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Operator Overloading Tutorial

Visual Studio .NET 2003

This tutorial demonstrates how user-defined classes can overload operators.

Sample Files

See Operator Overloading Sample to download and build the sample files discussed in this tutorial.

Further Reading

Tutorial

Operator overloading permits user-defined operator implementations to be specified for operations where one or both of the operands are of a user-defined class or struct type. The tutorial contains two examples. The first example shows how to use operator overloading to create a complex number class that defines complex addition. The second example shows how to use operator overloading to implement a three-valued logical type.

Example 1

This example shows how you can use operator overloading to create a complex number class Complex that defines complex addition. The program displays the imaginary and the real parts of the numbers and the addition result using an override of the ToString method.

// complex.cs
using System;

public struct Complex 
{
   public int real;
   public int imaginary;

   public Complex(int real, int imaginary) 
   {
      this.real = real;
      this.imaginary = imaginary;
   }

   // Declare which operator to overload (+), the types 
   // that can be added (two Complex objects), and the 
   // return type (Complex):
   public static Complex operator +(Complex c1, Complex c2) 
   {
      return new Complex(c1.real + c2.real, c1.imaginary + c2.imaginary);
   }
   // Override the ToString method to display an complex number in the suitable format:
   public override string ToString()
   {
      return(String.Format("{0} + {1}i", real, imaginary));
   }

   public static void Main() 
   {
      Complex num1 = new Complex(2,3);
      Complex num2 = new Complex(3,4);

      // Add two Complex objects (num1 and num2) through the
      // overloaded plus operator:
      Complex sum = num1 + num2;

     // Print the numbers and the sum using the overriden ToString method:
      Console.WriteLine("First complex number:  {0}",num1);
      Console.WriteLine("Second complex number: {0}",num2);
      Console.WriteLine("The sum of the two numbers: {0}",sum);
 
   }
}

Output

First complex number:  2 + 3i
Second complex number: 3 + 4i
The sum of the two numbers: 5 + 7i

Example 2

This example shows how operator overloading can be used to implement a three-valued logical type. The possible values of this type are DBBool.dbTrue, DBBool.dbFalse, and DBBool.dbNull, where the dbNull member indicates an unknown value.

Note   Defining the True and False operators is only useful for types that represent True, False, and Null (neither True nor False), as used in databases.
// dbbool.cs
using System;

public struct DBBool
{
   // The three possible DBBool values:
   public static readonly DBBool dbNull = new DBBool(0);
   public static readonly DBBool dbFalse = new DBBool(-1);
   public static readonly DBBool dbTrue = new DBBool(1);
   // Private field that stores -1, 0, 1 for dbFalse, dbNull, dbTrue:
   int value; 

   // Private constructor. The value parameter must be -1, 0, or 1:
   DBBool(int value) 
   {
      this.value = value;
   }

   // Implicit conversion from bool to DBBool. Maps true to 
   // DBBool.dbTrue and false to DBBool.dbFalse:
   public static implicit operator DBBool(bool x) 
   {
      return x? dbTrue: dbFalse;
   }

   // Explicit conversion from DBBool to bool. Throws an 
   // exception if the given DBBool is dbNull, otherwise returns
   // true or false:
   public static explicit operator bool(DBBool x) 
   {
      if (x.value == 0) throw new InvalidOperationException();
      return x.value > 0;
   }

   // Equality operator. Returns dbNull if either operand is dbNull, 
   // otherwise returns dbTrue or dbFalse:
   public static DBBool operator ==(DBBool x, DBBool y) 
   {
      if (x.value == 0 || y.value == 0) return dbNull;
      return x.value == y.value? dbTrue: dbFalse;
   }

   // Inequality operator. Returns dbNull if either operand is
   // dbNull, otherwise returns dbTrue or dbFalse:
   public static DBBool operator !=(DBBool x, DBBool y) 
   {
      if (x.value == 0 || y.value == 0) return dbNull;
      return x.value != y.value? dbTrue: dbFalse;
   }

   // Logical negation operator. Returns dbTrue if the operand is 
   // dbFalse, dbNull if the operand is dbNull, or dbFalse if the
   // operand is dbTrue:
   public static DBBool operator !(DBBool x) 
   {
      return new DBBool(-x.value);
   }

   // Logical AND operator. Returns dbFalse if either operand is 
   // dbFalse, dbNull if either operand is dbNull, otherwise dbTrue:
   public static DBBool operator &(DBBool x, DBBool y) 
   {
      return new DBBool(x.value < y.value? x.value: y.value);
   }

   // Logical OR operator. Returns dbTrue if either operand is 
   // dbTrue, dbNull if either operand is dbNull, otherwise dbFalse:
   public static DBBool operator |(DBBool x, DBBool y) 
   {
      return new DBBool(x.value > y.value? x.value: y.value);
   }

   // Definitely true operator. Returns true if the operand is 
   // dbTrue, false otherwise:
   public static bool operator true(DBBool x) 
   {
      return x.value > 0;
   }

   // Definitely false operator. Returns true if the operand is 
   // dbFalse, false otherwise:
   public static bool operator false(DBBool x) 
   {
      return x.value < 0;
   }

   // Overload the conversion from DBBool to string:
   public static implicit operator string(DBBool x) 
   {
      return x.value > 0 ? "dbTrue"
           : x.value < 0 ? "dbFalse"
           : "dbNull";
   }

   // Override the Object.Equals(object o) method:
   public override bool Equals(object o) 
   {
      try 
      {
         return (bool) (this == (DBBool) o);
      }
      catch 
      {
         return false;
      }
   }

   // Override the Object.GetHashCode() method:
   public override int GetHashCode() 
   {
      return value;
   }

   // Override the ToString method to convert DBBool to a string:
   public override string ToString() 
   {
      switch (value) 
      {
         case -1:
            return "DBBool.False";
         case 0:
            return "DBBool.Null";
         case 1:
            return "DBBool.True";
         default:
            throw new InvalidOperationException();
      }
   }
}

class Test 
{
   static void Main() 
   {
      DBBool a, b;
      a = DBBool.dbTrue;
      b = DBBool.dbNull;

      Console.WriteLine( "!{0} = {1}", a, !a);
      Console.WriteLine( "!{0} = {1}", b, !b);
      Console.WriteLine( "{0} & {1} = {2}", a, b, a & b);
      Console.WriteLine( "{0} | {1} = {2}", a, b, a | b);
      // Invoke the true operator to determine the Boolean 
      // value of the DBBool variable:
      if (b)
         Console.WriteLine("b is definitely true");
      else
         Console.WriteLine("b is not definitely true");   
   }
}

Output

!DBBool.True = DBBool.False
!DBBool.Null = DBBool.Null
DBBool.True & DBBool.Null = DBBool.Null
DBBool.True | DBBool.Null = DBBool.True
b is not definitely true

See Also

C# Tutorials

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