你调用的对象是空的

Question

In this line of code

  <% var tmp = int.Parse(ViewData["numOfGroups"].ToString()); %>

I have error: Object reference not set to an instance of an object . How correctly convert

ViewData["numOfGroups"] to int ?

Answer 1

You should first make sure that your controller action is setting this variable:

public ActionResult Index()
{
    ViewData["numOfGroups"] = "15";
    return View();
}

Once you've done this you should no longer get a NullReferenceException and your code should work.

Of course as I've already written it multiple times here you should prefer strongly typed view instead of ViewData . Also you should type your model properties accordingly. It is not the responsibility of the view to parse strings. So:

public ActionResult Index()
{
    var model = new MyModel
    {
        NumOfGroups = 15
    };
    return View(model);
}

And in your view:

<% var tmp = Model.NumOfGroups; %>

By the way this should also be avoided as I have the feeling that you are declaring variables in your view which means that you have the intent of using them. Views are not for declaring variables and writing C# code. They are markup.

Answer 2

If the error was related to converting ViewData["numOfGroups"] to int then you would get FormatException . Check that you are really passing data to the view and that it contains numOfGroups key.

Answer 3

Seems ViewData["numOfGroups"] is null. It is quite possible that problem is not in conversion itself but in ViewData["numOfGroups"].ToString() .

Answer 4

Since ViewData dictionary contains <string, object> you have to do unboxing on the value:

int tmp = (int)ViewData["numOfGroups"];

but check if the object is null first or surround by try/catch if there's any chance the conversion will not work... ...or use TryParse() that returns bool if the conversion succeeded or not.