Ways to cast objects to a generic type

Question

In relation to Casting generic type "as T" whilst enforcing the type of T

And with the following example

private static T deserialize<T>(string streng) where T : class
{
    XmlSerializer ser = new XmlSerializer(typeof(T));
    StringReader reader = new StringReader(streng);
    return ser.Deserialize(reader) as T;
}

and

private static T deserialize<T>(string streng)
{
    XmlSerializer ser = new XmlSerializer(typeof(T));
    StringReader reader = new StringReader(streng);
    return (T)ser.Deserialize(reader);
}

I'm used to doing the object as Type casting, so I was a little confused when I found that I couldn't just do that with T. Then I found the question above and in that a solution to the as T compiler error.

But why is where T : class necessary when using object as T and not when using (T)object? What is the actual difference between the two ways of casting the object?

Answer 1

Because as implies the cast could fail and return null. Without a : class, T could be int etc - which can't be null. With (T)obj it will simply explode in a shower of sparks; no need to handle a null.

As an aside (re struct), note you can use as if it is known you are casting to a Nullable<> - for example:

static T? Cast<T>(object obj) where T : struct
{
    return obj as T?;
}