Question

I have a situation that I cannot change: one database table (table A) accepts 6 decimal places, while a related column in a different table (table B) only has 3 decimal places.

I need to copy from A to B, but if A has more than 3 decimal places the extra data will be lost. I cant change the table definition but I can add a workaround. So I’m trying to find out how to check if a decimal has more than 3 decimal places or not?

eg

Table A
Id, Qty,  Unit(=6dp)
1,  1,     0.00025
2,  4000,  0.00025

Table B
Id, TotalQty(=3dp)

I want to be able to find out if Qty * Unit from Table A has more than 3 decimals (row 1 would fail, row 2 would pass):

if (CountDecimalPlaces(tableA.Qty * tableA.Unit) > 3)
{
    return false;
}
tableB.TotalQty = tableA.Qty * tableA.Unit;

How would I implement the CountDecimalPlaces(decimal value) {} function?

Answer 1

This works for 3 decimal places, and it can be adapted for a generic solution:

static bool LessThan3DecimalPlaces(decimal dec)
{
    decimal value = dec * 1000;
    return value == Math.Floor(value);
}
static void Test()
{
    Console.WriteLine(LessThan3DecimalPlaces(1m * 0.00025m));
    Console.WriteLine(LessThan3DecimalPlaces(4000m * 0.00025m));
}

For a real generic solution, you’ll need to “deconstruct” the decimal value in its parts – take a look at Decimal.GetBits for more information.

Update: this is a simple implementation of a generic solution which works for all decimals whose integer part is less than long.MaxValue (you’d need something like a “big integer” for a trully generic function).

static decimal CountDecimalPlaces(decimal dec)
{
    Console.Write("{0}: ", dec);
    int[] bits = Decimal.GetBits(dec);
    ulong lowInt = (uint)bits[0];
    ulong midInt = (uint)bits[1];
    int exponent = (bits[3] & 0x00FF0000) >> 16;
    int result = exponent;
    ulong lowDecimal = lowInt | (midInt << 32);
    while (result > 0 && (lowDecimal % 10) == 0)
    {
        result--;
        lowDecimal /= 10;
    }

    return result;
}

static void Foo()
{
    Console.WriteLine(CountDecimalPlaces(1.6m));
    Console.WriteLine(CountDecimalPlaces(1.600m));
    Console.WriteLine(CountDecimalPlaces(decimal.MaxValue));
    Console.WriteLine(CountDecimalPlaces(1m * 0.00025m));
    Console.WriteLine(CountDecimalPlaces(4000m * 0.00025m));
}