Question

This is a question about the SYNTAX of c# and NOT about how we call/use IQueryable

Can someone please explain to me:

We have this declaration (System.Linq):

public static double Average<TSource>(this IQueryable<TSource> source,
                                      Expression<Func<TSource, int>> selector)

and to call the Average

double average = fruits.AsQueryable().Average(s => s.Length);

I understand how to call the Average and all the similar static method of IQueryable
but I don’t understand the syntax of the declaration.

public static double Average<TSource>(this IQueryable<TSource> source,
                                      Expression<Func<TSource, int>> selector)

What does the <TSource> mean in Average<TSource>(
and also the this IQueryable<TSource> source.

since only one parameter passes when we call it and the actual lambda expression (s => s.Length);

Thanks in advance.

Answer 1

The <TSource> part declares the generic type parameters of the method – basically what kind of element the sequence contains. You should definitely understand generics before you get too far into LINQ.

Then,

this IQueryable<TSource> source

indicates the first parameter of the method:

• this indicates that it’s an extension method
• IQueryable<TSource> indicates the type of the parameter
• source is the name of the parameter

The fact that it’s an extension method is probably what’s confusing you. When you call

query.Average(s => s.Length)

that is converted by the compiler into

Queryable.Average(query, s => s.Length)