IEnumberable has an extension method Contains<T> which takes two parameters. The first parameter is the value to check for and the second is an implementation of IEqualityComparer. Looking at IEqualityComparer.Equals it takes two parameters named x and y, for the first and second objects to compare.
My question is X or Y the value from the IEnumerable?
Example
List<string> test = new List<String() { "a", "b", "c" };
test.Contains("d", myComparer);
When it calls to the Equals method for the first value will it be Equals("a","d") or Equals("d","a")?
It shouldn't matter - equality should be symmetric. From the docs for IEqualityComparer<T>.Equals:
The Equals method is reflexive, symmetric, and transitive. That is, it returns true if used to compare an object with itself; true for two objects x and y if it is true for y and x; and true for two objects x and z if it is true for x and y and also true for y and z.
I don't believe the usage in Enumerable.Contains is well-defined, i.e. it could change in a future version. If you just make your equality comparer obey the interface documentation, you'll be fine.