判斷url傳來的參數是正整數

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class Demo: System.Web.UI.Page
{

    public int UserId
    {
        set
        {
            if (value > 0)
            {
                ViewState["UserId"] = value;
            }
            else
            {
                string _temp=value.ToString();
                _temp = _temp.Replace("-", "");
                ViewState["UserId"] = int.Parse(_temp);
            }
        }

        get { return (int)ViewState["UserId"]; }
    }

    protected void Page_Load(object sender, EventArgs e)
    {
        UserId = GetParam("UserId");
        Response.Write(UserId);
    }

    //get url parameter
    private int GetParam(String param)
    {
        int _result = 0;  
        String _paramId = Request.QueryString[param];
        if (!string.IsNullOrEmpty(_paramId))
        {
            int.TryParse(_paramId.Trim(), out _result);
        } 
        return _result; 
    }
}