LINQ c# unique element in collection

Question

I have a method that is supposed to check whether there is exactly one element in a collection that holds true for some predicate (given as a Func).

public bool ExistsUnique(Func<T, bool> p)
    {
        var tempCol = from i in MyCollection where p(i) select i;  
        return (tempCol.Count() == 1);
    }

The problem with this is that when a second element that also holds true for the predicate is found (for example two of the same string exists in a collection) the count is still 1. Which means it either overrides the first element or never adds the second because it already exists.

Any ideas as to how I can fix this method? thx /Peter

I suspect this SO question and its answers may help you.

ssamuel
– ssamuel

2011-11-08 00:34:33 +00:00
Commented Nov 8, 2011 at 0:34 — ssamuel
– ssamuel, Commented Nov 8, 2011 at 0:34

shuniar · Accepted Answer · 2013-07-19 13:17:25Z

2

You can use the Single() method provided by LINQ like this:

public bool ExistsUnique(Func<T, bool> p)
{
    try
    {
        var temp = myCollection.Single(x => p(x));
    }
    catch(Exception e)
    {
        // log exception
        return false;
    }

    return true;
}

"Returns the only element of a sequence that satisfies a specified condition, and throws an exception if more than one such element exists."

From http://msdn.microsoft.com/en-us/library/bb535118.aspx

EDIT

To avoid throwing an exception, you may also use the SingleOrDefault() method:

public bool ExistsUnique(Func<T, bool> p)
{
    return myCollection.SingleOrDefault(x => p(x)) != null;
}

edited Jul 19, 2013 at 13:17

answered Nov 8, 2011 at 0:25

shuniar

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4 Comments

PNS Over a year ago

that is a great idea! I tried it though I still get the same problem and it returns true even though i tried adding two of the same string.

Paul Tyng Over a year ago

Throwing exceptions are pretty slow when they aren't necessary, may want to be careful with this technique when there are easy ways to do the same thing with no exception.

shuniar Over a year ago

Added an option without exception but left the exception in there as well. Both are viable options in my opinion depending on the desired results.

Ruben Bartelink Over a year ago

@shuniar I disagree but -1 removed. BTW when responding, people will know much much quicker if you put an at-theirname in your comment

Jim Mischel · Accepted Answer · 2011-11-08 00:32:53Z

1

There must be some other problem. I'd suspect your predicate. For example, this returns a count of 2, as expected:

        List<string> MyCollection = new List<string>()
        {
            "hello",
            "hello"
        };
        var tempCol = from i in MyCollection where i == "hello" select i;
        int count = tempCol.Count();

I doubt that it's the way you're calling it, either. The following works (returns false):

    static List<string> MyCollection = new List<string>()
        {
            "hello",
            "hello"
        };

    static bool ExistsUnique(Func<string, bool> p)
    {
        var tempCol = from i in MyCollection where p(i) select i;
        return tempCol.Count() == 1;
    }

    static void DoIt()
    {
        bool isUnique = ExistsUnique((s) => s.Equals("hello"));
        Console.WriteLine(isUnique);
    }

edited Nov 8, 2011 at 0:32

answered Nov 8, 2011 at 0:19

Jim Mischel

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3 Comments

PNS Over a year ago

I think you may be right. Though the predicate my_set.ExistsUnique(s => s.Equals("Hello")) I think is fine. I am using the exact same in my Exists method(just checking if at least one element holds true, which works). Could it be the way I write it p(i)?

Jim Mischel Over a year ago

@PNS: See my updated answer. I don't think the problem is with the way you're calling it.

PNS Over a year ago

Now I'm truly confused, the only difference from your methods and my own is fact that your ExistsUnique method takes string,bool whereas mine takes T,bool but nonetheless it should still work. right?

Peterdk · Accepted Answer · 2011-11-08 00:12:01Z

1

Are you sure tempCol has looped completely through MyCollection? is Count() a method that forces the complete loop or is it lazy?

Does for example tempCol.ToList().Count give the correct result?

answered Nov 8, 2011 at 0:12

Peterdk

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1 Comment

PNS Over a year ago

I tried saying p(i) || !p(i) where it added all the elements in the collection once (no duplicates), just to check if it actually iterated through all the elements which it did.

Paul Tyng · Accepted Answer · 2011-11-08 22:26:53Z

0

This implementation would make it so you don't have to actually enumerate the entire collection, so will save you some execution time.

public bool ExistsUnique(Func<T, bool> p)
{
    return MyCollection.Where(i => p(i)).Take(2).Count() == 1;
}

The Take(2) limits the Count to only enumerate the first two meeting the criteria.

answered Nov 8, 2011 at 22:26

Paul Tyng

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