Null Coalescing Operators ?? / ??=

// Returns the right-hand value if the left-hand side is null.
var result = variable ?? defaultValue;

// Assigns the right-hand value only when the variable is null.
variable ??= defaultValue;

// Accesses a member with a null check (returns null if the object is null).
var value = obj?.Property;
var value = obj?.Method();

// Combines ?. and ??.
var value = obj?.Property ?? defaultValue;

using System;

// Use ?? to set a default value.
string name = null;
string displayName = name ?? "Guest";
Console.WriteLine(displayName); // Guest

string alias = "Tanaka";
Console.WriteLine(alias ?? "Guest"); // Tanaka

// Use ??= to assign only when the variable is null.
string cache = null;
cache ??= "initial value";
Console.WriteLine(cache); // initial value

cache ??= "not overwritten";
Console.WriteLine(cache); // initial value (unchanged because it is not null)

// Use ?. for null-safe member access.
string text = null;
int? length = text?.Length; // No exception even when text is null.
Console.WriteLine(length); // (empty — outputs null)

string realText = "Hello";
Console.WriteLine(realText?.Length);       // 5
Console.WriteLine(realText?.ToUpper());    // HELLO

// Combine ?. and ??.
string input = null;
int len = input?.Length ?? 0;
Console.WriteLine(len); // 0

// Example with a class.
class User { public string Address { get; set; } }
User user = null;
string address = user?.Address ?? "No address registered";
Console.WriteLine(address); // No address registered

Combining ?? and ?. significantly reduces the number of null-checking if statements. Verbose code like if (obj != null) return obj.Value; else return default; can be written in a single line.

To make value types (such as int or bool) nullable, see Nullable<T> / Nullable Types. For type checking and casting, see is / as / Pattern Matching.