Extend the options of LINQ

Question

I would like to be able to write the following in a LINQ query:

from item in list
collate by item.Property
select item;

to use this code, I have the following method:

public static IEnumerable<T> CollateBy<T,TKey>(
                                 this IEnumerable<T> arr, 
                                 Func<T, TKey> keySelector)
{
    foreach (var group in arr.GroupBy(keySelector))
    {
        foreach (var item in group)
            yield return item;
    }
}

How do I write this method so that the query stated above would work?

Answer 1

You can't do this. from, in, by and select are C# keywords. So that you ask is equivalent to define a custom keyword, which is not possible.

For the record, each LINQ query written in query syntax (like the one you have mentioned) under the hood is compiled in method calls that have the same result.

For instance the following query:

var adultCustomers = from customer in customers
                     where customer.Age > 18;

is equivalent to the query below:

var adultCustomers = customers.Where(customer=>customer.Age>18);

Under the hood the compiler initially translates the first query to the second one. The first version is just syntactic sugar.

In your case the following query (method syntax) is might that you want:

var result = list.CollateBy(item => item.Property);