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.

Answer 2

?. is a new operator that helps a developer to omit excessive checks for null.

You can read more here: https://msdn.microsoft.com/en-us/library/dn986595.aspx

Answer 3

collection.First() takes the first item from the collection. If that is not null it will call somePropA. If it is null (here comes the purpose of this operator), it will return null: it is just a smart way to do a null check. It is called the null-conditional operator.

That single character prevents the need for checking each and every property or return value for null.

Another way to write this:

var a = ( collection != null && collection.First() != null
          ? collection.First().somePropA : null
        ) ?? new A();

Or:

A a;

if (collection != null && collection.First() != null)
{
    a = collection.First().somePropA;
}
else
{
    a = null;
}

if (a == null)
{
    a = new A();
}