Converting object to long

Question

I have the following function:

    public virtual long AsLong(object originalValue,long defaultValue)
    {
        double buffer = defaultValue;
        if (originalValue != null)
        {
            double result;
            var readValueIsconverted = double.TryParse(originalValue.ToString(), out result);
            if (readValueIsconverted)
                buffer = result;
        }

        var roundedValue = Math.Round(buffer, 0);
        var convertedValue = (long) roundedValue;
        return convertedValue;
    }

I have used double in order to allow conversion of 14.4! I have the following failing test:

    [Fact]
    public void CanConvertLongMaxValue()
    {
        var cellValue = new Converter();
        const long longValue = 0x7FFFFFFFFFFFFFFF;
        var result = cellValue.AsLong(longValue, 12);

        Assert.Equal(longValue, result);

    } 

I have traced the code and roundedValue is positive but the convertedValue is negative. so what is the problem?

Answer 1

The problem is that you're trying to hold an integer value with 19 significant digits (in decimal) in a double which has 15-16 significant digits.

Hence, it's not possible to represent the value exactly in the double. Apparently rounding causes the value to overflow when converted into a long, making it a negative value.

You can confirm this like so:

var convertedValue = checked((long)roundedValue);

If you absolutely must deal with this case, I would suggest using decimal instead of double, or splitting the string on the decimal point (or whatever is used in your locale) and dealing with rounding that way.