Web服务如何找出客户端用来调用此web方法的URL？

Question

假设我有一个webservice（http：//myWebservice/CodeGenerator/CodeValidator.asmx），如下所示

[WebMethod]
public string ValidateCode(string sCode)
{
    // I need to check requested url here. The call comes from different sites.
    // For example www.abc.com/accesscode.aspx. I need to get this url. 
}