Question

I want to check a business rule before I bind the data the user enters into a textbox? Before I bind the data to the model, I need to ensure it meets a certain criteria. This would be very easy to do if I could execute a method before the binding occurs. Is there a way for me to do this?

Answer 1

You can implement ValidationRule:

public class CustomValidationRule : ValidationRule
{
    private static bool IsValid(string value)
    {
        // implement you business rule checking logic here
        // if valid
        //     return true;
        // else
        //     return fase;
    }

    public override ValidationResult Validate(object value, CultureInfo cultureInfo)
    {
        var val = (string)value;
        if(IsValid(val))
        {
            return ValidationResult.ValidResult;
        }
        else
        {
            return new ValidationResult(false, "Value is not valid");
        }
    }
}

And use it in your binding:

<TextBox>
    <TextBox.Text>
        <Binding Path="ViewModelProperty" UpdateSourceTrigger="PropertyChanged">
            <Binding.ValidationRules>
                <validation:CustomValidationRule />
            </Binding.ValidationRules>
        </Binding>
    </TextBox.Text>
</TextBox>

Where validation is a XML namespace alias for namespace where CustomValidationRule is defined (add xmlns:validation="clr-namespace:NAMESPACE_NAME_HERE" to your XAML).