Somewhere in your code, you are using floats, not doubles.

Try the following in a console app (C#):

static void Main(string[] args)
{
   double a = 0.1;
   double b = 0.2;
   double c = a + b;
   double d = 0.1 + 0.2;
   float e = 0.1f;
   float f = 0.2f;
   double g = e + f;
   
   
   Console.WriteLine("a={0}", a);
   Console.WriteLine("b={0}", b);
   Console.WriteLine("c={0}", c);
   Console.WriteLine("d={0}", d);
   Console.WriteLine("e={0}", e);
   Console.WriteLine("f={0}", f);
   Console.WriteLine("g={0}", g);
   Console.Read();
}

Your expectations are wrong.

The numbers 0.1 and 0.2 do not have exact representations in binary floating
point - which is what .NET uses for floats and doubles.  Consequently, when
you add a number very close to 0.1 to a number very close to 0.2, the best
you can hope for is a number very close to 0.3.  Four double, which you're
apparently using here, you can expect about 17 digits of accuracy.

When comparing floating point numbers for "equality", you should always
compare for equality wiithin some small range, e.g. instead of x == y, use
Math.Abs(x-y) < 0.0000000001.

When displaying floating point numbers, you should always specify how many
digits after the decimal point, according to your needs.

If you need to be able to add 0.1 to 0.2 and get exactly 0.3 (e.g. you're
doing financial calculations), then you should use the System.Decimal type
instead.  It is a decimal floating point number and can represent negative
powers of 10 (e.g. 0.1) exactly.  Be aware though - decimal is orders of
magnitude slower than double, so unless you really need absolute decimal
accuracy, you're probably better off using double.

You should also take a look at the paper at the following url.  It goes into
the details of working with binary floating point numbers in great detail.

http://docs.sun.com/source/806-3568/ncg_goldberg.html

-cd

That is how "double" type works. See .NET framework document. It is the same
to other language where "duoble" or similar floating type is implemented. I
have been seeing the same or similar questions appearing in NGs for
classical VB every other week or so in many years.

If the rounding precision is an issue to your app, you might want to
consider to use "decimal" type instead of "double" or "single".