使用webservice的有关问题

[System.Web.Services.WebServiceBindingAttribute(Name = "HelloWorld", Namespace = "http://tempuri.org/")]
public partial class Soap : System.Web.Services.Protocols.SoapHttpClientProtocol
{
    public Soap()
    {
        this.Url = "http://localhost/webservice.asmx";
    }

    public DateTime GetDate()
    {
        object[] results = this.Invoke("HelloWorld", new object[] { });
        return ((DateTime)(results[1]));
    }
}

异常详细信息: System.ArgumentException: HelloWorld Web 服务方法名无效。

源错误: 


行 65:     public DateTime GetDate()
行 66:     {
行 67:         object[] results = this.Invoke("HelloWorld", new object[] { });
行 68:         return ((DateTime)(results[1]));
行 69:     }