MySqlConnection critical error e0434352 (no replies)

VS 2013, NET 4.5
MySQL Connector Net 6.7.4
UnmanagedExports.1.2.6 (RGiesecke nuget package)

C unmanaged accessing NET DLL.
calling conventions work with ms sql db.
just don't understand why I am having the trouble with MySQL.

Creating a new connection causes critical error e0434352.

return connectionString is correct if return before MySqlConnection.

[DllExport("connect",
CallingConvention = CallingConvention.StdCall)]
public static string connect([MarshalAs(UnmanagedType.LPStr)]string _url,
[MarshalAs(UnmanagedType.LPStr)]string _database,
[MarshalAs (UnmanagedType.LPStr)]string _login,
[MarshalAs(UnmanagedType.LPStr)]string _password)
{
MySqlConnection connection;
connectionString = "Server=127.0.0.1;Uid=root;Pwd=;";

// connectionString = "SERVER=" + _url + ";" + "DATABASE=" +
// _database + ";" + "UID=" + _login + ";" + "PASSWORD=" + _password + ";";

connection = new MySqlConnection(connectionString);
return ("after connection");

try
{
connection.Open();
return ("Ok");
} catch( Exception ex )
{
return (ex.ToString());
}
}