1public static string ResolveUrl(string originalUrl)
2{
3    if (!string.IsNullOrEmpty(originalUrl) && '~' == originalUrl[0])
4    {
5        int index = originalUrl.IndexOf('?');
6        string queryString = (-1 == index) ? null : originalUrl.Substring(index);
7        if (-1 != index) originalUrl = originalUrl.Substring(0, index);
8        originalUrl = VirtualPathUtility.ToAbsolute(originalUrl) + queryString;
9    }
10
11    return originalUrl;
12}

string.Format("", ResolveUrl("folder/file.aspx"))