how to use session variable in asp using c#

Assuming that your code is in the page (either inline or in the code behind) you can just use...

DataType someValue = (DataType)Session["SessionVariableNameHere"]; //Getter
Session["SessionVariableNameHere"] = someNewValue; //Setter

Obviously you'll need to name your session variable properly, and cast to the appropriate data type when you get it back out of the session.

EDIT - A Full Example

protected void Login1_LoggedIn(object sender, EventArgs e)
{
    Session["LoginTime"] = DateTime.Now;
}

and later in a page load...

protected void Page_Load(object sender, EventArgs e)
{
    Literal1.Text = "Last Online: " + ((DateTime)Session["LoginTime"]).ToString("yyyy-MM-dd");
}

When user enters correct username & password. create a session which will hold flag

if(userLoggedInSuccessfully)
{
          Session["SessionVariableName"] = "Flag";
}

If you are using master page in your page just check on page_load

page_load()
{
                 if(Session["SessionVariableName"] != null)
                 {
                       if(Session["SessionVariableName"]=="Flag")
                       {
                              //Valid User
                       }
                       else
                       {
                                  //Invalid user
                       }
                 }
                 else
                 {
                           //Session expired
                 }

}

I usually define a (base) page-level property and try to avoid hard-coding the session variable name every time I have to reference it. Here's an example:

In Constants.cs:

public static class Constants
{
  public static class SessionKeys
  {
    public static string MY_SESSION_VARIABLE = "MySessionVariable";  //Or better yet - a newly generated GUID.
  }
}

In the page's code-behind, define your property:

protected MyType MyVariable
{
  get
  {
    MyType result = null;

    object myVar = Session[Constants.SessionKeys.MY_SESSION_VARIABLE];
    if (myVar != null && myVar is MyType)
    {
      result = myVar as MyType;
    }

    return result;
  }
  set 
  {
    Session[Constants.SessionKeys.MY_SESSION_VARIABLE] = value;
  }
}

In the page's code-behind, reference the property:

//set
MyVariable = new MyType();

//get
string x = MyVariable.SomeProperty;