在代码隐藏中声明和创建变量

namespace NS
{
    public partial class A: System.Web.UI.UserControl
    private Variable v;
    protected void Page_Load(object sender, EventArgs e){
        if (!Page.IsPostBack) v= new Variable();
        ....
    }
}

namespace NS
{
    public partial class A: System.Web.UI.UserControl
    private Variable v = new Variable();
    protected void Page_Load(object sender, EventArgs e){
    }
}

namespace NS
{
    public partial class A: System.Web.UI.UserControl
    private Variable v;
    protected void Page_Load(object sender, EventArgs e){
        v = new Variable();
    }
}

对于第二个场景,每次何时创建变量“v”?第二种情况是否等同于第三种情况?