Lambda expression syntax

Question

Is it mandatory that lambda expression need to use when LINQ will be used, or are lambda expressions optional?

In lambda expressions, the sign => is always used. What does it mean?

 customers.Where(c => c.City == "London"); 

Here c => is used but why? What kind of meaning of using c =>. Please discuss in detail because I don't know lambda.

Answer 1

No, you don't have to use a lambda expression. For example, your Where example could be written as:

private static bool IsLondon(Customer customer)
{
    return customer.City == "London";
}

...

var londoners = customers.Where(IsLondon);

That's assuming LINQ to Objects, of course. For LINQ to SQL etc, you'd need to build an expression tree.

As to why "=>" is always used in a lambda expression, that's simply because that's the way the operator is written - it's like asking why "+" is used for addition.

A lambda expression of "c => ..." is effectively giving the lambda expression a parameter called c... in this case generic type inference provides the type of c. The body provides either an action to perform or some calculation to return a value based on c.

A full-blown description of lambda expressions is beyond the scope of this answer. As a blatant plug for my book, however, they're covered in detail in chapter 9 of C# in Depth.

Answer 2

The lambda expression

c => c.City == "London"

is shorthand for something like

bool IsCustomerInLondon(Customer c)
{
  return (c.City == "London");
}

It's just a very concise way of writing a simple function that returns a value. It's called an "anonymous function" because it's never assigned a name or a formal definition (the parameter types and the return type are inferred from the context).

(Actually, it's not just shorthand; lambda expressions are related to some other constructs called closures, which are very cool and powerful tools.)

Answer 3

Think about lambdas as anonymous of functions. I'll try to explain it with following code.

public bool IsLondon(Customer customer)
{
    return customer.City == "London";
}

Now we strip down function name and get rid of braces:

public bool Customer customer    
    return customer.City == "London";

We don't need return type, because compiler is able to deduce type from expression:

 customer.City == "London";

In the same manner compiler knows about input type, so we don't need to specify it.

So basically, what we left with is

customer
   return customer.City == "London";

And lambda syntax in c# is basically:

(parameter list) => "expression"

You can also write "multi-line" expressions, but then you have to surround your code with curly braces. Also you will need to use "return" statement, like you usually do.