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Why is floating point arithmetic in C# imprecise?

I have been dealing with some numbers and C#, and the following line of code results in a different number than one would expect:

double num = (3600.2 - 3600.0);

I expected num to be 0.2, however, it turned out to be 0.1999999999998181. Is there any reason why it is producing a close, but still different decimal?

Answer 1

This is because double is a floating point datatype.

If you want greater accuracy you could switch to using decimal instead.

The literal suffix for decimal is m, so to use decimal arithmetic (and produce a decimal result) you could write your code as

var num = (3600.2m - 3600.0m);

Note that there are disadvantages to using a decimal. It is a 128 bit datatype as opposed to 64 bit which is the size of a double. This makes it more expensive both in terms of memory and processing. It also has a much smaller range than double.