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What is the difference between Func, Action and Predicate? What is a delegate in C#, and what does it represent conceptually? How do you write and use a lambda expression as a shorthand for a delegate? What is a delegate in C#, and how does it enable passing methods as values? How do events differ from plain delegates? What is an event, and how does it differ from a plain public delegate field? What are multicast delegates, and in what order do their subscribed methods get invoked? How does variable capture (closures) work in a C# lambda expression, and what pitfall occurs when capturing a loop variable? What causes event-handler memory leaks and how do you prevent them? What is the difference between multicast delegate invocation and exception handling?
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What is a closure and what is a common pitfall?

Intermediate

Answer

A closure is a lambda that captures variables from its enclosing scope. A classic pitfall is capturing a loop variable by reference so all lambdas see its final value.
var fns = new List<Func<int>>();
foreach (var i in Enumerable.Range(0,3)) fns.Add(() => i); // each captures its own i (C# 5+)
Real-world example Building event handlers in a loop that must remember the item they were created for.
All 17 Delegates, Events & Lambdas questions