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Delegates

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A delegate is a C# type that describes a callable method signature. A delegate value pairs that signature with a method and, for a closed instance method, its target object. The value can then be stored, passed around, combined with other delegates, and invoked later.

This makes delegates the common mechanism behind callbacks, LINQ operators, strategy-style APIs, and events. Any static or instance method with a compatible signature can be assigned to the delegate type.

static decimal StandardPrice(int q, decimal p) => q * p;
 
PriceCalculator calc = StandardPrice;
var total = calc(3, 19.99m); // 59.97
 
public delegate decimal PriceCalculator(int quantity, decimal unitPrice);

Built-in Generic Delegates

  • Action<T...>: returns void
  • Func<T..., TResult>: returns a value
  • Predicate<T>: returns bool for one input. Its signature matches Func<T, bool>, but it remains a different delegate type.
Func<int, int, int> add = (a, b) => a + b;
Action<string> log = s => Console.WriteLine(s);
Predicate<int> isEven = n => n % 2 == 0;

Multicast Delegates

A delegate can represent more than one method. += combines invocation lists, -= removes an entry, and invoking the result calls its handlers in registration order.

Action pipeline = () => Console.WriteLine("Step 1");
pipeline += () => Console.WriteLine("Step 2");
pipeline += () => Console.WriteLine("Step 3");
 
pipeline();

Three details matter at runtime:

  • If one handler throws, later handlers are not invoked.
  • For non-void delegates, only the last handler’s return value is returned.
  • -= removes the last matching handler from the invocation list.

Variance

Delegate compatibility supports covariance for return values and contravariance for parameters:

  • Covariance: a method may return a more derived type.
  • Contravariance: a method may accept a less derived parameter type.
Func<Dog> dogFactory = () => new Dog();
Func<Animal> animalFactory = dogFactory; // covariance
 
Action<Animal> inspectAnimal = a => Console.WriteLine(a.GetType().Name);
Action<Dog> inspectDog = inspectAnimal;   // contravariance
 
class Animal { }
class Dog : Animal { }

Anonymous Methods and Lambdas

Anonymous methods (delegate(...) { ... }) and lambdas ((...) => ...) can be converted to delegate instances. Both forms may capture local variables.

int threshold = 10;
Func<int, bool> greaterThanThreshold = x => x > threshold;

The captured variable becomes shared closure state. It is not copied when the delegate is created.

Closures

A closure holds variables captured from an outer scope by a lambda or anonymous method. Every delegate that closes over the same variable sees the same storage, including later updates.

var handlers = new List<Action>();
 
for (int i = 0; i < 3; i++)
{
    handlers.Add(() => Console.WriteLine(i));
}
 
handlers.ForEach(h => h()); // 3, 3, 3

The lambda captures the single for variable i, not its value during each iteration.

var handlers = new List<Action>();
 
for (int i = 0; i < 3; i++)
{
    int copy = i; // capture per-iteration value
    handlers.Add(() => Console.WriteLine(copy));
}
 
handlers.ForEach(h => h()); // 0, 1, 2

The extra local gives each iteration separate storage. This is needed whenever later mutation of shared captured state would change the delegate’s meaning, with for loop indices being the usual example.

Allocation, Captures, and Function Pointers

  • Delegates are immutable. += and -= create a new multicast delegate with an updated invocation list through Delegate.Combine or Delegate.Remove. Frequent subscription changes on a hot path therefore allocate.
  • A capturing lambda needs closure state. A static lambda (C# 9) cannot capture locals, so the compiler rejects accidental captures and may reuse the delegate instance.
  • Static method-group conversions can be cached by the compiler. C# 11 added cached static method-group conversions, avoiding a fresh delegate allocation when the same conversion is evaluated repeatedly.
  • DynamicInvoke uses late-bound invocation. It performs runtime checks and can box arguments, so a typed call is preferable. For an instance method, a closed delegate binds its target instance, while an open delegate receives that instance as its first argument.
  • Function pointers (delegate*, C# 9) remove the delegate object. They fit narrow unsafe or interop paths where the calling convention and lifetime are controlled. They give up delegate safety and composition to get there.

Pitfalls

  • One exception stops a multicast call. Later handlers are skipped. Isolating handlers requires enumerating GetInvocationList() and invoking each one inside its own try/catch.
  • A non-void multicast call exposes only the last return value. Collecting every result requires enumerating the handlers, invoking each one separately and keeping each return value.
  • Captured loop state can drift. Closures capture variables rather than snapshots, so several handlers may observe the same final value. A per-iteration local creates the intended snapshot.
  • The legacy asynchronous delegate pattern is unsupported on modern .NET. Calling delegate BeginInvoke or EndInvoke throws PlatformNotSupportedException. Task-based APIs are the replacement.

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