如何使用LINQ查找与2个逗号分隔的字符串匹配

Question

I am new to LINQ.

I am trying to compare 2 comma separated strings to see if they contain a matching value.

I have a string that contains a list of codes. masterFormList = "AAA,BBB,CCC,FFF,GGG,HHH"

I am trying to compare it to a list of objects. In a given field FormCode contains a comma separated string of codes. I want to see if this at lease one code in this string is in the masterFormList. How would I write linq to accomplish this?

Right now I have:

resultsList = (from r in resultsList
where r.FormCodes.Split(',').Contains(masterFormList)
select r).ToList();

It does not return any matching items from the list.

Please advise

Answer 1

You'd need to build a collection of the items to search for, then check to see if there are any contained within that set:

var masterSet = new HashSet<string>(masterFormList.Split(','));

resultsList = resultsList
                 .Where(r => r.FormCodes.Split(',')
                              .Any(code => masterSet.Contains(code)))
                 .ToList();

Answer 2

var masterFormList = "AAA,BBB,CCC,FFF,GGG,HHH";
var otherList = "XXX,BBB,YYY";

bool match = otherList.Split(',').Intersect(masterFormList.Split(',')).Any();

or if you want the matching items

var matches = otherList.Split(',').Intersect(masterFormList.Split(',')).ToList();

Answer 3

To answer the question as stated, this will find all the matches between two strings:

var matches =
    from masterCode in masterFormList.Split(',')
    join formCode in formCodes.Split(',') on masterCode equals formCode 
    select formCode;
foreach (string match in matches)
{
    Console.WriteLine(match);
}

but that's overkill if all you want to know is that one exists. You could just do this with the same query:

Console.WriteLine(matches.Any());

However, that's likely to do more work than strictly necessary. A modification to Reed Copsey's answer might be simplest (if we're looking to answer the question in the title of your post):

var masterSet = new HashSet<string>(masterFormList.Split(','));
bool atLeastOneMatch = formCodes.Split(',').Any(c => masterSet.Contains(c));

While these are reasonably idiomatic LINQ solutions to the problem you stated ("I am trying to compare 2 comma separated strings to see if they contain a matching value") they're probably not a great match for what you actually seem to want, which is to take a list of objects, and find just the ones in which a particular property meets your criteria. And a join is probably the wrong approach for that because it looks rather unwieldy:

resultList =
    (from formItem in resultList
     from code in formItem.FormCodes.Split(',')
     join masterCode in masterFormList.Split(',') on code equals masterCode
     group code by formItem into matchGroup
     select matchGroup.Key)
    .ToList();

or if you prefer:

resultList =
    (from formItem in resultList
     from code in formItem.FormCodes.Split(',')
     join masterCode in masterFormList.Split(',') on code equals masterCode into matchGroup
     where matchGroup.Any()
     select formItem)
     .Distinct()
     .ToList();

These solutions have little to commend them...

So given the problem evident from your code (as opposed to the problem defined in the question title and the first 3 paragraphs of your post), Reed Copsey's solution is better.

The one tweak I'd make is that if your master set is fixed, you'd only want to build that HashSet<string> once, to amortize the costs. So either you'd put it in a static field:

private readonly static HashSet<string> masterSet =
    new HashSet<tring>(masterFormList.Split(',');

or use Lazy<T> to create it it on demand.

(Edited 8/8/2013 after Reed pointed out to me in the comments that the problem evident from the code example was not the same as the problem stated in the question.)