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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 is the difference between multicast delegate invocation and exception handling? How do the built-in Func<>, Action<>, and Predicate<T> generic delegate types differ, and when would you use each? How would you implement the 'weak event pattern' to prevent event subscribers from causing a memory leak by keeping the publisher alive indefinitely? How does a lambda expression get compiled differently when it captures NO outer variables versus when it DOES capture outer state? How do delegate covariance and contravariance work with method group conversions in C#? How would you implement a custom EventHandler using async/await, avoiding the common 'async void' anti-pattern while still matching the standard EventHandler delegate signature?
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C# › Delegates, Events & Lambdas

What causes event-handler memory leaks and how do you prevent them?

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Answer

A long-lived publisher holding a delegate to a subscriber keeps the subscriber alive. Prevent it by unsubscribing (-=), using weak event patterns, or scoping lifetimes.
source.Changed += Handler;
// later, to allow GC:
source.Changed -= Handler;
Real-world example A screen that subscribes to a singleton service must unsubscribe on close or it never gets collected.
All 17 Delegates, Events & Lambdas questions