I have a very vanilla understanding of Delegates. I know that you use them to callback to a function in your function. But why can you just call the method instead of using a delegate. How does using a delegate make it more dynamic and how did Delegates evolve into Lambda Expressions? Thanks !

Delegates are used when you want save the method address in a variable, and call via the variable rather than by the name. Many languages have an address of operator, but methods call are highly typed. When you declare a delegate variable, it meant to both define the method signature and store the value, the delegate type overrides the plus operator to allow adding multiple callbacks. originally you needed to define the delegate type before using, but after generics were added, the handy Func<> and Action<> generics can be used to define a delegate variable. Action<string> myCallback; // variable to hold address of a method that take a string value Methods need to named members of a class. Lambda are a way to define an anonymous method, that is one without name. As they don’t have a name, the definition of a lambda returns a variable that can be used as a delegate variable. In other words a lambda defines an expression that return a lambda, rather than a method definition. For example: myCallback = s => Console.WriteLine(s); then later call: myCallback(“hello”); note: lambda also implement closure, which is very handy in callbacks, and often makes more useful than defined methods.

Delegates and Callback - Microsoft Q&A

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Bruce (SqlWork.com) 69,976 Reputation points

2023-06-30T23:59:38.89+00:00

Delegates are used when you want save the method address in a variable, and call via the variable rather than by the name. Many languages have an address of operator, but methods call are highly typed. When you declare a delegate variable, it meant to both define the method signature and store the value, the delegate type overrides the plus operator to allow adding multiple callbacks.

originally you needed to define the delegate type before using, but after generics were added, the handy Func<> and Action<> generics can be used to define a delegate variable.

Action<string> myCallback; // variable to hold address of a method that take a string value

Methods need to named members of a class. Lambda are a way to define an anonymous method, that is one without name. As they don’t have a name, the definition of a lambda returns a variable that can be used as a delegate variable. In other words a lambda defines an expression that return a lambda, rather than a method definition.

For example:

myCallback = s => Console.WriteLine(s);

then later call:

myCallback(“hello”);

note: lambda also implement closure, which is very handy in callbacks, and often makes more useful than defined methods.
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Ronald Rex 1,666 Reputation points

2023-07-01T14:14:20.1866667+00:00

Thanks for your help. I wanted to share a paragraph because I am not clear on some things. A "callback" is a term that refers to a coding design pattern, available in any language that has function pointers, or an analogue to function pointers (which is kinda what a delegate is)

In this pattern, you pass a pointer to a function to another function, so that within the called function, it can "call back" the function you passed to it. In this way you can control a large chunk of the internal behavior of a method from outside the method, by passing pointers to different "callback" function each time you call it... An example of a callback is when you have a sorting algorithm that has to be passed a pointer to a function that will "compare" any arbitrary pair of objects in the list to be sorted, to determine which goes before the other. On one call to the sort method, you might pass a callback function that compares by object name, and another time pass a function that compares by object weight, or whatever... I am not sure what the statement In this way you can control a large chunk of the internal behavior of a method from outside the method, by passing pointers to different "callback" function each time you call it means. Thanks !!!

Bruce (SqlWork.com) 69,976 Reputation points

2023-07-01T17:40:51.38+00:00

The most common example is handling the click event if a UI. The ui component knows how to display and capture events. But the click event callback allows the ui control to also execute application logic to perform.

without callback support, you application would need to subclass every control to implement click processing.

another area where callbacks are handy is the notion of dynamic injection, passing callbacks to the constructor. This all the call to method implementations defined by the caller.

say you passed a data repository. In a test you can pass a test version, in the actual code the real one.

the linq library is a good example of passing delegates. The Were() method can be passed any delegate that returns bool.

if you call Where() twice in you code you can do different compares.
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