Predicate in Net 3.5

24 Nov 2007 2:46 AM

shapper

Hello,

I have a List(Of MyClass).
MyClass has two properties: Name and City.

I want to find, without using a loop, if there is a item where name =
NameParameter.

I know that I can use a Predicate but in .NET 2.0 Predicates do not
accepted Parameters.

Because of that I used a Predicate Wrapper.

With .NET 3.5 I know this has changed and now it is much easier to do
something like this.

However, I can't find any example of it.

Can someone, please, help me out?

Thanks,
Miguel

24 Nov 2007 4:14 AM

Nicholas Paldino [.NET/C# MVP]

Miguel,

    Well, like it or not, you are going to have to have a loop somewhere, it
just might not be apparent.

    In 3.5, if you want to see if there is any item in the list where
NameParameter = some value, you can do this:

// The list.
List<MyClass> myList = ...;

// The value to check against.
string nameParameterValue = ...;

// Does it contain the value?
bool containsValue = myList.Any(item => item.NameParameter ==
nameParameterValue);

    The Any extension method is going to perform the loop for you.

--
- Nicholas Paldino [.NET/C# MVP]
- mvp@spam.guard.caspershouse.com

Show quote

"shapper" <mdmo***@gmail.com> wrote in message
news:c65821ec-8a35-4ff3-9207-dcbadc033032@e6g2000prf.googlegroups.com...
> Hello,
>
> I have a List(Of MyClass).
> MyClass has two properties: Name and City.
>
> I want to find, without using a loop, if there is a item where name =
> NameParameter.
>
> I know that I can use a Predicate but in .NET 2.0 Predicates do not
> accepted Parameters.
>
> Because of that I used a Predicate Wrapper.
>
> With .NET 3.5 I know this has changed and now it is much easier to do
> something like this.
>
> However, I can't find any example of it.
>
> Can someone, please, help me out?
>
> Thanks,
> Miguel

24 Nov 2007 7:50 AM

Jon Skeet [C# MVP]

shapper <mdmo***@gmail.com> wrote:

> I have a List(Of MyClass).
> MyClass has two properties: Name and City.
>
> I want to find, without using a loop, if there is a item where name =
> NameParameter.
>
> I know that I can use a Predicate but in .NET 2.0 Predicates do not
> accepted Parameters.

You can use an anonymous method though:

public bool CheckForItem(string name)
{
    return items.Exists(delegate (MyClass item)
        { return item.Name==name;}
    );
}

> However, I can't find any example of it.

With C# 3 (with or without .NET 3.5) you could use:

public bool CheckForItem(string name)
{
return items.Exists(item => item.Name==name);
}

--
Jon Skeet - <sk***@pobox.com>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
World class .NET training in the UK: http://iterativetraining.co.uk

24 Nov 2007 11:26 PM

bob clegg

On Fri, 23 Nov 2007 18:46:26 -0800 (PST), shapper <mdmo***@gmail.com>
wrote:

Show quote

>Hello,
>
>I have a List(Of MyClass).
>MyClass has two properties: Name and City.
>
>I want to find, without using a loop, if there is a item where name =
>NameParameter.
>
>I know that I can use a Predicate but in .NET 2.0 Predicates do not
>accepted Parameters.
>
>Because of that I used a Predicate Wrapper.
>
>With .NET 3.5 I know this has changed and now it is much easier to do
>something like this.
>
>However, I can't find any example of it.
>
>Can someone, please, help me out?
>
>Thanks,
>Miguel

Hi,
I am assuming that you are finding performance problems in this area.
Also assuming that the list contents are unique or you wouldn't be
asking "Is it in the list?"
Any chance of using Dictionary Class<myclass> instead of
List(myClass)?
The Dictionary.ContainsKey method approaches O(1) according to the
documentation.
If you can preSize the dictionary the add function also is O(1)
otherwise O(n).
hth.
Bob