The method "Except" in List does not give the right information in C#

Question

I have 2 lists of ManualReadTag objects: _manualReadTagList and mrt. The entries in mrt are a subset of _manualReadTagList. I need to get every entry from _manualReadTagList that is "NOT" present in the mrt. So this is how I did it:

IEnumerable<ManualReadTag> difference = _manualReadTagList.Except(mrt).ToList();

But this is not working. I get all the records in the _manualReadTagList and not just whatever is not in the mrt. Below is how I fill the mrt

var mrt = (from ManualReadTag row in ViewingGridFromComparison.ItemsSource
                select new ManualReadTag
                {
                    Plaza = Convert.ToInt16(row.Plaza),
                    Lane = Convert.ToInt16(row.Lane),
                    Trxn_DTime = Convert.ToDateTime(row.Trxn_DTime),
                    Tag_Number = row.Tag_Number
                }).ToList();

IEnumerable<ManualReadTag> difference = _manualReadTagList.Except(mrt).ToList();
ViewingGrid.ItemsSource = difference;

Can you please show me how to do this correctly. Thank you.

Answer 1

The problem, if you haven't overridden Equals etc, is that in creating mrt you've created new objects by copying data items out of your old objects. This means the default provisions of Equals and GetHashCode, which are based on the memory address of your ManualReadTag objects, are different and the objects are not considered equal:

ManualReadTag a = new ManualReadTag(){ SomeProperty = 123 };
ManualReadTag b = new ManualReadTag(){ SomeProperty = a.SomeProperty };
ManualReadTag c = a;

a.Equals(b) //false even though they have identical property values, memory addresses are different
a.Equals(c); //true

You need to either need to reuse the same objects so that memory address based equality assertion is true:

//only works if ItemsSource was made from _manualReadTagList
_manualReadTagList.Except( ViewingGridFromComparison.ItemsSource)

Or you need to override Equals and GetHashCode on ManualReadTag (for best performance Except will build a hash set from the mrt then iterate the _mnualReadTagList hashing everything in it and looking it up in the hashset created from mrt. It thus needs to be able to calculate the hash of a ManualReadTag and then, because it is theoretically possible to have two different objects have the same hashcode, use Equals to verify absolutely whether they are the same or not should the hashcodes turn out equal)

Or you need to create your own IEqualityComparer class (see the documentation for an example: https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.except - this code example could also be used as a base to override Equals and GetHashCode on ManualReadTag)

Or you need to ditch Except and roll your own; turn mrt into a hashset or dictionary (something that has a fast lookup) based on what properties you consider make two ManualReadTag instances the same, and do what Except does by iterating over _manualReadTagList checking every element for being in mrt - Michal did something like this, though that LINQ solution probably uses nested loops and could have poor performance if the mrt list is large

Answer 2

Well, you somehow know how to compare two classes, you mentioned:

You understood it wrong _manualReadTagList has 1,2,3,4,5,6 and mrt has 2,4 so I should just get 1,3,5,6

Where are these numbers coming from??? I guess it is the field Tag_Number in ManualReadTag class (if it's different, just replace the field in the code below and it'll work). So you need following code:

_manualReadTagList
  .Where(m => !mrt.Select(i => i.Tag_Number).Contains(m.Tag_Number))
  .ToList();

Other way would be to override Equals method in ManualReadTag to check equality of instances based on particular field (I assume Tag_Number).