MonitorDropDownList_SelectedIndexChanged函数不能实现与全局变量之间的传值

protected void MonitorDropDownList_SelectedIndexChanged(object sender, EventArgs e)
    {
        this.Label1.Text = this.MonitorDropDownList.SelectedValue;
    }
    protected void search_Click(object sender, EventArgs e)
    {
        GetMoniterPlaceData();
    }
public void GetMoniterPlaceData()
    {
        //获取监测点编号
          int moniterplaceid = Convert.ToInt32(Label1.Text.ToString());

        //把对象函数添加到IList<>函数中
        IList<FWSync.Model.MoniterPlaceInfo> MP = new List<FWSync.Model.MoniterPlaceInfo>();
        MoniterPlace moniterplace = new MoniterPlace();
        MP.Add(moniterplace.GetMoniterPlaceByMoniterPlaceID(moniterplaceid));
        for (int i = 0; i < MP.Count; i++)
        {
            MoniterPlaceNameLabel.Text = MP[i].MoniterPlaceName;
        }
    }
这样通过this.Label1.Text 是能实现protected void MonitorDropDownList_SelectedIndexChanged(object sender, EventArgs e)这个函数传值的，
但是换成全局变量就这个函数就不执行了，就不能实现了，为什么？？？？换成全局代码的如下：
int Mpid;//全局变量
protected void MonitorDropDownList_SelectedIndexChanged(object sender, EventArgs e)
    {
        Mpid = Convert.ToInt32(this.MonitorDropDownList.SelectedValue);
    }
protected void search_Click(object sender, EventArgs e)
    {
        GetMoniterPlaceData();
    }
public void GetMoniterPlaceData()
    {
        //获取监测点编号
          int moniterplaceid = Mpid;

        //把对象函数添加到IList<>函数中
        IList<FWSync.Model.MoniterPlaceInfo> MP = new List<FWSync.Model.MoniterPlaceInfo>();
        MoniterPlace moniterplace = new MoniterPlace();
        MP.Add(moniterplace.GetMoniterPlaceByMoniterPlaceID(moniterplaceid));
        for (int i = 0; i < MP.Count; i++)
        {
            MoniterPlaceNameLabel.Text = MP[i].MoniterPlaceName;
        }
    }
这样通过this.Label1.Text 是能实现protected void MonitorDropDownList_SelectedIndexChanged(object sender, EventArgs e)这个函数传值的，
但是换成全局变量就这个函数就不执行了，就不能实现了，为什么？？？？请各位高手指点一下，在此先谢谢各位了！！！ label 全局变量对象

补充：.NET技术 ,  ASP.NET