Enumerable.Where method

Guest User

14y2.1kC#

Hi

Given the following code

List fruits = new List
{ "apple", "passionfruit", "banana", "mango", "orange", "blueberry", "grape", "strawberry"};

IEnumerable query = fruits.Where(fruit => fruit.Length < 6);

Im having difficulty in matching this up to the actual Where method declaration as below:-

public static IEnumerable Where(this IEnumerable source,Func predicate

From what I can work out so far:-

IEnumerable is IEnumerable and the Where(this IEnumerable source, Func predicate, is the (fruit => fruit.Length < 6); Linq & Lambda part?

Regards

Steven

6 Replies

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Vulpes14y ago
Accepted answer
Well, the first thing to note is that 'Where' is an extension method (the first parameter, source, is prefixed by 'this') and so it's applied to an object which implements IEnumerable as though it were an instance method of that object.

The object here is 'fruits' which, as a List, implements IEnumerable.

The second parameter, predicate, is then a delegate of type Func which takes a parameter of type string and returns a value of type bool.

So, the delegate (a lambda expression is used here) takes every string in source (assigned to the parameter 'fruit') and returns true if fruit.Length < 6.

The return value of the Where method is then the sequence of strings (an IEnumerable) within 'source' which satisfies the predicate. So, in other words, Where acts as a filter.

Clear as mud, eh :)
Vulpes14y ago
Yes, 'Where' is a generic method and all generic methods need to have their type parameters declared in angle brackets next to the method name.

So TSource is its (only) type parameter and is used in the declaration of its parameter types.

It may also be used in the body of the method though, as it's part of the .NET Framework, this is not exposed to us except by using special tools such as Reflector.
Guest User14y ago
Vulpes

Just to re-visit this one quickly.

Why does the Where extension method have to have the generic declaration beside it?

Is it due to the fact that the implementation type ie IEnumerable has the generic defined as the Type parameter?

Regards

Steven
Guest User14y ago
Yep I was aware of that, thanks again Vulpes!
Vulpes14y ago
Although you've probably worked it out already, one thing I should have mentioned is that the compiler 'infers' the type of TSource to be string because Where is applied to an IEnumerable.
Guest User14y ago
I actually understand fully. Thanks for the detailed description Vulpes!