Why use lambda expression with LINQ in this case

Notso · August 8th, 2013, 08:24 AM

Hi,
I am reading through some materials on ASP.NET MVC 4. In the example, they want to sort through some "product" objects, and grab the 3 with the highest prices. They are using LINQ's dot notation to do it. Here is the LINQ dot notation they give.

Code:

var foundProducts = products.OrderByDescending(e => e.Price)
     .take(3)
     .Select(e => new {
          e.Name
          e.Price
      });

My question involves the lambda expressions used. Maybe I do not understand them well enough yet, but I do not see why they are needed in this case. Why can you not do the following?

Code:

var foundProducts = products.OrderByDescending(e.Price)
     .take(3)
     .Select(new {
          e.Name
          e.Price
      });

It might be a dumb question because I am not sure how the system knows what e is without the lambda, but can someone please explain that (why it is needed) to me, and/or explain to me why the lambdas are useful here?

Thanks in advance

Oleg Kolpashchikov · August 14th, 2013, 02:36 PM

You need the lambda-expression here.
Lamba is a shorthand form for calling anonymous methods.

It comes from assigning anonymous methods as handlers for events:

Code:

SubmitButton.Click += delegate (object sender, EventArgs e)
    { MessageBox.Show("Click!") };

Here we have an anonymous method (i.e. method without a name) that will be called that Click-event fires.

Another way to write it is with lambdas:

Code:

SubmitButton.Click += (s, e) => MessageBox.Show("Click!");

"s" and "e" are parameters (object sender & EventArgs e); the compiler can infer them from the "Click"-event and does not need details.
"=>" - is the lambda that means "goes to". Parameters "s" and "e" go into this method. We can use "MessageBox(s.SomeProperty)" because "s" stands for incoming object.

"OrderByDescending()"-method expects a delegate Func<TSource, TKey> and writing a lambda-expression actually provides this TSource. (Func is a special built-in delegate in .NET)
It has nothing to do with ASP.NET MVC - it is the same for all LINQ queries.

If all that sounds rather confusing, you may either just ignore it, or dig up a book on C# and read about delegates, lambdas, extension methods and LINQ.

WebDev123 · August 15th, 2013, 09:43 PM

Interesting. Good reply!!