I want to check if an object is in a Queue before I enqueue it. If don't explicitly define an EqualityComparer, what does the Contains() function compare?
If it compares property values, that's perfect. If it compares to see if a reference to that object exists in the Queue then that defeats the purpose.
For classes, the default equality operation is by reference - it assumes that object identity and equality are the same, basically.
You can overcome this by overriding Equals and GetHashCode. I'd also suggest implementing IEquatable<T> to make this clear. Your hash code implementation should generate the hash code from the same values as the equality operation.
The default for reference types is to compare the reference.
However, if the type implements IEquatable<> it can be doing a different comparison. If you need to have a specific equality comparison in place, you need to create one yourself.