Question: I will like to validate user entry to ensure they are integers. How can I do it? I thought of using `IDataErrorInfo` which seems like the "correct" way to do validation in WPF. So I tried implementing it, in my ViewModel.

But the thing is my text box is bound to an integer field, and there isn't any need to validate if an `int` is an `int`. I noticed that WPF automatically adds a red border around the textbox to notify the user of the error. The underlying property doesn't change to an invalid value. But I would like to notify the user of this. How can I do it?

Answer: 
Another way is simply to not allow values that are not integers.
The following implementation is a little bit sucky, and I would like to abstract it later on in order for it to be more reusable, but here is what I did: 

in the code behind in my view (I know this is might hurt if you are a hardcore mvvm ;o) )
I defined the following functions :

```
  private void NumericOnly(System.Object sender, System.Windows.Input.TextCompositionEventArgs e)
{
    e.Handled = IsTextNumeric(e.Text);

}

private static bool IsTextNumeric(string str)
{
    System.Text.RegularExpressions.Regex reg = new System.Text.RegularExpressions.Regex("[^0-9]");
    return reg.IsMatch(str);

}

```

And in the XAML view, every textbox that was only supposed to accept integers
was defined like this: 

```
   <TextBox Padding="2"  TextAlignment="Right" PreviewTextInput="NumericOnly" Text="{Binding xxx.yyyy}" MaxLength="1" />

```

The key attribute being PreviewTextInput
We can do validation on text box changed event.
The following implementation prevents keypress input other than numeric and one decimal point.

```
private void textBoxNumeric_TextChanged(object sender, TextChangedEventArgs e)
{
        TextBox textBox = sender as TextBox;
        Int32 selectionStart = textBox.SelectionStart;
        Int32 selectionLength = textBox.SelectionLength;
        String newText = String.Empty;
        int count = 0;
        foreach (Char c in textBox.Text.ToCharArray())
        {
            if (Char.IsDigit(c) || Char.IsControl(c) || (c == '.' && count == 0))
            {
                newText += c;
                if (c == '.')
                    count += 1;
            }
        }
        textBox.Text = newText;
        textBox.SelectionStart = selectionStart <= textBox.Text.Length ? selectionStart : textBox.Text.Length;    
}

```