How to declare a namespace in WPF XAML?

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:

            <Binding Path="seconds" UpdateSourceTrigger="PropertyChanged">

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

How I should declare local:posintValidationRule in my XAML?


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:


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

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