I've been doing a fair amount of UI testing using WatiN recently – here’s some extension methods I've found useful.

This checks if a WatiN TextField is actually a hidden field. WatiN makes no distinction between text and hidden inputs, so this can come in handy if you render an input sometimes as hidden and sometimes as a visible text field. Note that this doesn't check if an input is visible (I've got another extension method for that in a moment), it checks if it’s hidden.

public static bool IsHiddenField(this TextField textField)
{
    if (textField == null || !textField.Exists)
    {
        return false;
    }

    var textFieldType = textField.GetAttributeValue("type");

    return (textFieldType != null) && textFieldType.ToLowerInvariant() == "hidden";
}

The next method quickly sets the value of a text field to a given string. By default WatiN types the text you give it into a text field one character at a time which can be necessary if you have behaviour you want to test which is triggered by individual key presses, but which most of time is just painfully slow; this method dumps the text in in one go. Note that if it's not a hidden field then it gives it focus first; this helps trigger validation once the value has been set and focus moves elsewhere.

public static void SetText(this TextField textField, string value)
{
    if ((textField == null) || !textField.Exists)
    {
         return;
    }

    if (!textField.IsHiddenField())
    {
        textField.Focus();
    }

    textField.Value = value;
}

Finally, here's a method which checks if an Element is currently visible. It does so by walking up the DOM and checking for a Style.Display of 'none' on any element between the one on which the method is invoked, and any of its ancestors.

public static bool IsElementVisible(this Element element)
{
    if ((element == null) || !element.Exists)
    {
        return false;
    }

    while ((element != null) && element.Exists)
    {
        if (element.Style.Display.ToLowerInvariant().Contains("none"))
        {
            return false;
        }

        element = element.Parent;
    }

    return true;
}

Hope they come in handy