How to convert Session Variable to Integer Type in C#

Try the magic code:

Session["LoginAttempt"] = ((int?)Session["LoginAttempt"] ?? 0) + 1;

This will convert the session variable Session["LoginAttempt"] to a nullable int (an int that can be null) the ?? 0 provides a value 0 if it is null, so the calculation succeeds.

The Session["LoginAttempt"] can be null if it is not initialized before.

You need to test to see if the Session variable exists before you can use it and assign to it.

Here you are doing an increment:

Session["LoginAttempt"] = ((int) Session["LoginAttempt"]) + 1;

But, if the Session["LoginAttempt"] does not exist, this will explain your error. A quick null test before the increment should sort it out.

if (Session["LoginAttempt"] != null)
    Session["LoginAttempt"] = ((int)Session["LoginAttempt"]) + 1;

Why not encapsulate the LoginAttempt value as a property and auto-assign a value:

protected int LoginAttempt
{
    get
    {
        if (Session["LoginAttempt"] == null)
        {
            Session["LoginAttempt"] = 1;
        }
        return Convert.ToInt32(Session["LoginAttempt"].ToString());
    }
    set
    {
        Session["LoginAttempt"] = value;
    }
}

so that the main body of the function is more readable:

if (LoginAttempt < 3)
{
}

It will do that the first time you try to set it if you haven't previously initialised it. Try this instead:

if (Session["LoginAttempt"] == null)
    Session["LoginAttempt"] = 1;
else
    ((int)Session["LoginAttempt"]) += 1;

Divide your nontrivial code in parts:

int sessionLogicAttempt = (int)Session["LoginAttempt"];
int someValue = sessionLogicAttempt + 1;
Session["LoginAttempt"] = someValue;

Also, add assertions to check for the values you are assuming.

//read
object attemptObj = Session["LoginAttempt"]
int attempt = 0;
if (attemptObj != null) attempt = (int)attemptObj ;

////write
Session["LoginAttempt"] = attempt++;

try make sure not to cast something that is possibly a null value.

int i = Session["val"] == null ? 0 : (int)Session["val"];

though this could mess you up as well if some other programmer uses your "val" session and put a non int value there so.

        int y = 0;
        if (int.TryParse(Session["val"] == null ? string.Empty : Session["val"].ToString(), out y))
        {
            // got your int value
        }
        else
        {
            // no int value in session val
        }