The connection was not closed the connection’s current state is open

by

I assume that the error is raised on this line:

con.Open(); // InvalidOperationException if it's already open

since you’re reusing a connection and you probably have not closed it last time.

You should always close a connection immediately as soon as you’re finished with it, best by using the using-statement:

public void run_runcommand(string query)   
{
    using(var con = new SqlConnection(connectionString))
    using(var cmd = new SqlCommand(query, con))
    {
        con.Open();
        // ...
    }  // close not needed since dispose also closes the connection
}

Note that you should not use a Catch block just to rethrow an exception. If you don’t do anything with it don’t catch it at all. It would be even better to use throw; instead of throw ex; to keep the stack trace. https://stackoverflow.com/a/4761295/284240

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