单选按钮已启用故障

Question

I'm setting a combobox to be enabled based on selection of Radioboxes. Currently it's producing this error Object reference not set to an instance of an object. for the following code below. I did it one at a time and to make the ComboBox False works. When I implemented Disc_OnChecked to Set to true it produced the error. If I can get help to bypass this error please.

private void Cont_OnChecked(object sender, RoutedEventArgs e)
{
    Cf.IsEnabled = false;

}

private void Disc_OnChecked(object sender, RoutedEventArgs e)
{
    Cf.IsEnabled = true;
}

Xaml code:

<GroupBox>
    <StackPanel Orientation="Horizontal">
        <TextBlock Text="Type:     "></TextBlock>
        <RadioButton Checked="Disc_OnChecked" GroupName="Group1" x:Name="Disc" IsChecked="true"  Content="Discrete" ></RadioButton>
        <RadioButton Checked="Cont_OnChecked" GroupName="Group1" x:Name="Cont" Content="Continuous"></RadioButton>
    </StackPanel>
</GroupBox>

<ComboBox x:Name="Cf" Width="125" SelectedIndex="1">
    <ComboBoxItem Content="Annual"></ComboBoxItem>
    <ComboBoxItem Content="Semi-annual"></ComboBoxItem>
</ComboBox>

Answer 1

Let me take a stab at it.

This is probably hapenning when your controls are getting wired up, and at this point the ComboBox has not been instantiated.

Simply check for null within your handlers:

private void Cont_OnChecked(object sender, RoutedEventArgs e)
{
    if (Cf != null)
       Cf.IsEnabled = false;

}

private void Disc_OnChecked(object sender, RoutedEventArgs e)
{
    if (Cf != null)
       Cf.IsEnabled = true;
}

Cheers

Answer 2

You can also bind the ComboBoxItem IsSelected property to the RadioButton IsChecked property

<StackPanel>
  <RadioButton x:Name="rbtnA" Content="A"/>
  <RadioButton x:Name="rbtnB" Content="B"/>
  <RadioButton x:Name="rbtnC" Content="C"/>
  <ComboBox>
      <ComboBoxItem Content="ComboBoxItem A" IsSelected="{Binding ElementName=rbtnA,Path=IsChecked}"/>
      <ComboBoxItem Content="ComboBoxItem B" IsSelected="{Binding ElementName=rbtnB,Path=IsChecked}"/>
      <ComboBoxItem Content="ComboBoxItem C" IsSelected="{Binding ElementName=rbtnC,Path=IsChecked}"/>
  </ComboBox>
</StackPanel>