Possible Duplicate:
How to validate Web Part property value?

below is a custom prop and a code that i am using for my custom web part, my question is, how to validate a custom web part and my requirement is that, when the user enter WebPartContentLink link i want to make sure that it starts with http:// any idea how to implement ?

private string _webPartContentLink = null;
        [WebDisplayName("Enter WebPart Content Link")]
        [WebDescription("Enter WebPart Content Link")]
        public string WebPartContentLink 
                if (_webPartContentLink == null)
                    _webPartContentLink = string.Empty;
                return _webPartContentLink;

                    _webPartContentLink = value;

PageViewerWebPart oWebPart = new PageViewerWebPart();
oWebPart.ContentLink = WebPart.WebPartContentLink.ToString();