How ViewBag in ASP.NET MVC works

ViewBag is of type dynamic but, is internally an System.Dynamic.ExpandoObject()

It is declared like this:

dynamic ViewBag = new System.Dynamic.ExpandoObject();

which is why you can do :

ViewBag.Foo = "Bar";

A Sample Expander Object Code:

public class ExpanderObject : DynamicObject, IDynamicMetaObjectProvider
{
    public Dictionary<string, object> objectDictionary;

    public ExpanderObject()
    {
        objectDictionary = new Dictionary<string, object>();
    }
    public override bool TryGetMember(GetMemberBinder binder, out object result)
    {
        object val;
        if (objectDictionary.TryGetValue(binder.Name, out val))
        {
            result = val;
            return true;
        }
        result = null;
        return false;
    }

    public override bool TrySetMember(SetMemberBinder binder, object value)
    {
        try
        {
            objectDictionary[binder.Name] = value;
            return true;
        }
        catch (Exception ex)
        {
            return false;
        }
    }
}