Question

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What is the “??” operator for?

What does the “??” operator perform in an expression ?

public NameValueCollection Metadata
{
    get { return metadata ?? (metadata = new NameValueCollection()); }
}

Answer 1

This is known as null-coalescing operator and it acts as following, assume a is a nullable int and b is a normal int

b = a ?? 1;

is equal to

b = (a != null ? (int)a : 1);

which is equal to

if(a != null)
    b = (int)a;
else
    b = 1;

Therefore

public NameValueCollection Metadata
{
    get { return metadata ?? (metadata = new NameValueCollection()); }
}

expanded should look like something like this

public NameValueCollection Metadata
{
    get
    {
        if(metadata == null)
            return (metadata = new NameValueCollection());
        else
            return metadata;
    }
}

which is some kind of a one liner singleton pattern, because the getter returns metadata (an initialized NameValueCollection object) every time its requested, expect the very first time which it’s null at that point, so it initializes it and then returns it. This is off topic but note that this approach to singleton pattern is not thread-safe.