I’m new to C# and WPFs, and honestly just started learning on my own this week. I’m trying to make a drop down menu that highlights red (signaling that this field is required) until an item is selected. I don’t have an item on the list that’s empty per se, but the default state is a blank option. Once one item is selected, you cannot reach the blank option again.

I’ve tried a bunch of different approaches, and believe that I am at the understanding that I need to make a style template that requests validation in order to change the border color. However, I’m encountering some issues with that. It never really sees a validation error when running. I think I must just have a fundamental misunderstanding of how to use validations (basically the syntax requirements of making my own validation and using it for this trigger).

If there is any way to accomplish this without validations that might be easier for me, but I didn’t see anything online describing that. Any help would be appreciated.

My XAML ComboBox template style:

 <Style x:Key="ComboBoxValidationStyle" TargetType="ComboBox">
     <Setter Property="Template">
         <Setter.Value>
             <ControlTemplate TargetType="ComboBox">
                 <Border x:Name="border" BorderBrush="Green" BorderThickness="1">
                     <ComboBox x:Name="PART_EditableTextBox"
                               ItemsSource="{TemplateBinding ItemsSource}" 
                               SelectedItem="{Binding SelectedModelNumber, Mode=TwoWay, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}">
                     </ComboBox>
                 </Border>
                 <ControlTemplate.Triggers>
                     <Trigger Property="Validation.HasError" Value="True">
                         <Setter Property="ToolTip" Value="{Binding RelativeSource={RelativeSource Self}, Path=(Validation.Errors)[0].ErrorContent}"/>
                         <Setter Property="BorderBrush" TargetName="border" Value="Red"/>
                         <Setter Property="BorderThickness" TargetName="border" Value="1"/>
                     </Trigger>
                     <Trigger Property="IsMouseOver" Value="True">
                         <Setter Property="BorderBrush" TargetName="border" Value="#00aed7"/>
                     </Trigger>
                 </ControlTemplate.Triggers>
             </ControlTemplate>
         </Setter.Value>
     </Setter>
 </Style>

MY XAML ComboBox Code:

           <ComboBox Grid.Row="1" FontSize="15" Grid.Column="2" Width="110" Height="27" Margin="26,0,0,0" Style="{StaticResource ComboBoxValidationStyle}" ItemsSource="{Binding ModelNumbers}" Name="comboBox1">
               <ComboBox.SelectedItem>
                   <Binding Path="Name" Mode="TwoWay" RelativeSource="{RelativeSource Mode=TemplatedParent}" UpdateSourceTrigger="PropertyChanged" ValidatesOnDataErrors="True">
                       <Binding.ValidationRules>
                           <local:SelectionValidationRule></local:SelectionValidationRule>
                       </Binding.ValidationRules>
                   </Binding>
               </ComboBox.SelectedItem>
           </ComboBox>

My Validation code:

    public class SelectionValidationRule : ValidationRule
    {
        public override ValidationResult Validate(object value, CultureInfo cultureInfo)
        {
            if (value is string selectedItemString)
            {
                if (string.IsNullOrEmpty(selectedItemString))
                {
                    return new ValidationResult(false, null);
                }
            }
            else if (value == null || string.IsNullOrEmpty(value.ToString()))
            {
                return new ValidationResult(false, null);
            }
            else if (value is ComboBox comboBox)
            {
                if (comboBox.SelectedIndex == -1)
                {
                    return new ValidationResult(false, null);
                }
            }

            return new ValidationResult(true, null);
        }
    }

And yes, I did include the SelectionValidationRule class in my XAML code behind!