app.config上的连接字符串

<connectionStrings>
    <add name ="con_str" connectionString="~~~~~your connection string~~~~~~~~~~~"/>
</connectionStrings>

检查select语句

string myconnstring = ConfigurationManager.ConnectionStrings["con_str"].ConnectionString;

    SqlConnection conn = new SqlConnection(myconnstring);
        try
        {
            string sql = "UPDATE UsersTable SET Loggedin = '0' WHERE username = @userName";              
            SqlCommand cmd = new SqlCommand(sql, conn);          
            cmd.Parameters.AddWithValue("@userName", txtlogoutusername.Text);
            conn.Open();
            int rowsAffected =cmd.ExecuteNonQuery();
            Console.WriteLine("rows affected: " + rowsAffected);
        }
        catch(Exception ex)
        {

        }
        finally
        {
            conn.Close();
        }