c# - MySQL 检查用户是否存在 C#

Question

代码:

 static public void InsertUser(string userName, DataGridView DadataGridView1)
        {
            try
            {
                if (connection.State == ConnectionState.Closed)
                    connection.Open();

                MySqlCommand check_User_Name = new MySqlCommand("SELECT * FROM IP WHERE(Username = @username)", connection);
                check_User_Name.Parameters.AddWithValue("@username", userName);
                int UserExist = (int)check_User_Name.ExecuteScalar();

                if (UserExist > 0)
                {
                    MessageBox.Show("User Exist");
                }
                else
                {
                    MySqlCommand commandInsert = new MySqlCommand("INSERT INTO IP(Username) VALUES(@Username)", connection);
                    commandInsert.Parameters.AddWithValue("@username", userName);
                    commandInsert.ExecuteNonQuery();
                    commandInsert.Parameters.Clear();
                    MessageBox.Show("User Inserted sucessfuly");
                }

            }
            catch (MySqlException exception)
            {
                MessageBox.Show(exception.ToString());

            }
            finally
            {
                connection.Close();

            }

我想检查数据库中的重复项，但遇到问题

int UserExist = (int)check_User_Name.ExecuteScalar();

System.NullReferenceException was unhandled Message=Object reference not set to an instance of an object.

Answer 1

使用SELECT count(*) FROM IP WHERE(Username = @username)代替SELECT * FROM IP WHERE(Username = @username)，因为。 ExecuteScalar() 只能处理单个值。 * 将获取所有列值。在这种情况下，您不能使用 .ExecuteScalar()。

所以你的代码将是这样的:

 MySqlCommand check_User_Name = new MySqlCommand("SELECT count(*) FROM IP WHERE Username = @username", connection);
                check_User_Name.Parameters.AddWithValue("@username", userName);
                int UserExist = (int)check_User_Name.ExecuteScalar();    
                if (UserExist > 0)
                {
                    MessageBox.Show("User Exist");
                } 
                else
                  {
                //execute insert here
                  }