Lambda Expressions in C#

1 - What is a lambda expression (Lambda expression)

As we saw in the post about delegates, lambda expressions are another way to represent anonymous methods using a special syntax.

Here we can see an example of an anonymous method that checks if the age is of majority:

delegate(int edad) { return edad >= 18 };

The same code can be represented using a Lambda expression as follows

edad => edad >= 18;

As you can see, the code has changed a bit and we are going to explain why. 

1. We removed the keyword delegate and the function parameter type (in this case the delegate).
2. We include the lambda operator => to indicate it is a Lambda expression. It must be included between the input parameter and the function body.
3. Finally, the compiler is smart enough to detect that parentheses or braces are not needed, so we can remove them. 

1.1 - Multiple parameters in a Lambda Expression

Lambda expressions allow us to include multiple parameters if we enclose them in parentheses:

(edad, limiteEdad) => edad >= 18 && edad <= limiteEdad;

1.2 - Lambda expression with no parameters

Lambda expressions also allow us to omit parameters. For this, we simply include empty parentheses when declaring the expression. 

() => Console.WriteLine("Expresión Lambda sin parámetros");

1.3 - Lambda expression with a body

Finally, we have the option to include a body with more than one line. To do this, just use braces, one to open and another to close: { ///Codigo }

edad => {
    Console.WriteLine("comprobando la edad");
    int edadMinima = 18;
    return edad >= edadMinima;
}

As you can see, it is also possible to declare variables inside the body of a lambda expression.

2 - Using lambda expressions with delegates

A lambda expression can be assigned to delegates as we saw in the post about delegates: Func<in T, out TResult>, Action<T>, Predicate<in T> . 

Func<int, bool> mayorDeEdad = edad => edad >= 18;
Persona p = new Persona(){ Nombre = "Ivan", Edad = 27 };
bool esMayorDeEdad = mayorDeEdad(p.Edad); //return true

We can see that we can also call it as a method. 

3 - Using lambda expressions in LINQ

The most common case for using lambda expressions is within LINQ queries. LINQ is a query language for objects within C#. We will see a specific module dedicated to this, so I will not go into more detail now. I will leave the link here once I write that post. 

For now, we just need to know that LINQ contains an extension method thanks to the implementation of the IEnumerable<T> interface, which is called .Where() and accepts the delegate Func<T, bool>, which means it will return true or false depending on what we provide as a predicate. 

List<Persona> personas = new List<Persona>() { ... }
Func<int, bool> mayorDeEdad = edad => edad >= 18;
List<Persona> mayoresDeEdad = personas.Where( a=> mayorDeEdad(a.Edad));

Running the code, we can see how it filters all people who are not of legal age. 

Conclusion

• Understanding and getting used to using lambda expressions is fundamental, as they are continuously used in our everyday work.
• The operator => executes the expression, it does not assign a value. 
• The syntax is parameters => function body.
• A lambda expression can have multiple parameters, or none at all.
• A lambda expression can be assigned to Func, Action, and Predicate delegates.