Handling session timeout in ajax calls

You could write a custom [Authorize] attribute which would return JSON instead of throwing a 401 exception in case of unauthorized access which would allow client scripts to handle the scenario gracefully:

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
public class MyAuthorizeAttribute : AuthorizeAttribute
{
    protected override void HandleUnauthorizedRequest(AuthorizationContext filterContext)
    {
        if (filterContext.HttpContext.Request.IsAjaxRequest())
        {
            filterContext.Result = new JsonResult
            {
                Data = new 
                { 
                    // put whatever data you want which will be sent
                    // to the client
                    message = "sorry, but you were logged out" 
                },
                JsonRequestBehavior = JsonRequestBehavior.AllowGet
            };
        }
        else
        {
            base.HandleUnauthorizedRequest(filterContext);
        }
    }
}

then decorate your controller/actions with it and on the client:

$.get('@Url.Action("SomeAction")', function (result) {
    if (result.message) {
        alert(result.message);
    } else {
        // do whatever you were doing before with the results
    }
});

I wouldn't change JsonRequestBehavior to AllowGet. Instead I suggest:

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
public sealed class MyAuthorizeAttribute : AuthorizeAttribute
{
    public override void OnAuthorization(AuthorizationContext filterContext)
    {
        base.OnAuthorization(filterContext);
        OnAuthorizationHelp(filterContext);
    }

    internal void OnAuthorizationHelp(AuthorizationContext filterContext)
    {

        if (filterContext.Result is HttpUnauthorizedResult)
        {
            if (filterContext.HttpContext.Request.IsAjaxRequest())
            {
                filterContext.HttpContext.Response.StatusCode = 401;
                filterContext.HttpContext.Response.End();
            }
        }
    }
}

and add global js ajax errors handler:

   $(document).ajaxError(function (xhr, props) {
        if (props.status === 401) {
            location.reload(); 
        }
   }

Even though this is well past answered, I think this is the shortest and sweetest answer if you are using .NET 4.5. Little property called SuppressFormsAuthenticationRedirect which was added. Set to true and it will not perform the 302 Redirect to login page.

http://msdn.microsoft.com/en-us/library/system.web.httpresponse.suppressformsauthenticationredirect.aspx

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, Inherited = true, AllowMultiple = true)]
public class AjaxAuthorizeAttribute : AuthorizeAttribute
{
    protected override void HandleUnauthorizedRequest(AuthorizationContext filterContext)
    {
        // returns a 401 already
        base.HandleUnauthorizedRequest(filterContext);
        if (filterContext.HttpContext.Request.IsAjaxRequest())
        {
            // we simply have to tell mvc not to redirect to login page
            filterContext.HttpContext.Response.SuppressFormsAuthenticationRedirect = true;
        }
    }
}

Assuming you plan on handling the ajax requests fail/error callback, in which you will get a 401 Unauthorized.

On Master page add this jquery script ------------

<script type="text/javascript">

   $.ajaxSetup({
        statusCode: {
            403: function () {
                window.location.reload();
            }
        }
    });


    OR


    $.ajaxSetup({
        error: function (x, e) {
            if (x.status == 403) {
                window.location.reload(); 
            }
        }
    });

</script>

Add a cs file named with TraceFilter in your project and write a seald class TraceFilterAttribute inheriting to ActionFilterAttribute. Add TraceFilterAttribute class in FilterConfig.cs available in App_Start folder of your project by writing below line.

filters.Add(new TraceFilterAttribute());

Override method OnActionExecuting() in TraceFilterAttribute class. This will automatically check session and if finds session null then calls script available in master page and from their you can go to your choice page.

[AttributeUsage(AttributeTargets.All)]
public sealed class TraceFilterAttribute : ActionFilterAttribute
{
public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        if (filterContext != null)
        {
HttpSessionStateBase objHttpSessionStateBase = filterContext.HttpContext.Session;
                var userSession = objHttpSessionStateBase["etenetID"];
if (((userSession == null) && (!objHttpSessionStateBase.IsNewSession)) || (objHttpSessionStateBase.IsNewSession))
                {
                    objHttpSessionStateBase.RemoveAll();
                    objHttpSessionStateBase.Clear();
                    objHttpSessionStateBase.Abandon();
                    if (filterContext.HttpContext.Request.IsAjaxRequest())
                    {
                        filterContext.HttpContext.Response.StatusCode = 403;
                        filterContext.Result = new JsonResult { Data = "LogOut" };
                    }
                    else
                    {
                        filterContext.Result = new RedirectResult("~/Admin/GoToLogin");
                    }

                }


}
}

}

I was having a similar issue and found this

Instead of returning any JSON, just before the response is sent back, force ASP.NET to return a 401 code. In Global.asax:

protected void Application_EndRequest()
    {
        var context = new HttpContextWrapper(Context);
        if (context.Request.IsAjaxRequest() && context.Response.StatusCode == 302)
        {
            Context.Response.Clear();
            Context.Response.Write("**custom error message**");
            Context.Response.StatusCode = 401;
        }
    }

Then you can let the client deal with it in JavaScript/jQuery or whatever you are using