How to Multiply Long by Double

Question

Sorry for asking such a beginner question but I can't figure this out.

I have a long integer that I'd like to divide by 1.28 then round it up or down to the nearest integer.

long size = 24524343254;
double ratio = 1.28;
size = size * 1.28; //Error Cannot implicitly convert type 'double' to 'long'

Answer 1

You need to explicitly cast the double result back to long (as the compiler states - there is not implicit conversion):

size = (long)Math.Round(size * 1.28);

You need Math.Round if you want to round to nearest (there is a special case for when the number to round is halfway between two numbers, then it's rounded towards the nearest even by default). You can also simply cast the result back (long)(size * 1.28) if you only want to round towards 0.

As pointed out by @CodeInChaos the implicit cast from long to double (size * 1.28 will cast size into a double first) can result in the loss of precision as doubles only have a precision of 53 bits but long is 64bit.