How to declare a namespace in WPF XAML?

Question

I am trying to use in WPF a validating input of databound controls with validation rules. In the code behind file of a wpf window I have a class:

public class posintValidationRule : ValidationRule
{
    public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)
    {
        string _strInt = value.ToString();
        int _int = -1;
        if (!Int32.TryParse(_strInt, out _int))
            return new ValidationResult(false, "Value must be an integer");
        if (_int < 0)
            return new ValidationResult(false, "Value must be positive");
        return new ValidationResult(true, null);
    }
}

In XAML there is also a style error template.

When I put a textbox with validation in XAML:

<TextBox.Text>
    <Binding Path="seconds" UpdateSourceTrigger="PropertyChanged">
        <Binding.ValidationRules>
           <local:posintValidationRule/> 
        </Binding.ValidationRules>
    </Binding>
</TextBox.Text>

I get a compile time error: ''local' is an undeclared namespace.' XML is not valid.

How I should declare local:posintValidationRule in my XAML?

Solution

At the top of your XAML file, you need to declare what your "local" namespace is; alongside the default Microsoft XAML stuff. Something like this:

xmlns:local="clr-namespace:YourApplication"

Note this assumes that "posintValidationRule" is defined at the root namespace in "YourApplication".