Using the null-coalescing operator (??) allows you to specify a default value for a nullable type if the left-hand operand is null.

string testString = null;
Console.WriteLine("The specified string is - " + (testString ?? "not provided"));

Live Demo on .NET Fiddle

This is logically equivalent to:

string testString = null;
if (testString == null)
{
    Console.WriteLine("The specified string is - not provided");
}
else
{
    Console.WriteLine("The specified string is - " + testString);
}

or using the ternary operator (?:) operator:

string testString = null;
Console.WriteLine("The specified string is - " + (testString == null ? "not provided" : testString));

The left-hand operand must be nullable, while the right-hand operand may or may not be. The result will be typed accordingly.

Non-nullable

int? a = null;
int b = 3;
var output = a ?? b;
var type = output.GetType();  

Console.WriteLine($"Output Type :{type}");
Console.WriteLine($"Output value :{output}");

Output:

Type :System.Int32
value :3

View Demo

Nullable

int? a = null;
int? b = null;
var output = a ?? b;

output will be of type int? and equal to b, or null.

Multiple Coalescing

Coalescing can also be done in chains:

int? a = null;
int? b = null;
int c = 3;
var output = a ?? b ?? c;

var type = output.GetType();    
Console.WriteLine($"Type :{type}");
Console.WriteLine($"value :{output}");

Output:

Type :System.Int32
value :3

View Demo

Null Conditional Chaining

The null coalescing operator can be used in tandem with the null propagation operator to provide safer access to properties of objects.

object o = null;
var output = o?.ToString() ?? "Default Value";

Output:

Type :System.String
value :Default Value

View Demo

The null coalescing operator makes it easy to ensure that a method that may return null will fall back to a default value.

Without the null coalescing operator:

string name = GetName();

if (name == null)
    name = "Unknown!";

With the null coalescing operator:

string name = GetName() ?? "Unknown!";

A common usage scenario that this feature really helps with is when you are looking for an object in a collection and need to create a new one if it does not already exist.

IEnumerable<MyClass> myList = GetMyList();
var item = myList.SingleOrDefault(x => x.Id == 2) ?? new MyClass { Id = 2 };

private List<FooBar> _fooBars;

public List<FooBar> FooBars
{
    get { return _fooBars ?? (_fooBars = new List<FooBar>()); }
}

The first time the property .FooBars is accessed the _fooBars variable will evaluate as null, thus falling through to the assignment statement assigns and evaluates to the resulting value.

Thread safety

This is not thread-safe way of implementing lazy properties. For thread-safe laziness, use the Lazy<T> class built into the .NET Framework.

C# 6 Syntactic Sugar using expression bodies

Note that since C# 6, this syntax can be simplified using expression body for the property:

private List<FooBar> _fooBars;

public List<FooBar> FooBars => _fooBars ?? ( _fooBars = new List<FooBar>() );

Subsequent accesses to the property will yield the value stored in the _fooBars variable.

Example in the MVVM pattern

This is often used when implementing commands in the MVVM pattern. Instead of initializing the commands eagerly with the construction of a viewmodel, commands are lazily initialized using this pattern as follows:

private ICommand _actionCommand = null;
public ICommand ActionCommand =>
   _actionCommand ?? ( _actionCommand = new DelegateCommand( DoAction ) );