Compare difference of two lists, .Except() algorithm to replace LINQ

Question

I used to use .Except() to compare the difference of two List in C# by using LINQ like following.

List<string> APromotionProduct= GetAPromotionProduct(PrdA);
List<string> BPromotionProduct = GetBPromotionProduct<string>(PrdB);
List<string> tempList = new List<string>();
tempList = PromotionProduct.Except(XMLPromotionProduct).ToList();

However, my company is not using LINQ, we are on .NET 3. Therefore I can't use Enumerable.Except. How can I achieve the same purpose or how can I write the algorithm for .Except().

What version of the .NET framework your company is using? Why you can't use one >= 3.5? — Tim Schmelter, Dec 15 '17 at 10:03
"how can I write the algorithm for .Except()." Think about it... in 2 loops you would compare one list with another and extract only the elements that are not present in the other. — Mong Zhu, Dec 15 '17 at 10:05
If you can't think of a loop yourself, try checking the [reference source](http://referencesource.microsoft.com/System.Core/R/64071682ee3bf309.html) — René Vogt, Dec 15 '17 at 10:06
Can't access what library? It's part of the .NET Framework. Is your application C# or not? Are you asking to rewrite Except in another language or something? — Equalsk, Dec 15 '17 at 10:12
@ChanYoongHon: it has nothing to do with the IDE used. What .NET framework are you using? — Tim Schmelter, Dec 15 '17 at 10:17
@Equalsk My application is C# with .Net Version 3.0. My company IDE can't use LINQ at all. — Chan Yoong Hon, Dec 15 '17 at 10:17
Right, now we've got it. That is a key piece of information, put it in your question. — Equalsk, Dec 15 '17 at 10:19

Tim Schmelter · Accepted Answer · 2017-12-15T10:16:37.673

If you can't use LINQ for whatever reason, this is the source:

static IEnumerable<TSource> ExceptIterator<TSource>(IEnumerable<TSource> first, IEnumerable<TSource> second, IEqualityComparer<TSource> comparer) {
    Set<TSource> set = new Set<TSource>(comparer);
    foreach (TSource element in second) set.Add(element);
    foreach (TSource element in first)
        if (set.Add(element)) yield return element;
}

So you could use a HashSet<string>(ok, still .NET 3.5 needed):

public static IEnumerable<TSource> ExceptNoLinq<TSource>(this IEnumerable<TSource> first, IEnumerable<TSource> second, IEqualityComparer<TSource> comparer) 
{
    HashSet<TSource> set = new HashSet<TSource>(comparer);
    foreach (TSource element in second) set.Add(element);
    foreach (TSource element in first)
        if (set.Add(element)) yield return element;
}

Then you can use:

var exceptItems = PromotionProduct.ExceptNoLinq(XMLPromotionProduct);
List<string> resultList = new List<string>(exceptItems);

If you are even on .NET 2:

public static IEnumerable<TSource> ExceptDotNet2<TSource>(this IEnumerable<TSource> first, IEnumerable<TSource> second, IEqualityComparer<TSource> comparer)
{
    var dict = new Dictionary<TSource, bool>(comparer);
    foreach (TSource element in second) dict.Add(element, true);
    foreach (TSource element in first)
    {
        if (!dict.ContainsKey(element))
        {
            dict.Add(element, true);
            yield return element;
        }
    }
}

Thanks, you really help me. LINQ make people lazy to think like me. — Chan Yoong Hon, Dec 15 '17 at 10:29
@ChanYoongHon: If you are on .NET3 you should be able to use the last dictionary approach. — Tim Schmelter, Dec 15 '17 at 10:38

score 1 · Answer 2 · answered Dec 15 '17 at 10:13

1

Except() is an extension method from System.Linq. If you can refer this namespace in your file then you should be able to use it. This doesn't require a Visual studio. As long as you have access to the compiler you can compile and run your code.

answered Dec 15 '17 at 10:13

Libish Jacob

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Compare difference of two lists, .Except() algorithm to replace LINQ

2 Answers2