/// <summary>
/// Returns a site relative HTTP path from a partial path starting out with a ~.
/// Same syntax that ASP.Net internally supports but this method can be used
/// outside of the Page framework.
/// 
/// Works like Control.ResolveUrl including support for ~ syntax
/// but returns an absolute URL.
/// </summary>
/// <param name="originalUrl">Any Url including those starting with ~</param>
/// <returns>relative url</returns>
public static string ResolveUrl(string originalUrl)
{
	if (string.IsNullOrEmpty(originalUrl))
	    return originalUrl;

	// *** Absolute path - just return
	if (IsAbsolutePath(originalUrl))
	    return originalUrl;

	// *** We don't start with the '~' -> we don't process the Url
	if (!originalUrl.StartsWith("~"))
	    return originalUrl;

	// *** Fix up path for ~ root app dir directory
	// VirtualPathUtility blows up if there is a 
	// query string, so we have to account for this.
	int queryStringStartIndex = originalUrl.IndexOf('?');
	if (queryStringStartIndex != -1)
	{
	    string queryString = originalUrl.Substring(queryStringStartIndex);
	    string baseUrl = originalUrl.Substring(0, queryStringStartIndex);

	    return string.Concat(
		VirtualPathUtility.ToAbsolute(baseUrl),
		queryString);
	}
	else
	{
	    return VirtualPathUtility.ToAbsolute(originalUrl);
	}

}

/// <summary>
/// This method returns a fully qualified absolute server Url which includes
/// the protocol, server, port in addition to the server relative Url.
/// 
/// Works like Control.ResolveUrl including support for ~ syntax
/// but returns an absolute URL.
/// </summary>
/// <param name="ServerUrl">Any Url, either App relative or fully qualified</param>
/// <param name="forceHttps">if true forces the url to use https</param>
/// <returns></returns>
public static string ResolveServerUrl(string serverUrl, bool forceHttps)
{
	if (string.IsNullOrEmpty(serverUrl))
	    return serverUrl;

	// *** Is it already an absolute Url?
	if(IsAbsolutePath(serverUrl))
	    return serverUrl;

	string newServerUrl = ResolveUrl(serverUrl);
	Uri result = new Uri(HttpContext.Current.Request.Url, newServerUrl);

	if (!forceHttps)
	    return result.ToString();
	else
	    return ForceUriToHttps(result).ToString();
}

/// <summary>
/// This method returns a fully qualified absolute server Url which includes
/// the protocol, server, port in addition to the server relative Url.
/// 
/// It work like Page.ResolveUrl, but adds these to the beginning.
/// This method is useful for generating Urls for AJAX methods
/// </summary>
/// <param name="ServerUrl">Any Url, either App relative or fully qualified</param>
/// <returns></returns>
public static string ResolveServerUrl(string serverUrl)
{
	return ResolveServerUrl(serverUrl, false);
}

/// <summary>
/// Forces the Uri to use https
/// </summary>
private static Uri ForceUriToHttps(Uri uri)
{
	// ** Re-write Url using builder.
	UriBuilder builder = new UriBuilder(uri);
	builder.Scheme = Uri.UriSchemeHttps;
	builder.Port = 443;

	return builder.Uri;
}

private static bool IsAbsolutePath(string originalUrl)
{
	// *** Absolute path - just return
	int IndexOfSlashes = originalUrl.IndexOf("://");
	int IndexOfQuestionMarks = originalUrl.IndexOf("?");

	if (IndexOfSlashes > -1 &&
	     (IndexOfQuestionMarks < 0 ||
	      (IndexOfQuestionMarks > -1 && IndexOfQuestionMarks > IndexOfSlashes)
	      )
	    )
	    return true;

	return false;
}

http://www.west-wind.com/weblog/posts/2007/Sep/18/ResolveUrl-without-Page

You do not have Page object available everywhere to use Page.ResolveUrl. Here is an alternate.

static ResolveUrl  and ResolveServerUrl

url, aspnet

C#