How to connect to mirrored SQL Server after failover?

Fortunately, I fixed this problem. All I need to do is calling ClearPool method:

SqlConnection.ClearPool(conn);

This method will clear the connection pool. Then, the problem's gone. I'm so happy with it.

Thank you, all of you, for your support. :D

I'm not an expert about .net stuff but you need the SQL native client (SQLNCLI) to handle failover. "ado.net" may be SQLOLEDB

Up to Windows Server 2003 at least it's not installed by default. It's either standalone or part of SQL client tools, so I suspect you're using SQLOLEDB