C# – Understanding the Null-Conditional Operator

c#-6.0c++null-conditional-operatornullableroslyn

I have this very simple example:

class Program
{
    class A
    {
        public bool B;
    }

    static void Main()
    {
        System.Collections.ArrayList list = null;

        if (list?.Count > 0)
        {
            System.Console.WriteLine("Contains elements");
        }

        A a = null;

        if (a?.B)
        {
            System.Console.WriteLine("Is initialized");
        }
    }
}

The line if (list?.Count > 0) compiles perfectly which means that if list is null, the expression Count > 0 becomes false by default.

However, the line if (a?.B) throws a compiler error saying I can't implicitly convert bool? to bool.

Why is one different from the other?

Best Answer

list?.Count > 0: Here you compare an int? to an int, yielding a bool, since lifted comparison operators return a bool, not a bool?.
a?.B: Here, you have a bool?. if, however, requires a bool.

Related Solutions

C# 6.0 – What Does the Question Mark and Dot Operator ?. Mean?

It's the null conditional operator. It basically means:

"Evaluate the first operand; if that's null, stop, with a result of null. Otherwise, evaluate the second operand (as a member access of the first operand)."

In your example, the point is that if a is null, then a?.PropertyOfA will evaluate to null rather than throwing an exception - it will then compare that null reference with foo (using string's == overload), find they're not equal and execution will go into the body of the if statement.

In other words, it's like this:

string bar = (a == null ? null : a.PropertyOfA);
if (bar != foo)
{
    ...
}

... except that a is only evaluated once.

Note that this can change the type of the expression, too. For example, consider FileInfo.Length. That's a property of type long, but if you use it with the null conditional operator, you end up with an expression of type long?:

FileInfo fi = ...; // fi could be null
long? length = fi?.Length; // If fi is null, length will be null

C# – How to Use the Ternary Operator Inside an Interpolated String

According to the documentation:

The structure of an interpolated string is as follows:

{ <interpolationExpression>[,<alignment>][:<formatString>] }

The problem is that the colon is used to denote formatting, like:

Console.WriteLine($"The current hour is {hours:hh}")

The solution is to wrap the conditional in parenthesis:

var result = $"Descending {(isDescending ? "yes" : "no")}";

Best Answer

Related Solutions

C# 6.0 – What Does the Question Mark and Dot Operator ?. Mean?

C# – How to Use the Ternary Operator Inside an Interpolated String

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