Master C# Delegates – A Practical 2025 Guide

Are you tired of tangled event handlers and rigid method calls cluttering your codebase? What if you could swap behaviors on the fly – like Lego bricks – without rewriting a single if‑statement? Welcome to the world of C# delegates, where methods become first‑class citizens and flexibility meets performance.

What Exactly Is a Delegate?

Think of a delegate as a typed remote control that can point to any method matching its signature. Press the button (invoke the delegate) and the TV (method) turns on – no matter which brand you connect tomorrow.

// A remote control for any method that takes an int and returns void
public delegate void IntProcessor(int value);

Under the hood: A delegate is an immutable, sealed class derived from System.MulticastDelegate. It stores a reference to the target object (or null for static methods) and a method pointer.

Analogy

Imagine you’re hosting an exclusive tech meetup. The guest list (delegate) stipulates who can enter (methods with the right signature). Whether it’s Alice (static method) or Bob (instance method), the bouncer checks the list and lets them in – no extra code required.

Why and When Should You Use Delegates?

Anatomy of a Delegate

Declaring and Instantiating

// Declaration
delegate decimal DiscountPolicy(decimal amount);

// Instantiation (C# 10 target‑typed new syntax)
DiscountPolicy student = new(amount => amount * 0.8m);
DiscountPolicy vip     = LoyaltyDiscount; // method group conversion

Invocation

public static void Pay(decimal amount, DiscountPolicy policy)
{
    var final = policy(amount); // delegate invocation
    Console.WriteLine($"Charged {final:C}");
}

Pay(100m, student); // Charged €80.00

Multicast Delegates & Invocation Lists

Need to notify many listeners? Delegates chain like Spotify playlists.

Action onSave = SaveToDatabase;
onSave += SendAuditMail;
onSave += () => Console.WriteLine("Saved!");

onSave(); // Executes all three subscribers in order

Gotcha: Return values from multicast delegates come only from the last subscriber. Use ref parameters or custom result aggregation if you need more.

Built‑in Delegate Types: Action, Func, Predicate

Why declare new delegate types when generics already cover most cases?

Example – LINQ’s Where:

IEnumerable<int> evens = numbers.Where(static n => n % 2 == 0);

Where expects Func<int,bool> predicate – a built‑in delegate.

Delegates vs Events: Spot the Difference

An event is syntax sugar that exposes a multicast delegate while protecting its invocation list from outside tampering.

public event Action? Tick; // Only the declaring class can invoke Tick

Rule of thumb:

• Use delegates for single dependency injection (strategy pattern).
• Use events for one‑to‑many notifications (observer pattern).

Real‑World Delegate Patterns

Strategy Injection in ASP.NET Core Middleware

app.Use(async (ctx, next) =>
{
    RequestDelegate logging = async c =>
    {
        Console.WriteLine($"{c.Request.Path} at {DateTime.UtcNow}");
        await next();
    };

    await logging(ctx);
});

Dynamic Rule Engine with Delegates

var rules = new List<Func<Order,bool>>
{
    o => o.Total > 500,
    o => o.Customer.IsPrime,
    CustomRuleFromDb() // compiled via Roslyn at runtime
};

bool isApproved = rules.All(r => r(order));

UI Command Binding (MAUI / WPF)

public ICommand RefreshCommand => new RelayCommand(async () => await LoadAsync());

A command is essentially a delegate wrapped with CanExecute logic.

Common Pitfalls & Best Practices

1. Delegate Leaks – Unsubscribe from events to avoid memory leaks in long‑lived objects.
2. Exception Handling – One failing subscriber halts the entire multicast invocation. Wrap each handler or use GetInvocationList().
3. Avoid Anonymous Orphans – Keep references to anonymous delegates if you plan to unsubscribe later.
4. Prefer Built‑ins – Use Func/Action to reduce type noise unless a descriptive name clarifies intent.
5. Static Lambdas – Prefix lambdas with static (C# 9) to prevent accidental captures and heap allocations.

FAQ: Delegates Demystified

Conclusion: Delegates Unlock Composable Power

Mastering delegates turns your C# toolbox into a Swiss Army knife – compact yet capable of elegant callbacks, extensible architectures, and succinct functional pipelines. Start refactoring one rigid class today; inject a delegate and feel the code breathe. Ready to delegate your worries? Drop a comment below with your favorite use case or question!