Nullable types are represented by the Nullable<T> structure. An instance of Nullable<T> is boxed to either a null reference (if it doesn't have a value) or a boxed value of T (if it does).
The type parameter T has a value type constraint specified e.g., struct Nullable<T> where T : struct

// Implicit conversion from T to Nullable<T> (wrapping).
int? n1 = 8;                    
Nullable<int> n2 = 8;
 
// Nullable constructors.
var n3 = new Nullable<int>();  // n3 == null
var n4 = new Nullable<int>(8); // n4 == 8
 
// All three values x1, x2, and x3 are null.
int? x1 = new int?();
int? x2 = default(int?);
int? x3 = (int?)null;

int? n = 8;
 
// Explicit conversion from Nullable<T> to T (unwrapping).
int i1 = (int)n; // the same as n.Value
 
// Check if a nullable variable has a value.
if (n == null) { }
if (n.HasValue) { }
 
// Use the 'is' and 'as' operators.
if (n is Int32) { }
int? m = n as Int32?;
 
// Use Nullable<T>.GetValueOrDefault()
int? n1 = 8;
int i1 = n1.GetValueOrDefault();   // 8
int i2 = n1.GetValueOrDefault(88); // 8
 
int? n2 = null;
int i3 = n2.GetValueOrDefault();   // 0
int i4 = n2.GetValueOrDefault(88); // 88

The truth table for bool? variables determined by the C# compiler:

bool? x;
bool? y;

C# 8.0 introduced nullable and non-nullable reference types. Reference types can be configured to no longer allow the null value by setting a file- or project-level option.

Enable the feature at the project level:

<PropertyGroup>
  <Nullable>enable</Nullable>
</PropertyGroup>

Disable the feature at the file level:

#nullable disable

Enable the feature at the file level:

#nullable enable

Example of using a nullable reference type. Note that non-nullable fields (FirstName and LastName) have to have a value assigned:

#nullable enable
...
class PersonName
{
   public string FirstName = String.Empty;
   public string? MiddleName;   // a nullable reference field
   public string LastName = String.Empty;
}

Use the null-conditional operator to check if a member variable might be null:

var person = new PersonName();
...
// If the MiddleName field is null, the variable 'len' will be assigned null.
int? len = person.MiddleName?.Length; 

Use the null-coalescing operator to assign an alternative value if a variable is null:

var person = new PersonName();
...
// If the MiddleName field is null, the variable 'len' will have the value of 2.
int len = person.MiddleName?.Length ?? 2; 

• MSDN: Nullable value types
• MSDN: Nullable reference types
• MSDN: Null-conditional operators ?. and ?[]
• MSDN: Null-coalescing operators ?? and ??= operators
• Channel9: How to get rid of null reference exceptions forever