How to check if user is logged in or not in forms based authentication

Question

I want to implement forms based authentication manually in my website.

I am using Web.config file for data store

<authentication mode="Forms">
  <forms loginUrl="~/Login.aspx" 
         name=".ASPXAUTH"
         path="/"
         requireSSL="false"
         slidingExpiration="true"
         defaultUrl="~/Admin/OrderHistory.aspx"
         cookieless="UseDeviceProfile"
         enableCrossAppRedirects="false"
         >
    <credentials passwordFormat="Clear">
      <user name="Admin" password="adm123$"/>
      <user name="Administrator" password="adm234%"/>
    </credentials>
  </forms>
</authentication>
<authorization>
  <deny users ="?" />
  <allow users = "*" />
</authorization>

There is a Login.aspx page at root level in that im using ASP.NET login control to get username and password.

Everything works fine but when the user is logged in and manually go to login.aspx page, its not redirect the user to defaultUrl page.

I want to redirect the user to a specific page/defaultUrl page, if he is logged in and came manually to login.aspx page

How to do it?

Login Button-Click

if (FormsAuthentication.Authenticate(LoginUser.UserName, LoginUser.Password))
    {
        FormsAuthentication.RedirectFromLoginPage(LoginUser.UserName, true);

    }

Solution

    if(HttpContext.Current.User.Identity.IsAuthenticated)
    {

    //Redirect to Default page
    Response.Redirect("default.aspx");
    }

Comments

[user1509]

But, then how can one stay on that page only and not redirect to the Login.aspx page?

[Raghuveer]

Hi after adding the above code browser showing following message with error This webpage has a redirect loop

[Raab]

Use it on Login.aspx. not default.aspx

[Raghuveer]

i wrote that in login.aspx page load only

[Raghuveer]

may be we have to remove the ! in if statement

[L_J]

While this code may answer the question, providing information on how and why it solves the problem improves its long-term value.