Func, Action, Predicate — when to use which delegate type in C#?

All three are built-in generic delegates. They differ only in what they return and what they accept.

Delegate	Signature	Returns	Use for
Action	Action<T1..T16>	void	side-effecting operations
Func	Func<T1..T16, TResult>	TResult	mappers, factories
Predicate<T>	Predicate<T>	bool	filter conditions

Examples

Action<string> log = msg => Console.WriteLine(msg);

Func<int, int, int> add = (a, b) => a + b;

Predicate<Order> isPaid = o => o.Status == OrderStatus.Paid;

Why Predicate<T> exists if Func<T, bool> works

Historically Predicate<T> predates generics-heavy LINQ. Today prefer Func<T, bool> for interop — every LINQ method (Where, Any, All) takes Func<T, bool>, not Predicate<T>. List<T>.Find is one of the few APIs still on Predicate<T>.

Real-world rule of thumb

• Returns void → Action
• Returns bool for filtering → Func<T, bool> (LINQ-compatible)
• Returns anything else → Func<..., TResult>
• Defining a public API surface? Skip these and define a named delegate or interface — better stack traces, better tooling.

Closures gotcha

var handlers = new List<Action>();
for (int i = 0; i < 3; i++)
    handlers.Add(() => Console.WriteLine(i));  // all print 3

Capture i into a local inside the loop to fix.