Information Technology Reference
In-Depth Information
// Discussed below.
if
(
this
.GetType() != right.GetType())
return false
;
// Compare this type's contents here:
return this
.Equals(right
as
Foo
);
}
#region
IEquatable<Foo> Members
public bool
Equals(
Foo
other)
{
// elided.
return true
;
}
#endregion
}
First, Equals() should never throw exceptions—it doesn't make much
sense. Two variables are or are not equal; there's not much room for other
failures. Just return false for all failure conditions, such as null references
or the wrong argument types. Now, let's go through this method in detail
so you understand why each check is there and why some checks can be left
out. The first check determines whether the right-side object is null. There
is no check on this reference. In C#, this is never null. The CLR throws an
exception before calling any instance method through a null reference. The
next check determines whether the two object references are the same, test-
ing object identity. It's a very efficient test, and equal object identity guar-
antees equal contents.
The next check determines whether the two objects being compared are
the same type. The exact form is important. First, notice that it does not
assume that this is of type Foo; it calls this.GetType(). The actual type
might be a class derived from Foo. Second, the code checks the exact type of
objects being compared. It is not enough to ensure that you can convert the
right-side parameter to the current type. That test can cause two subtle bugs.
Consider the following example involving a small inheritance hierarchy:
public class
B
:
IEquatable
<
B
>
{
public override bool
Equals(
object
right)
{
// check null:
