Cast

In C# floating point values can be cast to ints. This has predictable results. We find out the result of a conversion to an integer from a value that cannot fit in an integer.

We show conversions from double, long and ulong to int. Casting a floating-point number removes the fractional part—so it rounds down (when positive).

Some input and output

Consider a double value with a fractional part, like 1.1. When we cast it, we expect to receive an integer equal to 1. Even 1.99 should result in 1.

(int)1.1   =  1
(int)1.5   =  1
(int)1.99  =  1
(int)-1.1  = -1

Example program

We perform many casts, and each is described in a separate section of the text. Casting some values can result in unexpected behavior.

// Part 1: cast 4 double values.
{
    double value1 = 1.1;
    int value2 = (int)value1;
    System.Console.WriteLine("{0} -> {1}", value1, value2);
}
{
    double value1 = 1.5;
    int value2 = (int)value1;
    System.Console.WriteLine("{0} -> {1}", value1, value2);
}
{
    double value1 = 1.99;
    int value2 = (int)value1;
    System.Console.WriteLine("{0} -> {1}", value1, value2);
}
{
    double value1 = -1.1;
    int value2 = (int)value1;
    System.Console.WriteLine("{0} -> {1}", value1, value2);
}
// Part 2: cast long values.
{
    long value1 = 10000000000;
    int value2 = (int)value1;
    System.Console.WriteLine("{0} -> {1}", value1, value2);
}
{
    ulong value1 = 10000000000;
    int value2 = (int)value1;
    System.Console.WriteLine("{0} -> {1}", value1, value2);
}
// Part 3: cast large double values.
{
    double value1 = 10000000000;
    int value2 = (int)value1;
    System.Console.WriteLine("{0} -> {1}", value1, value2);
}
{
    double value1 = -10000000000;
    int value2 = (int)value1;
    System.Console.WriteLine("{0} -> {1}", value1, value2);
}
1.1 -> 1
1.5 -> 1
1.99 -> 1
-1.1 -> -1
10000000000 -> 1410065408
10000000000 -> 1410065408
10000000000 -> -2147483648
-10000000000 -> -2147483648

Rounding

For more sophisticated rounding, check out the Math.Round method in the System namespace. We can call Math.Round, and then once the value is rounded as desired, cast to int.

using System;

double value1 = 1.5;
// Use int cast.
int value2 = (int)value1;
// Use Math.Round, then cast to int.
int value3 = (int)Math.Round(value1);
Console.WriteLine($"{value1} -> {value2} ... {value3}");
1.5 -> 1 ... 2

Checked context

When we cast longs, ulongs or doubles that cannot fit in the memory space of an int, we receive predictable but unusable results.

using System;

// This cast will return an invalid result.
double value = double.MaxValue;
int result = (int)value;
Console.WriteLine(result);

checked
{
    double value2 = double.MaxValue;
    // This will throw an exception.
    int result2 = (int)value2;
}
-2147483648
Unhandled exception. System.OverflowException: Arithmetic operation resulted in an overflow.
   at ...\Program.cs:line 12

Summary

Casting to int is often done in C#. When we cast positive fractional values, they are rounded down to the nearest integer. This means even the value 1.99 is changed to 1.