WPF C# 组合框默认值

Question

I have a problem with the set of a default value for a combobox:

<ComboBox x:Name="cmbAuthentification" Height="20" Grid.Row="1" Grid.Column="1" BorderBrush="#FF2BBFF0" FontFamily="Segoe UI Light" IsReadOnly="True" SelectionChanged="cmbAuthentification_SelectionChanged">
    <ComboBoxItem>Windows</ComboBoxItem>
    <ComboBoxItem>SQL Server</ComboBoxItem>
</ComboBox>

In the xmal, I have this code:

private void cmbAuthentification_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    if (cmbAuthentification.SelectedIndex == 0)
    {
        txtUserID.IsEnabled = false;
        txtUserID.Clear();
        txtUserPwd.IsEnabled = false;
        txtUserPwd.Clear();
    }

    if (cmbAuthentification.SelectedIndex == 1)
    {
        txtUserID.IsEnabled = true;
        txtUserPwd.IsEnabled = true;
    }
}

When I try to set SelectedIndex="0" in the ComboBox, I have an error at runtime: NullReferenceException : Object reference not set to an instance of an object.

Answer 1

Your issue is that the controls haven't yet been loaded when your event handler is executed for the first time.

You could check whether the value of the IsLoaded and hook up to the Loaded event if it returns false :

private void cmbAuthentification_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    if (!cmbAuthentification.IsLoaded)
    {
        cmbAuthentification.Loaded += (ss, ee) => cmbAuthentification_SelectionChanged(sender, e);
        return;
    }

    if (cmbAuthentification.SelectedIndex == 0)
    {
        txtUserID.IsEnabled = false;
        txtUserID.Clear();
        txtUserPwd.IsEnabled = false;
        txtUserPwd.Clear();
    }

    if (cmbAuthentification.SelectedIndex == 1)
    {
        txtUserID.IsEnabled = true;
        txtUserPwd.IsEnabled = true;
    }
}