math - Strange subtraction in c# -

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i doing c# program.. must simple mathematical operation.

72057594037927936.0 - 255.0 = .....

both numbers double... obtain 

72057594037927680.0

instead of 

72057594037927681.0

can explain me please how possible?

thanx

the exact result of subtraction,

72057594037927681 = 0xffffffffffff01

needs 56 bits of precision, double has 53, hence result rounded nearest representable number.


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