How to redirect back to that page where session was expired in ASP.Net

I have created the sample with two pages

I have set the Home Page as Startup page and on page load i am checking if user is loged In or not.

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        if (Session["UserName"] == null)
        {
            Response.Redirect("Login.aspx?url=http://localhost:26540/RedirectBackToPageAfterLogin/Home.aspx");
        }
    }
}

If he is not logedIn then user will be redirected to LoginPage with QueryString(Url of this currectPage).

LoginPage

<form id="form1" runat="server">
<div>
    <table border="0" cellpadding="0" cellspacing="0">
        <tr>
            <td>
                User Name
            </td>
            <td>
                <asp:TextBox ID="txtUserName" runat="server" Text="Test" />
            </td>
        </tr>
        <tr>
            <td>
                Password
            </td>
            <td>
                <asp:TextBox ID="txtPassword" Text="abc" TextMode="Password" runat="server" />
            </td>
        </tr>
        <tr>
            <td>
            </td>
            <td>
                <asp:Button Text="Login" OnClick="Login" runat="server" />
            </td>
        </tr>
    </table>
</div>
</form>

C#

protected void Login(object sender, EventArgs e)
{
    if (this.txtUserName.Text.Trim() == "Test")
    {            
        Session["UserName"] = this.txtUserName.Text.Trim();
        if (!string.IsNullOrEmpty(Request.QueryString["url"]))
        {
            Response.Redirect("Home.aspx");
        }
    }
}