Clearing the property value by calling ClearValue does not necessarily give a dependency property its default value. Clearing the property only specifically clears whatever local value may have been applied, but other factors in dependency property precedence (such as styles, or an animation) might still be applying a value. For more information on this concept, see Dependency Property Value Precedence.

The following example checks for an existing local value with ReadLocalValue. If there is a local value, as indicated by not returning UnsetValue, then the existing local value is removed by calling ClearValue.

public static bool ClearSetProperty(DependencyObject targetObject, DependencyProperty targetDP)
{
    if (targetObject == null || targetDP == null)
    {
        throw new ArgumentNullException();
    }
    object localValue = targetObject.ReadLocalValue(targetDP);
    if (localValue == DependencyProperty.UnsetValue)
    {
        return false;
    }
    else
    {
        targetObject.ClearValue(targetDP);
        return true;
    }
}

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

Reference