Why does this cast change the result when even VS says it is redundant?

Question

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I have found a bug (in my code) and I am struggling to find correct understanding of it.

It all boils down to this specific example taken from the immediate window while debugging:

x
0.00023569075
dx
-0.000235702712
x+dx+1f < 1f
true

(float) (x+dx+1f) < 1f
false

x and dx are both of type float. So why is the boolean value different when I do a cast?

In the actual code i had:

x+=dx
if( x+1f < 1f) // Add a one to truncate really small negative values (originally testing x < 0)
{
    // do actions accordingly

    // Later doing
    x+=1; // Where x<1 has to be true, therefore we have to get rid of really small negatives where x+=1 will give x==1 as true and x<1 false.
}

but I am now trying with the cast

x+=dx;
if( (float)( x+1f) < 1f) // Add a one to truncate really small negative values (originally testing x < 0)
{
    // do actions accordingly
    // Later doing
    x+=1; // Where x<1 has to be true, therefore we have to get rid of really small negatives where x+=1 will give x==1 as true and x<1 false.
}

Visual studio says that the cast is redundant but it DO get a false positve without it as de immediate window also told me when:

x+dx+1f < 1f
true

I am currently running my code to see if I get the bug again with the cast and I will update as soon as I get convinced either way.

In the meanwhile I hope someone can sort out whats going on here? Can I expect the Cast to do something?

Update - Variables My variables x and dx are components of a Vector2 (Xna/monogame). So in the code you should read

Vector2 coord; // the x (and y) components are floats.
Vector2 ds;
coord.X // where it says x 
ds.X    // where it says dx

I thought this would not matter, but maybe it does.

Update 2 - Drop the above example

Seeing that the cast did change the outcome I made this simple demonstration

class Program
{
    static void Main(string[] args)
    {
        float a = -2.98023224E-08f; // Just a small negative number i picked...

        Console.WriteLine(((a + 1f) < 1f) ? "true" : "false");          //true

        Console.WriteLine(((float)(a + 1f) < 1f) ? "true":"false");     //false

        // Visual Studio Community 2015 marks the above cast as redundant
        // but its clearly something fishy going on!

        Console.Read();
    }
}

So, why does this cast change the result when even VS says it is redundant?

Why does this cast change the result when even VS says it is redundant?

3 Answers