WPF Combo Box validation

I’ve been working on a small WPF project that requires some business rule validation. I thought this would be relatively simple but proved to be slightly more complicated than expected. At the end of the day, I found two relatively simple ways to perform the validation.

The first is to simply bind ComboBox.Selected item to a property and do the validation in the set block.

    private MyDataObject _someData;
    public MyDataObject SomeData
    {
        get
        {
            return _ someData;
        }
        set
        {
            _ someData = value;
            if (value == null || string.IsNullOrEmpty(value.MyProperty))
                throw new ApplicationException("SomeData is required");
        }
    }

Then, in the XAML, you just need to hook it up to treat exceptions as validation errors.

    <ComboBox.SelectedItem>
        <Binding Path="SomeData" ElementName="Window">
            <Binding.ValidationRules>
                <ExceptionValidationRule />
            </Binding.ValidationRules>
        </Binding>
    </ComboBox.SelectedItem>

The second method gives more flexibility but requires the creation of a custom ValidationRule class.

    public class MyCustomValidationRule : ValidationRule
    {
        public override ValidationResult Validate(object value, CultureInfo cultureInfo)
        {
            if (value is MyDataObject)
            {
                var myDataObj = (MyDataObject)value;
                if (myDataObj.CheckCustomBusinessRules())
                    return new ValidationResult(true, null);
            }
 
            return new ValidationResult(false, "Invalid selection!");
        }
    }

And then you hook up the validation rules to use the new custom class.

    <ComboBox.SelectedItem>
        <Binding Path="SomeData" ElementName="Window">
            <Binding.ValidationRules>
                <local:PersonValidation />
            </Binding.ValidationRules>
        </Binding>
    </ComboBox.SelectedItem>

(Note that the “local” prefix requires an additional namespace at the top of your XAML file, xmlns:local=”clr-namespace:MyNamespace”)

One last tip that threw me off for a bit is that I was missing x:Name=”Window” in my Window tag at the top of my XAML; this caused the ComboBox’s SelectedItem to not bind properly and the validation rules consequently didn’t work.