Delegates — Type-Safe Function Pointers

A delegate is a type-safe object that holds a reference to a method — a “typed function pointer.” You can pass delegates as parameters, store them in variables, and invoke them later without knowing which specific method will run. This is the mechanism behind every callback pattern in .NET: LINQ’s Where(predicate), ASP.NET Core’s middleware pipeline, event handlers, and async continuations. Understanding delegates is the prerequisite for understanding lambdas, events, and the entire functional programming side of C#.

Declaring and Using Delegates #

// ── Declare a delegate type — defines the method signature ────────────────
public delegate bool Validator(string value);
public delegate void Logger(string message, LogLevel level);
public delegate int Transformer(int input);

// ── Create delegate instances — point to methods ──────────────────────────
bool IsValidEmail(string value) => value.Contains('@') && value.Contains('.');
bool IsNotEmpty(string value)   => !string.IsNullOrWhiteSpace(value);

Validator emailValidator = IsValidEmail;
Validator notEmpty       = IsNotEmpty;

// ── Invoke delegates ───────────────────────────────────────────────────────
bool result1 = emailValidator("alice@example.com");  // true
bool result2 = emailValidator("notanemail");          // false

// Null-safe invocation
bool result3 = emailValidator?.Invoke("test@test.com") ?? false;

// ── Delegate as method parameter (callback pattern) ───────────────────────
public static string Process(string input, Validator validate, Transformer transform)
{
    if (!validate(input))
        throw new ArgumentException($"Invalid input: {input}");
    return transform(int.Parse(input)).ToString();
}

// ── Multicast delegates — chain multiple methods ──────────────────────────
void LogToConsole(string msg, LogLevel lvl) => Console.WriteLine($"[{lvl}] {msg}");
void LogToFile   (string msg, LogLevel lvl) => File.AppendAllText("app.log", $"[{lvl}] {msg}\n");

Logger multiLogger  = LogToConsole;
multiLogger        += LogToFile;    // both methods called on invoke

multiLogger("Application started", LogLevel.Information);
// Calls LogToConsole AND LogToFile

multiLogger -= LogToFile;          // remove one handler
multiLogger("Now only console", LogLevel.Information);

Delegate Compatibility #

// Any method with the matching signature can be assigned to a delegate
public delegate int MathOp(int a, int b);

int Add(int a, int b) => a + b;
int Subtract(int a, int b) => a - b;
int Multiply(int a, int b) => a * b;

MathOp op = Add;
Console.WriteLine(op(10, 3));   // 13

op = Subtract;
Console.WriteLine(op(10, 3));   // 7

// Store in a dictionary for a command dispatch table
var operations = new Dictionary<string, MathOp>
{
    ["+"] = Add,
    ["-"] = Subtract,
    ["*"] = Multiply,
};

string symbol = "+";
int result = operations[symbol](10, 5);   // 15

Common Mistakes #

Mistake 1 — Declaring custom delegate types when Action/Func suffice #

❌ Wrong — unnecessary custom delegate type:

public delegate void Processor(string input);   // redundant

✅ Correct — use built-in Action:

Action<string> processor = input => DoWork(input);

Mistake 2 — Not checking for null before invoking a delegate #

❌ Wrong — NullReferenceException if no handlers are subscribed:

myDelegate("data");   // throws if myDelegate is null

✅ Correct — null-conditional invocation:

myDelegate?.Invoke("data");   // safe — does nothing if null