c# creating a custom "double" type

Question

In my application I want all my properties storing money amounts to be rounded to n decimal places.

For code clarity, I'd rather have a custom type MoneyAmount which all my corresponding fields would have, instead of having to put a `Math.Round(value, n)' in all the property getters/setters.

Is there a neat way to achieve this?

I saw this post about overloading assignment operators - is this the suggested approach?

EDIT: Given the multiple views, I post the full code I derived here:

public struct MoneyAmount {
const int N = 4;
private readonly double _value;

public MoneyAmount(double value) {
  _value = Math.Round(value, N);
}

#region mathematical operators
public static MoneyAmount operator +(MoneyAmount d1, MoneyAmount d2) {
  return new MoneyAmount(d1._value + d2._value);
}

public static MoneyAmount operator -(MoneyAmount d1, MoneyAmount d2) {
  return new MoneyAmount(d1._value - d2._value);
}

public static MoneyAmount operator *(MoneyAmount d1, MoneyAmount d2) {
  return new MoneyAmount(d1._value * d2._value);
}

public static MoneyAmount operator /(MoneyAmount d1, MoneyAmount d2) {
  return new MoneyAmount(d1._value / d2._value);
}
#endregion

#region logical operators
public static bool operator ==(MoneyAmount d1, MoneyAmount d2) {
  return d1._value == d2._value;
}
public static bool operator !=(MoneyAmount d1, MoneyAmount d2) {
  return d1._value != d2._value;
}
public static bool operator >(MoneyAmount d1, MoneyAmount d2) {
  return d1._value > d2._value;
}
public static bool operator >=(MoneyAmount d1, MoneyAmount d2) {
  return d1._value >= d2._value;
}
public static bool operator <(MoneyAmount d1, MoneyAmount d2) {
  return d1._value < d2._value;
}
public static bool operator <=(MoneyAmount d1, MoneyAmount d2) {
  return d1._value <= d2._value;
}
#endregion

#region Implicit conversions
/// <summary>
/// Implicit conversion from int to MoneyAmount. 
/// Implicit: No cast operator is required.
/// </summary>
public static implicit operator MoneyAmount(int value) {
  return new MoneyAmount(value);
}

/// <summary>
/// Implicit conversion from float to MoneyAmount. 
/// Implicit: No cast operator is required.
/// </summary>
public static implicit operator MoneyAmount(float value) {
  return new MoneyAmount(value);
}

/// <summary>
/// Implicit conversion from double to MoneyAmount. 
/// Implicit: No cast operator is required.
/// </summary>
public static implicit operator MoneyAmount(double value) {
  return new MoneyAmount(value);
}

/// <summary>
/// Implicit conversion from decimal to MoneyAmount. 
/// Implicit: No cast operator is required.
/// </summary>
public static implicit operator MoneyAmount(decimal value) {
  return new MoneyAmount(Convert.ToDouble(value));
}
#endregion

#region Explicit conversions
/// <summary>
/// Explicit conversion from MoneyAmount to int. 
/// Explicit: A cast operator is required.
/// </summary>
public static explicit operator int(MoneyAmount value) {
  return (int)value._value;
}

/// <summary>
/// Explicit conversion from MoneyAmount to float. 
/// Explicit: A cast operator is required.
/// </summary>
public static explicit operator float(MoneyAmount value) {
  return (float)value._value;
}

/// <summary>
/// Explicit conversion from MoneyAmount to double. 
/// Explicit: A cast operator is required.
/// </summary>
public static explicit operator double(MoneyAmount value) {
  return (double)value._value;
}

/// <summary>
/// Explicit conversion from MoneyAmount to decimal. 
/// Explicit: A cast operator is required.
/// </summary>
public static explicit operator decimal(MoneyAmount value) {
  return Convert.ToDecimal(value._value);
}
#endregion
}

Answer 1

I'd suggest the following:

1. Create a new struct, called MoneyAmount.
2. It contains one field: A double.
3. The constructor with one double parameter, this constructor rounds the value and assigns it to the internal field.
4. Add the members/operators you might need to your struct so it has all the same operations as the double, like +, -, etc. But also casts/conversions from/to other types. Every operation produces a new instance of MoneyAmount with a rounded value.
5. Also consider implementing the interfaces IFormattable, IComparable and IConvertible.

Short example:

public struct MoneyAmount
{
    const int N = 4;
    private readonly double _value;

    public MoneyAmount(double value)
    {
        _value = Math.Round(value, N);
    }

    // Example of one member of double:
    public static MoneyAmount operator *(MoneyAmount d1, MoneyAmount d2) 
    {
        return new MoneyAmount(d1._value * d2._value);
    }

    /// <summary>
    /// Implicit conversion from double to MoneyAmount. 
    /// Implicit: No cast operator is required.
    /// </summary>
    public static implicit operator MoneyAmount(double value)
    {
        return new MoneyAmount(value);
    }

    /// <summary>
    /// Explicit conversion from MoneyAmount to double. 
    /// Explicit: A cast operator is required.
    /// </summary>
    public static explicit operator double(MoneyAmount value)
    {
        return value._value;
    }

    /// <summary>
    /// Explicit conversion from MoneyAmount to int. 
    /// Explicit: A cast operator is required.
    /// </summary>
    public static explicit operator MoneyAmount(int value)
    {
        return new MoneyAmount(value);
    }

    /// <summary>
    /// Explicit conversion from MoneyAmount to int. 
    /// Explicit: A cast operator is required.
    /// </summary>
    public static explicit operator int(MoneyAmount value)
    {
        return (int)value._value;
    }

    // All other members here...
}

I realize: The double has a lot of members...

With these operators, the following code is possible:

MoneyAmount m = 1.50; // Assignment from a double.
MoneyAmount n = 10; // Assignment from an integer.
m += n; // Mathematical operation with another MoneyAmount .
m *= 10; // Mathematical operation with an integer.
m -= 12.50; // Mathematical operation with a double.

EDIT

All conversion methods you may want to implement:

• Explicit MoneyAmount --> int
• Explicit MoneyAmount --> float
• Explicit MoneyAmount --> double

• Explicit MoneyAmount --> decimal

• Implicit int--> MoneyAmount

• Implicit float --> MoneyAmount
• Implicit double--> MoneyAmount
• Implicit decimal --> MoneyAmount

All mathematical operations you may want to implement:

• MoneyAmount + MoneyAmount
• MoneyAmount - MoneyAmount
• MoneyAmount * MoneyAmount
• MoneyAmount / MoneyAmount

All relational operations you may want to implement:

• MoneyAmount == MoneyAmount
• MoneyAmount != MoneyAmount
• MoneyAmount > MoneyAmount
• MoneyAmount >= MoneyAmount
• MoneyAmount < MoneyAmount
• MoneyAmount <= MoneyAmount

With all these operations your have all basics covered.

Answer 2

This gets big very quickly. Writing a struct is easy, as demonstrated in @MartinMulder's answer, but consider that you will want to overload a number of combinations of operators, as well as including a few implicit/explicit casts as well.

Mathematical & Logical Operation

Consider that you may want to do mathematical operations on MoneyAmount

• MoneyAmount + MoneyAmount
• MoneyAmount + double
• MoneyAmount + int
• MoneyAmount + decimal

That is 4 overloads of the + operator. Rinse and repeat for -,/,* (and possibly %). You'll also want to overload <,<=, == and >, >=. Thats something like 30 operator overloads. Phew! Thats a lot of static methods.

public static MoneyAmount operator +(MoneyAmount d1, double d2) 
{
    return new MoneyAmount((decimal)(d1._value + d2));
}

Explicit/Implicit casts

Now consider that instead of this code

MoneyAmount m = new MoneyAmount(1.234);

You wanted to do this:

MoneyAmount m = 1.234;

That can be achieved with an implicit cast operator.

public static implicit operator MoneyAmount(double d)
{
    return new MoneyAmount((decimal)d);
}

(You'll need one for every type you want to allow implicit casts)

Another one:

int i = 4;
MoneyAmount m = (MoneyAmount)i;

This is done with an explicit cast operator overload.

public static explicit operator MoneyAmount(double d)
{
    return new MoneyAmount((decimal)d);
}

(Again, 1 for every type you want to allow explicit casts)