Question

Below is a code that does not work. I made it work without validation, I mean I set a criterion inside MyClass‘s set and it did work. However, when I try a validation rule it does not work.

class MyClass
{
    private string num;

    public string MyString
    {
        get
        {                    
            return num;
        }
        set
        {
            num = value;             
        }
    }
}

public class MyValidationRule : ValidationRule
{
    public override ValidationResult Validate(object value, CultureInfo cultureInfo)
    {
        float num = System.Convert.ToSingle(value);

        // Is positive?
        if (num < 0)
        {                    
            return new ValidationResult(false, "Must be positive");
        }

        // Number is valid
        return new ValidationResult(true, null);
    }
}

public MainWindow()
{
    InitializeComponent();

    mc = new MyClass();
    Binding b = new Binding(mc.MyString);
    b.Mode = BindingMode.TwoWay;
    b.ValidationRules.Add(new MyValidationRule());
    b.UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged;

    MyTextBox.DataContext = mc;
}

Answer 1

Your binding seems broken. This:

Binding b = new Binding(mc.MyString);

…should be:

Binding b = new Binding("MyString");

Also i do not even see any call to SetBinding.

(How to debug bindings)