How to translate "var a = collection?.First()?.somePropA ?? new A();"

Question

What is the purpose of single question mark on the right side of assignment in expression with null-coalescing operator? Example:

var a = collection?.First()?.somePropA ?? new A();

Answer 1

The single quotation mark (?.) is newly added in C# 6.0 and represents a null check.

So, for example, the following code is the same;

var foo = bar?.bar2;

Or

var foo = null;
if (bar != null)
    foo = bar.bar2;
else
    foo = null;

However, both can still return null. Therefore one can use the ?? check to assign a value if the result indeed is null.

That being said, one can write the following code;

var foo = bar?.bar2 ?? new Bar2();

which is basically the same as

var foo = null;
if (bar != null)
    foo = bar.bar2;
else
    foo = new Bar2();

Using ?. multiple times can really shorten your code. Consider, for example, the following line;

var foo = bar?.bar2?.bar3?.bar4?.bar5 ?? new Bar5();

this is syntactic sugar for

var foo = null;
if (bar != null)
{
    if (bar.bar2 != null)
    {
        if (bar.bar2.bar3 != null)
        {
            if (bar.bar2.bar3.bar4 != null)
            {
                if (bar.bar2.bar3.bar4.bar5 != null)
                    foo = bar.bar2.bar3.bar4.bar5;
                else
                    foo = new Bar5();
            }
            else
                foo = new Bar5();
        }
        else
            foo = new Bar5();
    }
    else
        foo = new Bar5();
}
else
    foo = new Bar5();

Of course, there are already way better ways to write the above (for example, by initializing foo = new Bar5() before doing all the checks), but for clarity I kept it this way.