Type followed? Problems

First look at the example:

Static void Main (string [] args)
{
Int? Aa = null;
Console. WriteLine (aa ?? -1 );
Console. Read ();
}

Here, an empty integer aa is defined, assigned null to it, and then output.

A question mark indicates that this type can be null, and two question marks indicate that if this type is null, assign the value following the question mark to this type.

Static void Main (string [] args)
{
Int? Aa = null;
Int? Bb = 1;
Console. WriteLine (aa ?? -1 );
Console. WriteLine (bb ?? -1 );
Console. Read ();
}

Console output-1, 1.

This? Actually, it is a wildcard Nullable <T>. msdn explains the specific application.