如何迭代集合并使用LINQ扩展方法更改值？

Question

Lets say I have a collection of Messages which has the properties "UserID" (int) and "Unread" (bool).

How can I use LINQ extension methods to set Unread = false, for any Message in the collection in whose UserID = 5?

So, I know I can do something like:

messages.Any(m => m.UserID == 5);

But, how do I set the Unread property of each of those with an extension method as well?

Note: I know I should not do this in production code. I'm simply trying to learn some more LINQ-fu.

Answer 1

Actually, this is possible using only the built-in LINQ extension methods without ToList .
I believe that this will perform very similarly to a regular for loop. (I haven't checked)

Don't you dare do this in real code.

messages.Where(m => m.UserID == 5)
        .Aggregate(0, (m, r) => { m.Unread = false; return r + 1; });

As an added bonus, this will return the number of users that it modified.

Answer 2

messages.Where(m => m.UserID == 5).ToList().ForEach(m => m.Unread = false);

然后提交更改。

Answer 3

Standard LINQ extension methods doesn't include side effects aimed methods. However you can either implement it yourself or use from Reactive Extensions for .NET (Rx) like this:

messages.Where(m => m.UserID == 5).Run(m => m.Unread = false);

Answer 4

As there is no explicit extension method that does a ForEach , you are stuck with either using a secondary library, or writing the foreach statement on your own.

foreach (Message msg in messages.Where(m => m.UserID == 5))
{
    msg.Unread = false;
}

If you really want to use a Linq statement to accomplish this, create a copy the collection using the ToList() method, accessing the ForEach() method of the List type:

messages.Where(m => m.UserID == 5).ToList().ForEach(m => m.Unread = false);

or place the side-effect in a Where() statement:

messages.Where(m =>
{
    if (m.UserID == 5) { m.Unread = false; return true; }
    return false;
});

In either case, I prefer to use the explicit foreach loop as it doesn't make unnecessary copies and is clearer than the Where hack.

Answer 5

With LINQ you can't because LINQ is a query language/extension. There is however a project called MoreLinq , which defines an extension method called ForEach which allows you to pass an action which will be performed on every element.

So, you could do with MoreLinq:

messages.Where(m => m.UserID == 5).ForEach(m => m.Unread = false);

Best Regards,
Oliver Hanappi

Answer 6

This answer is in the spirit of providing a solution. On could create an extension which does both the predicate ( Where extension) to weed out the items and the action needed upon those items.

Below is an extension named OperateOn which is quite easy to write:

public static void OperateOn<TSource>(this List<TSource> items, 
                                      Func<TSource, bool> predicate, 
                                      Action<TSource> operation)
{
    if ((items != null) && (items.Any()))
    {
        items.All (itm =>
        {
            if (predicate(itm))
                operation(itm);

            return true;
        });

    }
}

Here is it in action:

var myList = new List<Item>
                   { new Item() { UserId = 5, Name = "Alpha" },
                     new Item() { UserId = 5, Name = "Beta", UnRead = true },
                     new Item() { UserId = 6, Name = "Gamma", UnRead = false }
                   };


myList.OperateOn(itm => itm.UserId == 5, itm => itm.UnRead = true);

Console.WriteLine (string.Join(" ",
                               myList.Select (itm => string.Format("({0} : {1})",
                                                                   itm.Name,
                                                                   itm.UnRead ))));

/* Outputs this to the screen

(Alpha : True) (Beta : True) (Gamma : False)

*/

...

public class Item
{
    public bool UnRead { get; set; }
    public int UserId { get; set; }
    public string Name { get; set; }
}

Answer 7

You should be able to just do it in a Select(), remember the lambda is a shortcut for a function, so you can put as much logic in there as you want, then return the current item being enumerated. And... why exactly wouldn't you do this in production code?

messages = messages
    .Select(m => 
    {
        if (m.UserId == 5) 
            m.Unread = true;
        return m;
    });