C# Language Specification

Unary operators

The following rules apply to unary operator declarations, where T denotes the instance type of the class or struct that contains the operator declaration:

• 
  A unary +, -, !, or ~ operator must take a single parameter of type T or T? and can return any type.
  
• 
  A unary ++ or -- operator must take a single parameter of type T or T? and must return that same type or a type derived from it.
  
• 
  A unary true or false operator must take a single parameter of type T or T? and must return type bool.
  

The signature of a unary operator consists of the operator token (+, -, !, ~, ++, --, true, or false) and the type of the single formal parameter. The return type is not part of a unary operator’s signature, nor is the name of the formal parameter.

The true and false unary operators require pair-wise declaration. A compile-time error occurs if a class declares one of these operators without also declaring the other. The true and false operators are described further in §7.12.2 and §7.20.

The following example shows an implementation and subsequent usage of operator ++ for an integer vector class:

public class IntVector

{
public IntVector(int length) {...}

public int Length {...} // read-only property

public int this[int index] {...} // read-write indexer

public static IntVector operator ++(IntVector iv) {

IntVector temp = new IntVector(iv.Length);
for (int i = 0; i < iv.Length; i++)
temp[i] = iv[i] + 1;
return temp;
}
}

class Test

{
static void Main() {
IntVector iv1 = new IntVector(4); // vector of 4 x 0
IntVector iv2;

iv2 = iv1++; // iv2 contains 4 x 0, iv1 contains 4 x 1

iv2 = ++iv1; // iv2 contains 4 x 2, iv1 contains 4 x 2
}
}

Note how the operator method returns the value produced by adding 1 to the operand, just like the postfix increment and decrement operators (§7.6.9), and the prefix increment and decrement operators (§7.7.5). Unlike in C++, this method need not modify the value of its operand directly. In fact, modifying the operand value would violate the standard semantics of the postfix increment operator.

2.