c# - What's the point of a lambda expression?

Question

After reading this article, I can't figure out why lambda expressions are ever used. To be fair, I don't think I have a proper understanding of what delegates and expression tree types are, but I don't understand why anyone would use a lambda expression instead of a declared function. Can someone enlighten me?

Answer 1

First: brevity and locality:

Which would you rather write, read and maintain? This:

    var addresses = customers.Select(customer=>customer.Address);

or:

static private Address GetAddress(Customer customer)
{
     return customer.Address;
}

... a thousand lines later ...

    var addresses = customers.Select(GetAddress);

What's the point of cluttering up your program with hundreds or thousands of four-line functions when you could just put the code you need where you need it as a short expression?

Second: lambdas close over local scopes

Which would you rather read, write and maintain, this:

var currentCity = GetCurrentCity();
var addresses = customers.Where(c=>c.City == currentCity).Select(c=>c.Address);

or:

static private Address GetAddress(Customer customer)
{
     return customer.Address;
}

private class CityGetter
{
    public string currentCity;
    public bool DoesCityMatch(Customer customer)
    {
        return customer.City == this.currentCity;
    }
}

....

var currentCityGetter = new CityGetter();
currentCityGetter.currentCity = GetCurrentCity();
var addresses = customers.Where(currentCityGetter.DoesCityMatch).Select(GetAddress);

All that vexing code is written for you when you use a lambda.

Third: Query comprehensions are rewritten to lambdas for you

When you write:

var addresses = from customer in customers
                where customer.City == currentCity 
                select customer.Address;

it is transformed into the lambda syntax for you. Many people find this syntax pleasant to read, but we need the lambda syntax in order to actually make it work.

Fourth: lambdas are optionally type-inferred

Notice that we don't have to give the type of "customer" in the query comprehension above, or in the lambda versions, but we do have to give the type of the formal parameter when declaring it as a static method. The compiler is smart about inferring the type of a lambda parameter from context. This makes your code less redundant and more clear.

Fifth: Lambdas can become expression trees

Suppose you want to ask a web server "send me the addresses of the customers that live in the current city." Do you want to (1) pull down a million customers from the web site and do the filtering on your client machine, or (2) send the web site an object that tells it "the query contains a filter on the current city and then a selection of the address"? Let the server do the work and send you only the result that match.

Expression trees allow the compiler to turn the lambda into code that can be transformed into another query format at runtime and sent to a server for processing. Little helper methods that run on the client do not.