2+ it will be overwritten by the default provider. This is from the verbose logs for our app :

2025-09-13T16:33:21.7520936Z [78]: <sc|SqlAuthenticationProviderManager|SetProvider|Error>Failed to add provider Microsoft.Data.SqlClient.ActiveDirectoryAuthenticationProvider because a user-defined provider with type Microsoft.SqlServer.Management.UI.AadInteractiveAuthProvider.AadAuthenticationProvider already existed for authentication ActiveDirectoryInteractive.
2025-09-13T16:33:21.7531099Z [78]: <sc|ActiveDirectoryAuthenticationProvider|BeforeLoad|Info>being loaded into SqlAuthProviders for ActiveDirectoryInteractive.
2025-09-13T16:33:21.7531099Z [78]: <sc|SqlAuthenticationProviderManager|SetProvider|Info>Added auth provider Microsoft.Data.SqlClient.ActiveDirectoryAuthenticationProvider, overriding existed provider Microsoft.SqlServer.Management.UI.AadInteractiveAuthProvider.AadAuthenticationProvider for authentication ActiveDirectoryInteractive.

To reproduce

1. Create and register a custom auth provider as documented here https://learn.microsoft.com/en-us/sql/connect/ado-net/sql/azure-active-directory-authentication?view=sql-server-ver17#support-for-a-custom-sql-authentication-provider
2. Start up the application

Expected behavior

The custom auth provider is registered and called when Entra ID tokens are needed for authentication.

Further technical details

N/A

Additional context

This appears to have been introduced in https://github.com/dotnet/SqlClient/pull/1949/files#diff-d95a8fbf13f839abdfbc99fa02b0b3c2170eb8ddcb8747c729842f52854e56baL172

FYI @Wraith2

Previously, the method would return if a matching provider was already registered. But after the change the break just causes it to exit the loop, but then continue on to the following code which registers the default provider.

This was first introduced in MDS 5.2, but still seems to be an issue in 6.0 (I haven't verified this yet though except by looking at the code)

https://github.com/dotnet/SqlClient/blob/main/src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/SqlAuthenticationProviderManager.cs#L189

[paulmedynski]

Yep, that's definitely a regression.