In C#, user-defined types can define custom behaviors for certain operators with static member functions using the operator keyword. However, not all operators can be overloaded, and some of them have certain restrictions (Docs, 2015)

Operators	Description
+, -, !, ~, ++, –	Can be overloaded
+, -, *, /, %	Can be overloaded
==, !=, , <=, >=	Can be overloaded
&&, ||	Cannot be overloaded directly
+=, -=, *=, /=, %=	Cannot be overloaded directly
=, ., ?:, ->, new, is, sizeof, typeof	Cannot be overloaded

Example: Fraction

public class Fraction
{
    private int _numerator;
    private int _denominator;

    public Fraction(int numerator, int denominator)
    {
        _numerator = numerator;
        _denominator = denominator;
    }

    public double Value => ((double)_numerator) / _denominator;

    public static Fraction operator +(Fraction first, Fraction second)
    {
        int numerator = first._numerator * second._denominator + second._numerator * first._denominator;
        int denominator = first._denominator * second._denominator;

        return new Fraction(numerator, denominator);
    }
}

To see the class in action,

Fraction f1 = new Fraction(2, 3);
Fraction f2 = new Fraction(3, 5);

Fraction result = f1 + f2;  // adding 2 fractions together
Console.WriteLine(result.Value);

Output:

1.26666666666667

which is equivalent to 2 / 3 + 3 / 5.

The method

public static Fraction operator +(Fraction first, Fraction second)

overload the operator + to provide custom logic for adding two fractions.

Live-code example

References

1. Docs, M. (2015, July 20). Overloadable Operators (C# Programming Guide). Retrieved from https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/statements-expressions-operators/overloadable-operators