Can you explain lambda expressions? [duplicate]

Question

I don't really get lambda expressions. While they've been around since the days of ALGOL, I didn't start hearing about them until fairly recently, when Python and Ruby became very popular. Now that C# has the => syntax, people in my world (.NET) are talking about lamdba expressions more and more.

I've read the Wikipedia article on the lambda calculus, but I'm not really a math guy. I don't really understand it from a practical perspective. When would I use lambda expressions? Why? How would I know that it's what I should be doing?

Can you show examples of how you would solve problems with lambda expressions, in a before-and-after format? Any imperative language is fine, but C# would be easiest for me to understand.

Answer 1

Basically as far as C# is concerned, lambda expressions are an easy way to create a delegate (or an expression tree, but let's leave those aside for now).

In C# 1 we could only create delegate instances from normal methods. In C# 2 we gained anonymous methods. In C# 3 we gained lambda expressions, which are like more concise anonymous methods.

They're particularly concise when you want to express some logic which takes one value and returns a value. For instance, in the context of LINQ:

                       // Only include children - a predicate var query = dataSource.Where(person => person.Age < 18)                         // Transform to sequence of names - a projection                       .Select(person => person.Name); 

There's a fuller discussion of this - along with other aspects - in my article on closures.