How to determine if a decimal/double is an integer?

How do I tell if a decimal or double value is an integer?

For example:

decimal d = 5.0; // Would be true
decimal f = 5.5; // Would be false

double d = 5.0; // Would be true
double f = 5.5; // Would be false

The reason I would like to know this is so that I can determine programmatically if I want to output the value using .ToString("N0") or .ToString("N2"). If there is no decimal point value, then I don't want to show that.

For floating point numbers, n % 1 == 0 is typically the way to check if there is anything past the decimal point.

public static void Main (string[] args)
{
    decimal d = 3.1M;
    Console.WriteLine((d % 1) == 0);
    d = 3.0M;
    Console.WriteLine((d % 1) == 0);
}

Output:

False
True

Update: As @Adrian Lopez mentioned below, comparison with a small value epsilon will discard floating-point computation mis-calculations. Since the question is about double values, below will be a more floating-point calculation proof answer:

Math.Abs(d % 1) <= (Double.Epsilon * 100)

There are any number of ways to do this. For example:

double d = 5.0;
bool isInt = d == (int)d;

You can also use modulo.

double d = 5.0;
bool isInt = d % 1 == 0;

How about this?

public static bool IsInteger(double number) {
    return number == Math.Truncate(number);
}

Same code for decimal.

Mark Byers made a good point, actually: this may not be what you really want. If what you really care about is whether a number rounded to the nearest two decimal places is an integer, you could do this instead:

public static bool IsNearlyInteger(double number) {
    return Math.Round(number, 2) == Math.Round(number);
}

Whilst the solutions proposed appear to work for simple examples, doing this in general is a bad idea. A number might not be exactly an integer but when you try to format it, it's close enough to an integer that you get 1.000000. This can happen if you do a calculation that in theory should give exactly 1, but in practice gives a number very close to but not exactly equal to one due to rounding errors.

Instead, format it first and if your string ends in a period followed by zeros then strip them. There are also some formats that you can use that strip trailing zeros automatically. This might be good enough for your purpose.

double d = 1.0002;
Console.WriteLine(d.ToString("0.##"));
d = 1.02;
Console.WriteLine(d.ToString("0.##"));

Output:

1
1.02

bool IsInteger(double num) {
    if (ceil(num) == num && floor(num) == num)
        return true;
    else
        return false;
}

Problemo solvo.

Edit: Pwned by Mark Rushakoff.

How to determine if a decimal/double is an integer?

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