全局变量不可以用Request.QueryString?

如题..那么我想用全局变量获取url的参数怎么办啊？

public partial class T1 : System.Web.UI.Page
{
public string a = Request.QueryString[ "a "]
protected void Page_Load(object sender, EventArgs e)
{
}

}

这样是报错的

[解决办法]
public partial class T1 : System.Web.UI.Page
{
public string a;
protected void Page_Load(object sender, EventArgs e)
{
a = Request.QueryString[ "a "];
}

}
[解决办法]
public partial class T1 : System.Web.UI.Page
{
public string a= " ";
protected void Page_Load(object sender, EventArgs e)
{
a= Request.QueryString[ "a "]
}

}

[解决办法]
public partial class T1 : System.Web.UI.Page
{
public string a= " ";
protected void Page_Load(object sender, EventArgs e)
{
a=Request.QueryString[ "a "]

}

}
[解决办法]
只要不写到！ispostback里就行