C# Compare Two Lists of Different Objects [duplicate]

Answer 1

The Except-method has an overload, which takes an IEqualityComparer<T> as the last argument.

There is a sample in the MSDN: https://msdn.microsoft.com/en-us/library/bb336390.aspx

EDIT
It is possible to create an list of anonymous objects and compare them with a special IEqualityComparer<T>. Here is the comparer class:

class MyEqualityComparer : IEqualityComparer<object>
{
    #region IEqualityComparer<object> Members

    bool IEqualityComparer<object>.Equals(object x, object y)
    {
        return (x as dynamic).Id == (y as dynamic).Id;
    }

    int IEqualityComparer<object>.GetHashCode(object obj)
    {
        return ((obj as dynamic).Id as object).GetHashCode();
    }

    #endregion
}

And the LINQ Expression should look like this:

var result = lst1.Select(x => new { Id = x.Id, Obj = x })
    .Except(lst2.Select(x => new { Id = x.Id, Obj = x }),
            new MyEqualityComparer())
    .Select(x => (x as dynamic).Obj as Type1);

I know, it is a bad style to use dynamic in this case, you can change it without any problem to a real type.

Answer 2

I would recommend converting the Ids of the 2 types to 2 HashSets. Then you can

HashSet<int> a = new HashSet<int>(firstList.Select(o => o.Id));

HashSet<int> b = new HashSet<int>(secondList.Select(o => o.Id));
if (a.IsSubsetOf(b) && b.IsSubsetOf(a))
{
    //Do your thing
}

Answer 3

I'm not sure about the available C#/Linq methods. From an algorithmic point of view, you could sort both lists (usually O(n*log(n))). Then you just need to scan through the lists (linear, aka O(m+n) where m is the number of elements in list 1 and n the number of elements in list 2). Assuming list 1 is the longer list, the total complexity is O(m*log(m)). Depending on the C# HashSet implementation, the answer by Murdock might be faster though.