C# Using Enumerable Range and Except with custom class to determine missing sequence number

Question

I have a List<MyClass>

The class is like this:

private class MyClass
        {

            public string Name{ get; set; }

            public int SequenceNumber { get; set; }

        }

I want to work out what Sequence numbers might be missing. I can see how to do this here however because this is a class I am unsure what to do?

I think I can handle the except method ok with my own IComparer but the Range method I can't figure out because it only excepts int so this doesn't compile:

Enumerable.Range(0, 1000000).Except(chqList, MyEqualityComparer<MyClass>);

Here is the IComparer:

 public class MyEqualityComparer<T> : IEqualityComparer<T> where T : MyClass
        {
            #region IEqualityComparer<T> Members

            public bool Equals(T x, T y)
            {
                return (x == null && y == null) || (x != null && y != null && x.SequenceNumber.Equals(y.SequenceNumber));
            }

            /// </exception>
            public int GetHashCode(T obj)
            {
                if (obj == null)
                {
                    throw new ArgumentNullException("obj");
                }

                return obj.GetHashCode();
            }

            #endregion
        }

Answer 1

You could always project the class to the sequence number, as that's what you're interested in from the class, which will give you an IEnumerable<int> that can be directly excepted from the range, e.g.

Enumerable.Range(0, 1000000).Except(chqList.Select(c => c.SequenceNumber));

This will return the missing sequence numbers. Note that this assumes chqList is a list of MyClass objects.