Four question marks you should know in C#

There are 4 question marks ? used in C# syntax which I think is pretty confusing sometimes. Here I will attempt to explain them to you.

1) The ?: Operator (since Visual Studio 2003)

The format : condition ? first_expression : second_expression;

If condition evaluates to true, first_expression shall be returned, otherwise second_expression.

Example

Random rnd = new Random();
var randomNo = rnd.Next();

var status = randomNo % 2 == 0 ? "Generated number is an even number." : "Generated number is an odd number.";
Console.WriteLine(randomNo);
Console.WriteLine(status);

2) The Nullable Types (since Visual Studio 2005)

Definition on msdn

Nullable types can represent all the values of an underlying type, and an additional null value.

How do you use a nullable, let say nullable integer. Simple

int? x = 0; // x can hold any integer values plus the null value
x = null;
var isXNull = (x == null) ? true: false;
Console.WriteLine(isXNull); // True

x = 15;
isXNull = (x == null) ? true: false;
Console.WriteLine(isXNull); // False

3) The Null-conditional Operators (since Visual Studio 2015)

Example of a pretty common mistake

public static void Main() {
 var student1 = CreateStudent("Quan", 27);
 Console.WriteLine(student1.Name); // Quan
 var student2 = CreateStudent("", 27); // student2 is null because no student name is given to the fiction method CreateStudent
 Console.WriteLine(student2.Name); // System.NullReferenceException: Object reference not set to an instance of an object.
}

The method CreateStudent returns a null value instead of creating a new student if the student name is not given. Trying to access its Name property after that will throw a NullReferenceException

What you can do (without the Null-conditional Operators)

var student2 = CreateStudent("", 27); // student2 is null
if (student2 != null){
 Console.WriteLine(student2.Name); // No more System.NullReferenceException
}

What you can do (with the Null-conditional Operators)

var student2 = CreateStudent("", 27); // student2 is null
Console.WriteLine(student2?.Name); // No more System.NullReferenceException

What happended? Thanks to the question mark after student2, you are telling C# that student2 may be null. If student2 is indeed null, C# will not try to access Name anymore, it returns null instead and Console.WriteLine will happily ignore the null value and only add a new line.

4) The ?? : Null-coalescing operator Operator (since Visual Studio 2005)

This is a special case of the ?: Operator in section 1

The format : asking_value ?? just_in_case;

if asking_value is null, just_in_case is returned, otherwise asking_value is returned.

Example

int? x = null; // x is nullable so it can be null
var y = x ?? 0; // if x is null, set y to 0, otherwise set y = x;
// which is essentially the same as
var y = (x == null) ? 0 : x;