将组合框索引设置为-1时的nullpointer

I have a combobox that gets it data from a database table.

我有一个组合框从数据库表中获取数据。

When Selected index changes I want to send the value of the selection to a textbox and then clear the selection.

选择索引更改时，我想将选择的值发送到文本框，然后清除选择。

private void ComboBox_SelectedIndexChanged(object sender, EventArgs e)
{
    MainTextBox.Text = ComboBox.SelectedValue.ToString();
    ComboBox.SelectedIndex = -1;
}

This gets the data to the textbox and clears the combobox, but also gives a null pointer exception.

这会将数据传输到文本框并清除组合框，但也会产生空指针异常。

This line by itself works fine:

这条线本身很好用：

MainTextBox.Text = ComboBox.SelectedValue.ToString();

This line by itself works fine:

这条线本身很好用：

ComboBox.SelectedIndex = -1;

How do I solve this?

我该如何解决这个问题？

1 个解决方案

#1

When you set ComboBox.SelectedIndex = -1 the ComboBox_SelectedIndexChanged function is called again as the index has been changed.

设置ComboBox.SelectedIndex = -1时，随着索引的更改，将再次调用ComboBox_SelectedIndexChanged函数。

When the function is called the second time, NullReferenceException is thrown as ComboBox.SelectedValue is set to null at SelectedIndex equal to -1.

当第二次调用该函数时，抛出NullReferenceException，因为在SelectedIndex中ComboBox.SelectedValue设置为null，等于-1。

Solution:

解：

private void ComboBox_SelectedIndexChanged(object sender, EventArgs e)
{
    if(ComboBox.SelectedIndex != -1)
    {
        MainTextBox.Text = ComboBox.SelectedValue.ToString();
        ComboBox.SelectedIndex = -1;
    }
}

#1