How does GetValueOrDefault work?

A NullReferenceException isn't thrown, because there is no reference. The GetValueOrDefault is a method in the Nullable<T> structure, so what you use it on is a value type, not a reference type.

The GetValueOrDefault(T) method is simply implemented like this:

public T GetValueOrDefault(T defaultValue) {
    return HasValue ? value : defaultValue;
}

So, to replicate the behaviour you just have to check the HasValue property to see what value to use.

GetValueOrDefault () prevents errors that may occur because of null. Returns 0 if the incoming data is null.

int ageValue = age.GetValueOrDefault(); // if age==null

The value of ageValue will be zero.

thing isn't null. Since structs can't be null, so Nullable<int> can't be null.

The thing is... it is just compiler magic. You think it is null. In fact, the HasValue is just set to false.

If you call GetValueOrDefault it checks if HasValue is true or false:

public T GetValueOrDefault(T defaultValue)
{
    return HasValue ? value : defaultValue;
}