C# method accepting a predicate - does this look ok?

Question

I'd like a method that has the following API:

//get all users with a role of admin
var users = myRepository.GetUsers(u => u.Role == Role.Admin);

Will something like this work?

IList<User> GetUsers(Func<User, bool> predicate)
{            
  var users = GetAllUsers();
  return users.Where(predicate).ToList();                                                          
}

If so, wil I be able to specify more complex predicates such as (pseudocode):

myRepository.GetUsers(u => u.CreatedDate is upto 14 days old);

An IList is an IEnumerable, so an IEnumerable is going to be less flexible. I think the comment may have been directed toward the possible unneeded call to ToList()? — Jace Rhea, Feb 05 '10 at 17:50

score 20 · Accepted Answer · answered Feb 05 '10 at 11:46

20

That looks absolutely fine. However, if you want to use the predicate with something like LINQ to SQL, you'd want to use:

IList<User> GetUsers(Expression<Func<User, bool>> predicate)

That means the lambda expression will be converted into an expression tree instead of a delegate - and that expression tree can then be converted into SQL.

answered Feb 05 '10 at 11:46

Jon Skeet

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Really appreciate your response. Thanks – Ben Aston Feb 05 '10 at 12:04
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+1 That explanation clarified my understanding of LINQ by a couple percent. Much obliged. :) – Dan J Jan 22 '11 at 00:28
Thanks much for this answer. I'm using LINQ to SQL, and using just Func throws an OutOfMemory exception. This works perfect. – AngieM Aug 24 '16 at 20:10

score 3 · Answer 2 · answered Feb 05 '10 at 11:44

Yes, this will work.

What you're telling the compiler is that you're going to pass a function that works on a User object, and returns a bool - what the function looks like is entirely up to you. You can get pretty complex nesting stuff using ors (||) and ands (&&) too, but if it gets too hard to see what the predicate is doing, you should probably consider refactoring.

But basically any lambda that takes for example u as a User object, and turns it into an expression that returns bool will work.

score 2 · Answer 3 · answered Feb 05 '10 at 11:47

2

One tip: you can use Predicate<T> instead of Func<T, bool>, which is a little clearer (Predicate<T> is built in to .net 3.5, namespace System).

answered Feb 05 '10 at 11:47

ljs

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`Predicate` was actually in .NET 2.0, but `Func` tends to be used in LINQ - partly because some methods have `Func`. – Jon Skeet Feb 05 '10 at 12:01
Ah I forgot that it was in 2.0; could you give me a few examples of methods using Func? Thanks – ljs Feb 05 '10 at 13:45

C# method accepting a predicate - does this look ok?

3 Answers3