null

Value Types

We cannot assign the value 'null' directly to a value type.
Value types by default are non-nullable

Reference Types

We can assign the value 'null' directly to a reference type

Null Conditional Operator (?.)

Also known as Null-Propagation operator

return myvalue?.SubString(0,5); 

Instead of

string smaller = myvalue; 
if (myvalue != null) 
{ 
   smaller = myvalue.SubString(0,5); 
} 
return smaller 

PropertyChanged?.Invoke(propertyChanged(this, new PropertyChangedEventArgs(nameof(Name)); 

Null Coalescing Operator (??)

Added in .NET 4.8

Before this operator you had to use the conditional operator.
If "a" is null then (b = -1).
If "a" is not null then (b = a)

int? a = null; 
int? b = (a == null) ? -1 : a; 
Console.WriteLine(b); 

Alternatively you can use the '??' operator to assign a nullable type to a non-nullable type using slightly less code.

int? a = null; 
int? b = a ?? -1; 
Console.WriteLine(b); 

Null Coalescing Assignment Operator (??=)

sometimes called the compound assignment operator - really

Nullable Types

Added in .NET 2.0

Nullable types can only be used with value types.
Nullable types can represent all the values of an underlying type plus an additional null value.

System.Nullable<T> variable; 

T can be any value type, including struct
T cannot be a referenced type

Every nullable type has two public readonly properties
HasValue - false when variable contains a null value
Value - the value when variable does not contain a null value
Use value property to get the value of nullable type.
Use HasValue property to check whether value is assigned to nullable type or not.
You can only use == and != operators with a nullable type.

Shorthand Syntax

You can use the '?' operator to shorthand the syntax e.g. int?, long? instead of using Nullable<T>.

T? variable; 
int? myInteger = null; 
double? myDouble = null; 
System.DateTime? myDateTime = null; 

Only Value Types

Nullable<T> and the ? suffix are for value types such as Int32, Double, DateTime, etc.
String is already a nullable type.