Delegate Comparers

Delegate comparers are also called "anonymous comparers". This kind of "anonymous comparer" is completely different than comparers for anonymous types.

Why You Shouldn't Use Delegate Comparers

• It can be difficult to maintain the comparer invariants with two unrestricted delegates. The invariants are:
• Compare(x, x) == 0
• Compare(x, y) == 0 implies Compare(y, x) == 0
• (Compare(x, y) == 0) && (Compare(y, z) == 0) implies Compare(x, z) == 0
• Compare(x, y) < 0 implies Compare(y, x) > 0
• Compare(x, y) > 0 implies Compare(y, x) < 0
• (Compare(x, y) < 0) && (Compare(y, z) < 0) implies Compare(x, z) < 0
• (Compare(x, y) > 0) && (Compare(y, z) > 0) implies Compare(x, z) > 0
• Compare(x, y) == 0 implies GetHashCode(x) == GetHashCode(y)
• All correctly-implemented anonymous comparers are some combination of key comparers (OrderBy/ThenBy), compound comparers (ThenBy), reverse comparers (Reverse), and sequence comparers (Sequence); and it's cleaner to use the fluent API to spell out the comparison algorithm explicitly.
• The fluent API will create the correct GetHashCode implementation automatically.

For these reasons, I strongly recommend against using the AnonymousComparer and AnonymousEqualityComparer classes. Anonymous comparers cannot be created using the normal Compare class; you have to create them explicitly.

Using Anonymous Comparers

Anonymous comparers can be created from a delegate:

// Treat all 0s and 1s as equivalent, all 2s and 3s as equivalent, etc.
IComparer<int> comparer = new AnonymousComparer<int>
{
  Compare = (x, y) => Compare<int>.Default().Compare(x / 2, y / 2),
};

However, it's highly recommended that you provide a corresponding delegate for GetHashCode as well:

// Treat all 0s and 1s as equivalent, all 2s and 3s as equivalent, etc.
IComparer<int> comparer = new AnonymousComparer<int>
{
  Compare = (x, y) => Compare<int>.Default().Compare(x / 2, y / 2),
  GetHashCode = x => Compare<int>.Default().GetHashCode(x / 2),
};

That allows hash-based collections and algorithms to work as expected.

Note that any AnonymousComparer can be written more succinctly with the fluent API. The code above can also be written as:

// Treat all 0s and 1s as equivalent, all 2s and 3s as equivalent, etc.
IComparer<int> comparer = Compare<int>.OrderBy(x => x / 2);

Not only is the code shorter and more clearly defines the comparer algorithm, it is also easier to maintain since it automatically creates a correct GetHashCode implementation.

For equality comparers, you can create a AnonymousEqualityComparer instance and set the Equals and GetHashCode delegates. Note that GetHashCode is not optional for equality comparers.