Why ViewBag.SomeProperty doesn't throw an exception when the property is not exists?

Question

In MVC, sometimes I'm setting specific property of ViewBag according to some condition.For example:

if(someCondition)
{
   // do some work
   ViewBag.SomeProperty = values;
}

return View();

In my View I'm checking whether the property is null like this:

@if(ViewBag.SomeProperty != null)
{
   ...
}

Until now I was thinking that should throw an exception because if my condition is not satisfied then SomeProperty is never get set.And that's why I was always using an else statement to set that property to null.But I just noticed, it doesn't throw an exception even if the property doesn't exists.For example in a Console Application if I do the following, I'm getting a RuntimeBinderException:

dynamic dynamicVariable = new {Name = "Foo"};

if(dynamicVariable.Surname != null) Console.WriteLine(dynamicVariable.Surname);

But it doesn't happen when it comes to ViewBag.What's the difference ?

Answer 1

AFAIK the ViewBag is a dynamic wrapper around ViewData. And ViewData itself retrieves the values as follows:

public object this[string key]
{
    get
    {
        object value;
        _innerDictionary.TryGetValue(key, out value);
        return value;
    }
    set { _innerDictionary[key] = value; }
}

hence, if key doesn't exist it returns default value for type and doesn't throw exception.

Answer 2

ViewBag inherits from DynamicViewDataDictionary which returns null for missing properties.