如何从double转换为Int64

Question

I am trying to convert a double to an Int64 and get some strange behaviors. To test my problem I tried the following code:

double temporary = Int64.MaxValue;
Int64 tem = Convert.ToInt64(temporary);

I still get an Overflow exception when converting the double to an Int64. Is this a bug? Or is there something I am not understanding?

Answer 1

The crux of this is that not all real numbers can be represented by finite floating point data types. In particular, there are 64 bit integer values which do not have exact representation as double precision floating point values.

The value of Int64.MaxValue is 9,223,372,036,854,775,807 . The closest double precision value to this is 9,223,372,036,854,775,808 . So when

double temporary = Int64.MaxValue;

is executed, the value assigned to temporary is actually 9,223,372,036,854,775,808.0 .

When you attempt to convert this double precision value to Int64 , an overflow occurs because the value exceeds Int64.MaxValue .

Answer 2

A double cannot exactly represent Int64.MaxValue , so it gets rounded up. Since a long can't represent the rounded-up value, you get the exception.