Question 0.3 / 3.0 why not equal to 0.1

[Tatb]

This code for test on both vs2017 (Console .Net Framework) and vs2019 (Console .Net Core) .

 static void Main(string[] args)
        {
            double a = 0.3d;
            double b = 3d;
            double c = a / basdf;

            Console.WriteLine($"a={a}, b={b}");
            Console.WriteLine(c);
            Console.ReadKey();
        }

Output :

a=0.3, b=3
c=0.1

a=0.3, b=3
c=0.09999999999999999

Why it is?
Should I keep using .Net Core? or I just avoid double type. or, Can I config IDE to fix them?

 

[Skydiver]

Welcome to the world of floating point math. Below is a must read.

What Every Computer Scientist Should Know About Floating-Point Arithmetic

 

[JohnH]

Floating-point numeric types - C# reference - C#

Learn about the built-in C# floating-point types: float, double, and decimal

docs.microsoft.com

there's no double or float instance that exactly represents 0.1

 

[ausm]

Should I keep using .Net Core? or I just avoid double type. or, Can I config IDE to fix them?

You can keep using .Net Core, it is the type you are using.
float and double will give you the best precition for a floating point number using the space that they are using in memory. You can use them e.g. in games for values where thier decimal representation doesn't matter that much.
If it does matter, e.g. in accounting applications, you better use decimal. Every number that can be assigned to decimal does have a decimal representation.
Check this out to see the effect in live.