How to check whether a string is a valid HTTP URL?

Try this to validate HTTP URLs (uriName is the URI you want to test):

Uri uriResult;
bool result = Uri.TryCreate(uriName, UriKind.Absolute, out uriResult) 
    && uriResult.Scheme == Uri.UriSchemeHttp;

Or, if you want to accept both HTTP and HTTPS URLs as valid (per J0e3gan’s comment):

Uri uriResult;
bool result = Uri.TryCreate(uriName, UriKind.Absolute, out uriResult) 
    && (uriResult.Scheme == Uri.UriSchemeHttp || uriResult.Scheme == Uri.UriSchemeHttps);