如何获取通用列表中的比较项目

Question

I have a custom class that implements that IComparable. This class is stored in a Generic List. I now need to compare to lists to see which objects are in list A but not in list B.

I thought the most simple way of doing this would be to iterate through list B and do A.contains().

I do not know how to get it to use my CompareTo() (or another method that I can override so that I can say if it contains a certain object or not). I could be wrong but as I understand it the contains checks if the objects are actually the same (ie points to the same place in memory).

Could anyone help me please?

Answer 1

Why don't you just override the Equals method of your class to be consistent in meaning with CompareTo(other) == 0 ? This is the simplest way and also the most idiomatic since, as you've noticed, Contains compares equality rather than using CompareTo . However, this check is done via Equals . It does not check whether the objects point to the same memory location.

/EDIT: Additionally, if you're using .NET 3.5 you can use the Contains overload that accepts an IEqualityComparer argument. You can use this to provide a class that implements a custom equality relation for your class type. However, I think the first method is more appropriate in your case.

Answer 2

Also if you are using framework 3.5 you can make a query like:

list notInB = From item in listA where (listB.find(function(x) x.property = item.property) = nothing) select item

This will return all the items of the listA that are not in listB.

The linq function find returns the item that matches de condition that you write in the lambda function. There you can add more conditions using and (&&)or or (||)

If you use this, there is no need to implement IComparable.

Answer 3

Thank you Konrad for such a quick and helpful reply. Worked a treat! I used IComparable originally because I needed to sort them on certain criteria but overriding equals worked perfectly for .contains.

:)

Answer 4

it the contains checks if the objects are actually the same (ie points to the same place in memory)

Note that object can be equal even if they do not point to the same place in memory. It depends how EqualsTo method is implemented.

I agree with Rudolph in that you first try to overload the EqualsTo().

Answer 5

There is also IEquatable and if your object implements that interface, then it is automatically used by all the generic collections if its implemented. Though I never quite understood why it existed in the first place, since the docs say that if you implement IEquatable, then you should also override Equals to match. Also remember if you are going to override Equals, that you need to also override GetHashCode as well.