IEnumerable的 <dynamic> linq表达

Question

I have dynamic list of customers IEnumerable<Customer>

now I want to have the disctinct company names from that list?

I thought I could do something like

 dynamic cur = (from c in result.Customers
                      select g.CompanyName).Distinct();

but learned today that I can't... how can I build a query like this?

Answer 1

What you are doing in code and what you are asking in the title of your question are two different things.

If you want IEnumerable<dynamic> you must do the following:

IEnumerable<dynamic> cur = (from c in result.Customers
               select g.CompanyName).Cast<dynamic>().Distinct();

from c in result.Customers select g.CompanyName returns IEnumerable<string> .
Cast<dynamic>() returns IEnumerable<dynamic> .
Distinct() returns distinct members of the enumerable.

Distinct() uses, by default, the default equality comparer EqualityComparer<T> . This examines the type being enumerated and tries to figure out how to handle it (the link describes this in detail).

This all works as advertised, unless the type being handled dynamically can't be handled by the default equality comparer. In this case, you'll have to use the override that takes a custom equality comparer.

Answer 2

As long as the Customer class has a member CompanyName , you can definitely do the following:

var companies = (from c in result.Customers
                 select c.CompanyName).Distinct();

There is no advantage to using the dynamic keyword over var here, other than the fact that it will prevent compiler errors from appearing.