What's the best way to compare Double and Int?

You really can't compare floating point and integral values in a naive way; particularly, since there's the classic floating point representation challenges. What you can do is subtract one from the other and see if the difference between them is less than some precision you care about, like so:

int iValue = 0;
double dValue = 0.0;

var diff = Math.Abs(dvalue - iValue);
if( diff < 0.0000001 ) // need some min threshold to compare floating points
   return true; // items equal

You really have to define for yourself what equality means to you. For example, you may want a floating point value to round towards the nearest integer, so that 3.999999981 will be "equal" to 4. Or you may want to truncate the value, so it would effectively be 3. It all depends on what you're trying to achieve.

EDIT: Note that i chose 0.0000001 as an example threshold value ... you need to decide for yourself what precision is sufficient for comparison. Just realize you need to be within the normal representational bounds of double which I believe is defined as Double.Epsilon.

It's an exceedingly bad idea to compare integers and floating-point numbers for equality in any language. It works for very simple cases, but after you do any math at all, the likliehood of the program doing what you want it to decreases dramatically.

It has to do with the way floating-point numbers are stored on a binary, digital system.

If you are very sure you want to use this, create a class to make you own number with fractions. use one int to maintain the whole number, and another int to maintain the fraction.