Passing Lambda selectors as an argument

Question

I am working on some reporting code and I have found myself repeating a certain piece of code which contains a where and select cause over and over again? The only piece of information changing is the target field.

    private static void UpdateResultsListInstructions(List<ManagementInfo> managementInfo, KeyValuePair<int, int> item)
    {
        managementInfo
            .Where(m => m.YearMonthNo == item.Key)
            .Select(m => m.VolumeOfInstructionsReceivedInMonth = item.Value)
            .ToList();
    }

    private static void UpdateResultsListClaims(List<ManagementInfo> managementInfo, KeyValuePair<int, int> item)
    {
        managementInfo
            .Where(m => m.YearMonthNo == item.Key)
            .Select(m => m.VolumeOfClaimsCancelled = item.Value)
            .ToList();
    }

Is it possible to pass in the selector as an argument so I can reuse one method?

Along the lines of

UpdateCommonResultsList(managementInfo, item, (m => m.VolumeOfClaimsCancelled = item.Value))

I can probably rejig all the code so that this is not necessary but now that I've had the thought I'd like to see if its possible. Any help appreciated.

Your current code is assigning values in the select clasuse. Is it your intention to cause this effect (it could be why you're doing a ToList and not returning anything). — René Wolferink, Nov 12 '12 at 17:18
Is there a particular reason you're using Linq extension methods to do what it seems a foreach loop would be better at? — cHao, Nov 12 '12 at 18:06
No particular reason other than i just read the chapter on Linq in the Jon Skeet book and thought I should give it a test drive :) When you say a foreach would be better in what respect? I thought that these linq extensions resolved to pretty much the same il code as a foreach. — fluent, Nov 12 '12 at 20:46
Not quite. The Linq stuff is a bunch of methods, whereas `foreach` is a language construct, so they'll never compile to the exact same thing. If you use a method that forces iteration, like `ToList()`, then yes, it'll end up doing a foreach internally. But frankly, why bother? You could say `.Select(...)` to modify your objects, but as noted in one of the comments above, it's unconventional and people will assume it's an accident. You could say `.ToList().ForEach(...)`, but you've gained nothing outside of a potentially massive List you won't even return. — cHao, Nov 12 '12 at 22:17

Rawling · Accepted Answer · 2012-11-12T17:53:11.210

4

Although I don't like the use of Select to update a list - I misread the question because it's not usual to do that! - you can do it as follows:

private static void UpdateCommonResultsList(
    List<ManagementInfo> managementInfo,
    KeyValuePair<int, int> item,
    Action<ManagementInfo, int> action)
{
    managementInfo
        .Where(m => m.YearMonthNo == item.Key)
        .Select(m => { action(m, item.Value); return item.Value; })
        .ToList();
}

Call as

UpdateCommonResultsList(
    managementInfo,
    item,
    (m, i) => m.VolumeOfClaimsCancelled = i);

Your code will be a lot more conventional if you do this as

private static void UpdateCommonResultsList(
    List<ManagementInfo> managementInfo,
    KeyValuePair<int, int> item,
    Action<ManagementInfo, int> action)
{
    foreach (ManagementInfo m in managementInfo
        .Where(m => m.YearMonthNo == item.Key))
    {
        action(m, item.Value);
    }
}

edited Nov 12 '12 at 17:53

answered Nov 12 '12 at 17:17

Rawling

49,248
7
89
127

Don't use `Action`, but `Func` as the Select requires a result. But ideally, in your last bit of code, do `action(m)` with the action being `Action` and then it doesn't need anything else. – René Wolferink Nov 12 '12 at 17:26
2

I've been sat on the Tube for 20 minutes going "I hope nobody notices that didn't compile before I can fix it"... Fat chance :p – Rawling Nov 12 '12 at 17:56
Awesome, many thanks for answering my question and pointing me in a better direction than using select for an update. Thanks again. – fluent Nov 12 '12 at 21:40

score 2 · Answer 2 · answered Nov 12 '12 at 17:17

2

The Select method takes an argument of type Func<TSource, TResult> so you can use that type in the method, and then pass it in as a lambda expression.

answered Nov 12 '12 at 17:17

ChrisPatrick

984
7
19

gabba · Answer 3 · 2012-11-12T17:37:08.057

1

You can pass action like this to update methods:

UpdateResultsListClaims(managementInfo,
  (m) => m == someItem.Key,
  (m, newData) => m.VolumeOfClaimsCancelled = someItem.Value)

private static void UpdateResultsListClaims(List<ManagementInfo> managementInfo,
Func<bool> updateCondition, Action<ManagementInfo> updateAction)
    {
        managementInfo
            .Where(m => updateCondition(m))
            .ToList()
            .ForEach(m => updateAction(m));
    }

UPDATE 1 For more readability you can call:

UpdateResultsListClaims(managementInfo,
  updateCondition: (m) => m == someItem.Key,
  updateAction: (m, newData) => m.VolumeOfClaimsCancelled = someItem.Value)

UPDATE 2 For easy use in many places you can make extension method like this

public static void UpdateResults(this List<ManagementInfo> managementInfo,
Func<bool> updateCondition, Action<ManagementInfo> updateAction)
    {
        managementInfo
            .Where(m => updateCondition(m))
            .ToList()
            .ForEach(m => updateAction(m));
    }

and call it like this

managementInfo.UpdateResults(
  updateCondition: (m) => m == someItem.Key,
  updateAction: (m, newData) => m.VolumeOfClaimsCancelled = someItem.Value)

edited Nov 12 '12 at 17:37

answered Nov 12 '12 at 17:23

gabba

2,815
2
27
48

1

Any particular reason not to say something like `foreach (var m in managementInfo.Where(updateCondition)) { updateAction(m); }` rather than creating a List? – cHao Nov 12 '12 at 18:01
cHao, the reason is just little less typing 8) By the way did some one know why in linq ForEach did't exists for IEnumerable? – gabba Nov 12 '12 at 19:03
here's discusson about foreac for ienumerable http://stackoverflow.com/questions/200574/linq-equivalent-of-foreach-for-ienumerablet – gabba Nov 12 '12 at 19:08
Because the biggest feature of IEnumerable, besides the extension methods, is right in the name -- it is designed to be *enumerable*; that is, it can be used in a foreach loop. A ForEach method wouldn't provide any functionality not already provided by C# and VB. – cHao Nov 12 '12 at 19:08
In case above, it can improve the readability and reduce typing. Even with codecomplete and snippets. But I vote up your comment both hands - list creation not optimal – gabba Nov 12 '12 at 19:19

score 0 · Answer 4 · answered Nov 12 '12 at 17:20

private static void UpdateCommonResultsList<T>(List<ManagementInfo> managementInfo, KeyValuePair<int, int> item, Func<ManagementInfo,T> updateFunc)
    {
        managementInfo
            .Where(m => m.YearMonthNo == item.Key)
            .Select(m => updateFunc(m))
            .ToList();
    }

And then invoke as:

UpdateCommonResultsList(managementInfo, item, (m => m.VolumeOfClaimsCancelled = item.Value));

Passing Lambda selectors as an argument

4 Answers4