An operator is overloaded by declaring an operator function. An operator function must be static, and at least one of the operands must be the type in which the operator function is declared.

In the following example, we define a struct called Note representing a musical note, and then overload the + operator:

public struct Note
{
  int value;

  public Note (int semitonesFromA)
   { value = semitonesFromA; }

  public static Note operator + (Note x, int semitones)
  {
    return new Note (x.value + semitones);
  }
}

This overload allows us to add an int to a Note:

Note B = new Note (2);
Note CSharp = B + 2;

Since we overrode +, we can use += too:

CSharp += 2;

Equality and comparison operators are often overridden when writing structs, and in rare cases with classes. Special rules and obligations come with overloading these operators:

Extending the previous example, here’s how we could overload Note’s equality operators:

public static bool operator == (Note n1, Note n2)
{
  return n1.value == n2.value;
}
public static bool operator != (Note n1, Note n2)
{
  return !(n1.value == n2.value);
}
public override bool Equals (object otherNote)
{
  if (!(otherNote is Note)) return false;
  return this == (Note)otherNote;
}
public override int GetHashCode()
{
  return value.GetHashCode();   // Use value's hashcode
}

Implicit and explicit conversions are overloadable operators. These conversions are typically overloaded to make converting between strongly related types (such as numeric types) concise and natural.

As explained in the discussion on types, the rationale behind implicit conversions is that they should always succeed and not lose information during conversion. Otherwise, explicit conversions should be defined.

In the following example, we define conversions between our musical Note type and a double (which represents the frequency in hertz of that note):

...
// Convert to hertz
public static implicit operator double (Note x)
{
  return 440 * Math.Pow (2,(double) x.value / 12 );
}

// Convert from hertz (accurate to nearest semitone)
public static explicit operator Note (double x)
{
  return new Note ((int) (0.5 + 12 * (Math.Log(x/440)
                  / Math.Log(2)) ));
}
...

Note n =(Note)554.37;  // explicit conversion
double x = n;          // implicit conversion

Custom conversions are ignored by the as and is operators.