Question

I have a custom type (Money) that has an implict conversion to decimal and an overloaded operator for +. When I have a list of these types and call the linq Sum method the result is decimal, not Money. How can I give the +operator presidence and return Money from the Sum?

internal class Test
{
    void Example()
    {
        var list = new[] { new Money(10, "GBP"), new Money(20, "GBP") };
        //this line fails to compile as there is not implicit 
        //conversion from decimal to money
        Money result = list.Sum(x => x);
    }
}


public class Money
{
    private Currency _currency;
    private string _iso3LetterCode;

    public decimal? Amount { get; set; }
    public Currency Currency
    {
        get {  return _currency; }
        set
        {
            _iso3LetterCode = value.Iso3LetterCode; 
            _currency = value; 
        }
    }

    public Money(decimal? amount, string iso3LetterCurrencyCode)
    {
        Amount = amount;
        Currency = Currency.FromIso3LetterCode(iso3LetterCurrencyCode);
    }

    public static Money operator +(Money c1, Money c2)
    {
        if (c1.Currency != c2.Currency)
            throw new ArgumentException(string.Format("Cannot add mixed currencies {0} differs from {1}",
                                                      c1.Currency, c2.Currency));
        var value = c1.Amount + c2.Amount;
        return new Money(value, c1.Currency);
    }

    public static implicit operator decimal?(Money money)
    {
        return money.Amount;
    }

    public static implicit operator decimal(Money money)
    {
        return money.Amount ?? 0;
    }
}

Answer 1

Sum only knows about the number types in System.

You can use Aggregate like this:

Money result = list.Aggregate((x,y) => x + y);

Because this is calling Aggregate<Money>, it will use your Money.operator+ and return a Money object.