What is Nullable Type?

• For value types (i.e. int), by default null cannot be used
• Nullable type is a special feature of C# that allows null value to be assigned to a value type
• A nullable value type T? represents all values of its underlying value type T and an additional null value
  • bool x: x can have only true or false
  • bool? x: x can have true, false, or null
• Any nullable value type is an instance of the generic System.Nullable<T> structure
• Nullable<T> and T? are interchangeable

Nullable Value Type

// Nullable types are VALUE types that can take null as value. The default value of a nullable type is null.
int? x = null;     // no value

char? letter = 'a';

bool? flag = null;

// An array of a nullable value type:
int?[] arr = new int?[10];

Nullable Value Type to Normal Value Type

• Use null-coalescing operator ??
• Or use Nullable<T>.GetValueOrDefault(T)

int? x = 10;
int foo = x ?? -1;
Console.WriteLine($"foo is {foo}");  // output: foo is 10

x = null;
int bar = x ?? -1;
Console.WriteLine($"bar is {bar}");  // output: bar is -1

Nullable Reference Type

• Reference types can have null as value by default
• If reference type gets null value, there might be null reference exception
• C# 8.0 introduces the nullable reference types feature
• Nullable reference types aren’t new class types, but rather annotations on existing reference types. The compiler uses those annotations to help you find potential null reference errors in your code
• There’s no runtime difference between a non-nullable reference type and a nullable reference type
• Details: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/nullable-reference-types

HasValue and Value

• Read-only properties to examine and get a value of a nullable value type variable
  • Nullable<T>.HasValue: indicates whether an instance of a nullable value type has a value of its underlying type
  • Nullable<T>.Value: gets the value of an underlying type if HasValue is true. If HasValue is false, the Value property throws an InvalidOperationException
• For Nullable Reference Type i.e. string? or T?
  • does not have “.HasValue” & “.Value” because nullable reference types aren’t new class types
  • just for compiler to help you find potential null reference errors in your code

For “string? x”, you might need to use “string.IsNullOrEmpty(x)”