Operator overloading - Define unary, arithmetic, equality, and comparison operators. - C# reference (original) (raw)

A user-defined type can overload a predefined C# operator. That is, a type can provide the custom implementation of an operation in case one or both of the operands are of that type. The Overloadable operators section shows which C# operators can be overloaded.

Use the operator keyword to declare an operator. An operator declaration must satisfy the following rules:

• It includes both a public and a static modifier.
• A unary operator has one input parameter. A binary operator has two input parameters. In each case, at least one parameter must have type T or T? where T is the type that contains the operator declaration.

The following example defines a simplified structure to represent a rational number. The structure overloads some of the arithmetic operators:

public readonly struct Fraction
{
    private readonly int num;
    private readonly int den;

    public Fraction(int numerator, int denominator)
    {
        if (denominator == 0)
        {
            throw new ArgumentException("Denominator cannot be zero.", nameof(denominator));
        }
        num = numerator;
        den = denominator;
    }

    public static Fraction operator +(Fraction a) => a;
    public static Fraction operator -(Fraction a) => new Fraction(-a.num, a.den);

    public static Fraction operator +(Fraction a, Fraction b)
        => new Fraction(a.num * b.den + b.num * a.den, a.den * b.den);

    public static Fraction operator -(Fraction a, Fraction b)
        => a + (-b);

    public static Fraction operator *(Fraction a, Fraction b)
        => new Fraction(a.num * b.num, a.den * b.den);

    public static Fraction operator /(Fraction a, Fraction b)
    {
        if (b.num == 0)
        {
            throw new DivideByZeroException();
        }
        return new Fraction(a.num * b.den, a.den * b.num);
    }

    public override string ToString() => $"{num} / {den}";
}

public static class OperatorOverloading
{
    public static void Main()
    {
        var a = new Fraction(5, 4);
        var b = new Fraction(1, 2);
        Console.WriteLine(-a);   // output: -5 / 4
        Console.WriteLine(a + b);  // output: 14 / 8
        Console.WriteLine(a - b);  // output: 6 / 8
        Console.WriteLine(a * b);  // output: 5 / 8
        Console.WriteLine(a / b);  // output: 10 / 4
    }
}

You could extend the preceding example by defining an implicit conversion from int to Fraction. Then, overloaded operators would support arguments of those two types. That is, it would become possible to add an integer to a fraction and obtain a fraction as a result.

You also use the operator keyword to define a custom type conversion. For more information, see User-defined conversion operators.

Overloadable operators

The following table shows the operators that can be overloaded:

Non overloadable operators

The following table shows the operators that can't be overloaded:

C# language specification

For more information, see the following sections of the C# language specification:

• Operator overloading
• Operators

See also

• C# operators and expressions
• User-defined conversion operators
• Design guidelines - Operator overloads
• Design guidelines - Equality operators
• Why are overloaded operators always static in C#?