Why does this simple double assertion fail in C#

Question

The following test will fail in C#

Assert.AreEqual<double>(10.0d, 16.1d - 6.1d);

The problem appears to be a floating point error.

16.1d - 6.1d == 10.000000000000002

This is causing me headaches in writing unit tests for code that uses double. Is there a way to fix this?

Answer 1

There is no exact conversion between the decimal system and the binary representation of a double (see excellent comment by @PatriciaShanahan below on why).

In this case the .1 part of the numbers is the problem, it cannot be finitely represented in a double (like 1/3 can't be finitely represented exactly as a decimal number).

A code snippet to explain what happends:

double larger = 16.1d; //Assign closest double representation of 16.1.
double smaller = 6.1; //Assign closest double representation of 6.1.
double diff = larger - smaller; //Assign closest diff between larger and  
                                //smaller, but since a smaller value has a  
                                //larger precision the result will have better  
                                //precision than larger but worse than smaller. 
                                //The difference shows up as the ...000002.

Always use the Assert.Equal overload which takes a delta parameter when comparing doubles.

Alternatively if you really need exact decimal conversion, use the decimal data type, that has another binary representation and would return exactly 10 in your example.

Answer 2

Floatingoint numbers are an estimate of the actual value based on an exponent so the test fails correctly. If you require exact equivalence in two decimal numbers you may need to check out the decimal data type.