This article was originally published on Hashnode and here you have a Spanish version that explains why 0.1 + 0.2 != 0.3 in C# and in most other languages
Many developers, even experienced ones, are surprised to see that in C# the expression Console.WriteLine(0.1 + 0.2 == 0.3) prints False
Why does this happen? The reason lies in how floating-point numbers are represented in binary. C# and most modern languages follow the IEEE 754 standard for float and double. Decimal numbers like 0.1 and 0.2 cannot be represented exactly in binary, so the sum is not exactly 0.3 but a close value, for example 0.30000000000000004
That small representation error makes 0.30000000000000004 != 0.3 and that is why the comparison with == returns False. This behavior is not exclusive to C#. In Python print(0.1 + 0.2 == 0.3) gives False. In JavaScript console.log(0.1 + 0.2 === 0.3) also gives False. In Java the comparison 0.1 + 0.2 == 0.3 returns False. They all use binary floating-point arithmetic and share the same limitation
What to do instead of using == directly? A good practice is to compare with an epsilon tolerance. For example, a function AreEqual(a, b, epsilon = 1e-10) can return abs(a - b) < epsilon. This technique is the recommended one for comparing floating-point values in most languages
In C#, when decimal precision is needed, such as in financial calculations, the decimal type should be used, which uses base 10 and avoids the rounding problems intrinsic to binary representation. For example decimal a = 0.1m; decimal b = 0.2m; then a + b == 0.3m will be True
Summary of good practices: do not compare floating-point numbers with ==; use an appropriate epsilon for comparisons; use decimal for monetary applications or when exact precision is required; remember that this behavior comes from the IEEE 754 standard and appears in many languages
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If you work with numbers and models that require precision, keep in mind the limitations of floating-point arithmetic and apply the appropriate techniques described here. At Q2BSTUDIO we can advise you on choosing numeric types, artificial intelligence implementations, AI agents, and Power BI integrations so that your custom applications are accurate and reliable
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