Problema risolto ragazzi ho provato con un altro codice migliore
codice:
string username = txtUserName.Text;
string password = txtPassword.Text;

if (ValidateUserNamePassword(username, password))
{
    // move to next form or do whatever you need to do after a successfull login
}
else
{
    MessageBox.Show("Invalid user name or password", "Invalid Login");
    return;
}
ed ecco qua
codice:
public bool ValidateUserNamePassword(string _username, string _password)
{
       string connectionString = "Data Source=[servername];Initial Catalog=[databaseName];User ID=[Admin Login];Password=[Admin Password];";

       using (SqlConnection cn = new SqlConnection(connectionString);
       {
          SqlCommand cmd = new SqlCommand();
          cmd.Connection = cn;
          cmd.CommandType = CommandType.StoredProcedure;
          cmd.CommandText = "tsp_GetUserNameAndPassword";

          SqlParameterCollection sqlParams = cmd.Parameters;
          sqlParams.AddWithValue("@UserName", _username);
          sqlParams.AddWithValue("@Password", _password);

          cn.Open();
          SqlDataReader dr = cmd.ExecuteReader(CommandBehavior.SingleRow);
          if (dr.Read())
          {
                // this will return true if a row matching the username and password is found.
                // this means that the user's input is valid
                return true;
          }
          else
          {
                return false;
           }

          dr.close();
          cn.close();
       }
}
e per finire
codice:
set ANSI_NULLS ON
set QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[tsp_GetUserNameAndPassword]
    @UserName varchar(15) -- or whatever the datatype is
    @Password varchar(25) -- or whatever the datatype is
AS

SELECT 
    UserName
    , Password
FROM [TableName]
WHERE
    UserName = @UserName
    Password = @Password
pero non ho capito la 2 e 3 parte dove vanno???