Vista - Force validation on button click

Hi there,

I've created a validation rule that checks if a field is empty, and
attached it to a TextBox.
However, the validation only occurs when I type something in and tab
out of the textbox. Is there a way to programmatically force
validation to occur when I click a button on a form?


public class NotEmptyValidationRule : ValidationRule
{
public override ValidationResult Validate(object value,
System.Globalization.CultureInfo cultureInfo)
{
string s = (string)value;
if (String.IsNullOrEmpty(s))
return new ValidationResult(false, "Cannot be
empty.");
return new ValidationResult(true, null);
}
}


<TextBox HorizontalAlignment="Right" Margin="0,133,19,152"
Name="textBox3" Width="120" >
<TextBox.Text>
<Binding Path="FirstName">
<Binding.ValidationRules>
<local:NotEmptyValidationRule />
</Binding.ValidationRules>
</Binding>
</TextBox.Text>
</TextBox>