How does this LINQ Expression work?

.net, c#, linq, linq-to-sql

Solution

The first part makes sense but it's the part in the brackets I don't understand.

What are you seeing here is a lambda expression. It's a very special anonymous delegate.

Is it mostly LINQ?

`Enumerable.SingleOrDefault` is a LINQ method, but the lambdas are something independent of LINQ; they just make LINQ incredibly more friendly then it otherwise would be.

Now, to get specific `IEnumerable<Subject>.SingleOrDefault(s => s.ID == ID)` returns the unique instance of `Subject` in the `IEnumerable<Subject>` that matches the predicate `s => s.ID == ID` or it returns `null` if there is no such instance. It throws an exception if there is more than one such instance. At compile-time `s => s.ID == ID` is translated into a full-blown delegate the eats objects of type `Subject` and returns a `bool` that is `true` if and only if `s.ID` is equal to `ID`.

How can we use `s` without declaring it?

The `=>` is the lambda operator. It basically separates the left-hand side of the lambda expression from the right-hand side. The left-hand side are the input variables. It's equivalent to the parameter list in an explicitly-defined method. That is `s` in the lambda expression plays the role of `s` below:

public bool Predicate(Subject s)

It's just that you don't have to declare the type of `s` as the compiler will infer it.

The right-hand side the lambda body. It's equivalent to the body below

public bool Predicate(Subject s) {
    return s.ID == ID;
}

What is more, you don't have to declare the return type; the compiler will infer that to.

So, in the end it is as if you did the following:

class MyPredicate {
    public string ID;
    public bool Predicate(Subject s) {
        return s.ID == this.ID;
    }
}

Then:

// elements is IEnumerable<Subject>
// ID is string
MyPredicate predicate = new MyPredicate();
predicate.ID = ID;
elements.SingleOrDefault(predicate.Predicate);

The nice thing is the compiler just automatically spits this out for you when you use a lambda expression.

Problem

Original source