What Are Lambda Expressions?

To work with LINQ, we need to be comfortable with delegates and lambda expressions. Let’s learn about them, starting with delegates.

What are delegates?

A delegate is a variable that references a method instead of a value or an object. In LINQ, Func and Action are the two built-in delegate types. We will be using Func a lot with LINQ.

The difference between Func and Action is the return type of the method they point to. The Action type references a void method. In other words, it references a method without return type. Conversely, Func references a method with a return type.

Let’s look at some Func and Action declarations.

• Action<Movie> holds a method that receives Movie as a parameter and returns no values.
• Action is a void method without any parameters.
• Func<Movie, string> represents a method that gets a Movie and returns a string.
• Func<Movie> doesn’t have any parameters and returns Movie.

When declaring Func delegates, the last type inside <> tells the return type of the method it accepts. The Action delegates don’t have a return type. Then, the types inside <> are the input parameter types.

Declaring a method that receives Func or Action

Both Func and Action are helpful when working with higher-order functions. These are functions that take functions as parameters or return another function as a result.

If you’ve worked with JavaScript callbacks or Python decorators, you’ve worked with higher-order functions. Guess what methods are also higher-order functions: LINQ methods!

Let’s see how to declare a method that uses Func or Action.

To declare a method that uses Func or Action as an input parameter, we use them like regular parameters in the method declaration. Then, to use it inside the method’s body, we have to either call Invoke on it or put parentheses next to the name passing the correct parameters.

Let’s see an example of a method that uses Func. Let’s imagine we have a PrettyPrint() method to print a movie out to the console. We’ll rely on a delegate to show the rating.