Casting float to decimal loses precision in C#

Question

In C# 4.0, the following cast behaves very unexpectedly:

(decimal)1056964.63f
1056965

Casting to double works just fine:

(double)1056964.63f
1056964.625

(decimal)(double)1056964.63f
1056964.625

Is this by design?

Answer 1

The problem is with your initial value - float is only accurate to 7 significant decimal digits anyway:

float f = 1056964.63f;
Console.WriteLine(f); // Prints 1056965

So really the second example is the unexpected one in some ways.

Now the exact value in f is 1056965.625, but that's the value given for all values from about 1056964.563 to 1056964.687 - so even the ".6" part isn't always correct. That's why the docs for System.Single state:

By default, a Single value contains only 7 decimal digits of precision, although a maximum of 9 digits is maintained internally.

The extra information is still preserved when you convert to double, because that's can preserve it without "interpreting" it at all - where converting it to a decimal form (either to print or for the decimal type) goes through code which knows it can't "trust" those last two digits.