数据为空.不能在空值上调用此方法或属性.(使用组合框) [英] Data is Null. This method or property cannot be called on null values.(using combo box)

问题描述

我在表格中有空值，将用于填充组合框.我不确定该怎么做.当我运行以下代码时，我得到了错误:

数据为空.不能在空值上调用此方法或属性.

我需要帮助，并且是mysql的新手

代码:

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    string constring = "datasource=localhost;port=3306;username=root;password=root";
    string Query = "SELECT * from database.check WHERE patientname IS NOT NULL";
    MySqlConnection conDataBase = new MySqlConnection(constring);
    MySqlCommand cmdDataBase = new MySqlCommand(Query, conDataBase);
    MySqlDataReader myReader;

    try
    {
        conDataBase.Open();
        myReader = cmdDataBase.ExecuteReader();

        while (myReader.Read())
        {
            string namethestore = myReader.GetString("namethestore");
            string checkername = myReader.GetString("checkername");
            this.textBox65.Text = namethestore;
            this.textBox66.Text = checkername;
        }
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
}

解决方案

当您的一个或多个字段包含NULL(DBNull.Value)时，就不能在它们上使用GetString.
您需要使用IsDBNull方法检查它们是否为空，然后选择要在文本框中输入的值.通常是一个空字符串

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    string constring = "datasource=localhost;port=3306;username=root;password=root";
    string Query = "SELECT * from database.check WHERE patientname IS NOT NULL";
    using(MySqlConnection conDataBase = new MySqlConnection(constring))
    using(MySqlCommand cmdDataBase = new MySqlCommand(Query, conDataBase))
    {
        try
        {
            conDataBase.Open();
            using(MySqlDataReader myReader = cmdDataBase.ExecuteReader())
            {
                int namePos = myReader.GetOrdinal("namethestore");
                int checkerPos = myReader.GetOrdinal("checkername");
                while (myReader.Read())
                {
                    string namethestore = myReader.IsDBNull(namePos) 
                                          ? string.Empty 
                                          : myReader.GetString("namethestore");
                    string checkername = myReader.IsDBNull(checkerPos) 
                                          ? string.Empty
                                          : myReader.GetString("checkername");
                    this.textBox65.Text = namethestore;
                    this.textBox66.Text = checkername;
                }
           }
      }
}

我还建议在一次性对象周围使用使用声明.这样可以确保在您不再需要它们时，也可以进行适当的关闭和处理，以防万一.

Hi I have nulls in a table that I will use to populate a combo box. I am not sure how to do this. When I run the below code I get the error:

Data is Null. This method or property cannot be called on null values.

I need help and I'm new to mysql

the code :

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    string constring = "datasource=localhost;port=3306;username=root;password=root";
    string Query = "SELECT * from database.check WHERE patientname IS NOT NULL";
    MySqlConnection conDataBase = new MySqlConnection(constring);
    MySqlCommand cmdDataBase = new MySqlCommand(Query, conDataBase);
    MySqlDataReader myReader;

    try
    {
        conDataBase.Open();
        myReader = cmdDataBase.ExecuteReader();

        while (myReader.Read())
        {
            string namethestore = myReader.GetString("namethestore");
            string checkername = myReader.GetString("checkername");
            this.textBox65.Text = namethestore;
            this.textBox66.Text = checkername;
        }
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
}

解决方案

When one or more of your fields contains a NULL (DBNull.Value) you cannot use GetString on them.
You need to check if they are null using the IsDBNull method and choose what value you want to put in the textbox instead. Usually it is an empty string

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    string constring = "datasource=localhost;port=3306;username=root;password=root";
    string Query = "SELECT * from database.check WHERE patientname IS NOT NULL";
    using(MySqlConnection conDataBase = new MySqlConnection(constring))
    using(MySqlCommand cmdDataBase = new MySqlCommand(Query, conDataBase))
    {
        try
        {
            conDataBase.Open();
            using(MySqlDataReader myReader = cmdDataBase.ExecuteReader())
            {
                int namePos = myReader.GetOrdinal("namethestore");
                int checkerPos = myReader.GetOrdinal("checkername");
                while (myReader.Read())
                {
                    string namethestore = myReader.IsDBNull(namePos) 
                                          ? string.Empty 
                                          : myReader.GetString("namethestore");
                    string checkername = myReader.IsDBNull(checkerPos) 
                                          ? string.Empty
                                          : myReader.GetString("checkername");
                    this.textBox65.Text = namethestore;
                    this.textBox66.Text = checkername;
                }
           }
      }
}

I suggest also to use the using statement around the disposable objects. This will ensure a proper closing and disposing when you don't need them anymore, also in case of exceptions.....

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