Lambda Expression: == vs. .Equals()

Question

This is a purely academic question, but what's the difference between using == and .Equals within a lambda expression and which one is preferred?

Code examples:

int categoryId = -1;
listOfCategories.FindAll(o => o.CategoryId == categoryId);

or

int categoryId = -1; 
listOfCategories.FindAll(o => o.CategoryId.Equals(categoryId));

Answer 1

They can be overloaded separately, so they could provide different answers. See http://msdn.microsoft.com/en-us/library/ms173147(VS.80).aspx which discusses how to overload each. Typically they will be the same, but there's no guarantee of that. So it depends on what type the lambda object is.

Answer 2

It depends on what defined for the object. If there's no operator== defined for the class, it will use the one from the Object class, which checks Object.ReferenceEquals before eventually calling Equals().

This shows an important distinction: if you say A.Equals(B) then A must be nun-null. if you say A == B, A may be null.

Answer 3

The Lambda is irrelevant here...

For value objects == and equals are the same For reference object == will be true if the objects are the same object (points to the same instance) while it is expected that equals compare the contents of the objects. This link explains it in another way.

Answer 4

This is more prominent in the Java world really. Basically '==' is operator overloading and .Equals() is the base method on Object class.

Answer 5

This is a duplicate of

http://stackoverflow.com/questions/814878/c-difference-between-and-equals

For some additional thoughts on different kinds of equality and how none of them do what you really want, see

http://blogs.msdn.com/ericlippert/archive/2009/04/09/double-your-dispatch-double-your-fun.aspx