Question

I am trying to understand specifically why it is neccessary to have this Where<TSource>

What does the type straight after Where tell you?

I understand the ‘this’ concept which means its an extension method but cannoth understand the type after Where

public static IEnumerable<TSource> Where<TSource>(
    this IEnumerable<TSource> source,
    Func<TSource, bool> predicate
)

Answer 1

Func<TSource, bool> is a pointer to a function which takes TSource as parameter and returns boolean. For example if you had the following function:

public bool Foo(SomeType abc)
{
    return abc.SomeProperty == "123";
}

you could pass it as argument to the Where method if you had a list of SomeType:

SomeType[] values = ...
var result = values.Where(Foo);

You could also use an anonymous function which avoids you the need to declare another function explicitly:

SomeType[] values = ...
var result = values.Where(x => x.SomeProperty == "123");

---

UPDATE:

I seem to have misunderstood the question. The type after the name of the function Where<TSource> indicates a generic function definition. It indicates that this function has a generic argument which can be of any type. So for example when you write:

SomeType[] values = ...
var result = values.Where(x => x.SomeProperty == "123");

TSource equals SomeType and the compiler is capable of automatically inferring it from the delegate. You could specify it explicitly but it’s too much of a writing:

SomeType[] values = ...
IEnumerable<SomeType> result = values.Where<SomeType>(x => x.SomeProperty == "123");