The predefined reference type equality operators are:
bool operator ==(object x, object y); bool operator !=(object x, object y);
The operators return the result of comparing the two references for equality or non-equality.
Since the predefined reference type equality operators accept operands of type object
, they apply to all types that do not declare applicable operator
==
and operator
!=
members. Conversely, any applicable user-defined equality operators effectively hide the predefined reference type equality operators.
The predefined reference type equality operators require the operands to be reference-type values or the value null
, and furthermore require that an implicit conversion exists from the type of either operand to the type of the other operand. Unless both of these conditions are true, a compile-time error occurs. Notable implications of these rules are:
A
and B
, and if neither A
nor B
derives from the other, then it would be impossible for the two operands to reference the same object. Thus, the operation is considered a compile-time error.For an operation of the form x
==
y
or x
!=
y
, if any applicable operator
==
or operator
!=
exists, the operator overload resolution (§7.2.4) rules will select that operator instead of the predefined reference type equality operator. However, it is always possible to select the reference type equality operator by explicitly casting one or both of the operands to type object
. The example
class Test { static void Main() { string s = "Test"; string t = string.Copy(s); Console.WriteLine(s == t); Console.WriteLine((object)s == t); Console.WriteLine(s == (object)t); Console.WriteLine((object)s == (object)t); } }
produces the output
True False False False
The s
and t
variables refer to two distinct string
instances containing the same characters. The first comparison outputs True
because the predefined string equality operator (§7.9.7) is selected when both operands are of type string
. The remaining comparisons all output False
because the predefined reference type equality operator is selected when one or both of the operands are of type object
.
Note that the above technique is not meaningful for value types. The example
class Test { static void Main() { int i = 123; int j = 123; Console.WriteLine((object)i == (object)j); } }
outputs False
because the casts create references to two separate instances of boxed int
values.