c#-how to convert arduino data to int array

array's indexes in c# start from 0.

this code line is your problem and the reason to the failure:

data[3] = Int32.Parse(serialPort.ReadLine());

change your code to:

int[] data = new int[3];
data[2] = Int32.Parse(serialPort.ReadLine());
data[1] = Int32.Parse(serialPort.ReadLine());
data[0] = Int32.Parse(serialPort.ReadLine());

Edit: (answer to the comment...)

This class provide you 2 extensions method one for unknown sequence length(but you have to create a stop condition) use the second method if you know the exact number of ints.

Note: 4 years ago i used SerialPort with a graphical tablet.
I used the event: DataReceived as a reading trigger. for more information see this example.

//to use extension methods do: serialPort.ReadKnownLength(3);

public static class SerialPortExtensions
{
    public static int[] ReadUnknownLength(this SerialPort serialPort)
    {
        var list = new List<int>();

        while (true)
        {
            try
            {
                var input = serialPort.ReadLine();

                //change the condition to your own break condition
                if (String.IsNullOrEmpty(input))
                    break;

                list.Add(int.Parse(input));
            }
            catch (TimeoutException)
            {
            }
        }

        // if you don't need to reorder
        //return list.ToArray();

        var result = new int[list.Count];

        for (var i = 0; i < result.Length; i++)
        {
            //reorder the input(you did it in your question...)
            result[i] = list[list.Count - (i + 1)];
        }

        return result;
    }

    //match your question behavior
    public static int[] ReadKnownLength(this SerialPort serialPort, int length)
    {
        var result = new int[length];

        for (int i = length - 1; i >= 0; i--)
        {
            result[i] = Int32.Parse(serialPort.ReadLine());
        }

        return result;
    }

}