Lambda expressions (how is the extension function Select defined?)

Question

var numbers=new int[]{1,2,3};
var numbers1=numbers.Select(n=>n);
var numbers2=numbers.Select(n=>n.ToString());
var numbers3=numbers.Select(n=>new {Number=n, Even=n%2==0});

How is it possible for the output of extension function SELECT to be of any type?

Answer 1

It is a generic extension method defined with the following signature:

public static IEnumerable<Tresult> Select<TSource, TResult>(
                this IEnumerable<TSource> source, 
                Func<TSource, TResult> selector);

With the type information present in the source and selector argument, the compiler can infer the types used so you don't need to explicitly name it.

Answer 2

The particular version of Select here takes a Func<T, TResult> selector, where T is the type of the input (in this case, int ), and TResult is the output. Based upon your lambda expression, the compiler is able to infer the types of TResult . In your case, the types are

• int
• string
• Anonymous

If you are curious about such anonymous functions, I encourage to you to check out the C# 4.0 language specification , perhaps starting at section 7.15.

Answer 3

IEnumerable.Select is a generic method with these signatures:

public static IEnumerable<TResult> Select<TSource, TResult>(
    this IEnumerable<TSource> source,
    Func<TSource, TResult> selector
)

public static IEnumerable<TResult> Select<TSource, TResult>(
    this IEnumerable<TSource> source,
    Func<TSource, int, TResult> selector
)

The function passed in must return a TResult and the Select itself returns IEnumerable<TResult> . The type of TResult can be deduced by the compiler (as done in the post), or it can be explicitly annotated.

More information about generics -- how TResult can be "an arbitrary but particular type", for instance -- can be found at the C# Generics Programming Guide . There are a number of SO questions that discuss the limits of C# type inference (including that of generics) as well.

Happy coding.