- Published on
Who cares about predicates without closures?
- Authors
-
-
- Name
- David Mohundro
- @drmohundro
-
I’ve been using the System.Collections.Generic
namespace an awful lot lately.
I really like it, too. I use the List<T>
collection all the time. It is so
much better than inheriting from the CollectionBase
to get a type-safe
collection. I won’t go into the reasons here, though. This post is about
Predicates
. If you have a List<T>
collection, you’ll notice it exposes a
Find
method that takes a Predicate
as an argument. A Predicate
is just a
delegate that takes an object of type T
and returns a boolean. When you call
the Find
method, it will call your Predicate
for each object in its
collection until your Predicate
returns true. Looks like a nice, clean way to
search collections, right?
Well, if you’re using C# anyway.
Compare the following VB.NET code and C# code:
VB.NET :
Module Module1
Sub Main()
Dim tests As New List(Of Test)
tests.Add(New Test("test"))
tests.Add(New Test("test2"))
tests.Add(New Test("test3"))
toSearchFor = "test2"
Console.WriteLine(tests.Find(AddressOf FindTest).ID)
End Sub
Private toSearchFor As String = ""
Private Function FindTest(ByVal val As Test) As Boolean
Return val.ID = toSearchFor
End Function
Private Class Test
Public ID As String
Public Sub New(ByVal id As String)
Me.ID = id
End Sub
End Class
End Module
C# :
using System;
using System.Collections.Generic;
using System.Text;
namespace CSPredicateExample
{
class Program
{
static void Main(string[] args)
{
List<Test> tests = new List<Test>();
tests.Add(new Test("test"));
tests.Add(new Test("test2"));
tests.Add(new Test("test3"));
string toSearchFor = "test2";
Console.WriteLine(tests.Find(delegate(Test val) { return val.ID == toSearchFor; }).ID);
}
class Test
{
public string ID;
public Test(string id)
{
this.ID = id;
}
}
}
}
Because of features like this, I am really looking forward to the next release of VB. Of course, with LINQ, will I even care about predicates then?
I would be very interested to know if anyone has any other suggestions on ways to effectively use the Predicate
model in VB.
UPDATE: Wow, go Paul Stovell! Instead of complaining like I did, Paul actually did something to get around this limitation in the current version of VB.NET and created an implementation of “Almost-anonymous” methods! His method also shows how to use the Widening overloaded operator in VB.NET, which I haven’t seen much of. Cool!