C＃索引不在数组[combobox]的范围内

Question

Below is the code that is resulting in error when I start debug. I receive a message pop up when I chose on the option from combobox1 or combobox2: Index was outside the bounds of the array. How could I solve this?

Thanks for your time reading. :)

public Form1()
{
    InitializeComponent();
    String[] arr1 = new String[2];

    arr1[0] = "SG";
    arr1[1] = "MY";

    comboBox1.Items.AddRange(arr1);
    comboBox2.Items.AddRange(arr1);        
}

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    double[,] arr = new double[2, 2];

    for(int i = 0; i <2; i++)
    {            
        arr[0, 0] = 1;
        arr[0, 1] = 1.24;
        arr[1, 0] = 0.80;
        arr[1, 1] = 1;
        label1.Text = 
            arr[comboBox1.SelectedIndex,
                comboBox2.SelectedIndex].ToString();//Index was outside the bounds of the array.         
    }
}

private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
{           
    double[,] arr = new double[2, 2];

    for(int i = 0; i < 2; i++)
    {
        arr[0, 0] = 1;
        arr[0, 1] = 1.24;
        arr[1, 0] = 0.80;
        arr[1, 1] = 1;
        label1.Text = 
            arr[comboBox1.SelectedIndex,
                comboBox2.SelectedIndex].ToString();//Index was outside the bounds of the array.
    }
}

Answer 1

You should step through it in debug mode. But I imagine it's because one of your ComboBoxes doesn't have a selected value, therefore SelectedIndex is returning -1 , which is invalid.

You could add a validation check at the start of each event to see if both ComboBoxes have selected values. Or better still, use a common function:

void CreateArray()
{
    //could also validate the values are not greater than 1 if you think that is worth it
    if(comboBox1.SelectedIndex == -1 || comboBox2.SelectedIndex == -1)
        return;

    double[,] arr = new double[2, 2];

    for(int i = 0; i < 2; i++)
    {
        arr[0, 0] = 1;
        arr[0, 1] = 1.24;
        arr[1, 0] = 0.80;
        arr[1, 1] = 1;
        label1.Text = arr[comboBox1.SelectedIndex, comboBox2.SelectedIndex].ToString();//Index was outside the bounds of the array.
    }

}

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    CreateArray();
}

private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
{
    CreateArray();
}

//or subscribe both events to the same handler

In this case, label1 will only be populated once bot ComboBox values have been set. Alternative, you could also set each ComboBox to have a default value first. Depends what your requirements are really.

---

A couple of other notes:

• Your for loop appears to be useless. You are just doing the exact same thing twice!
• It is better to create the array once, rather than on each event. Create a class level variable to hold the array (this is actually demonstrated in Sayse's answer)

Answer 2

this is partly an off topic answer but please consider the following..

private double[,] arr = new double[2,2]{{1,1.24},{0.80, 1}};

private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
{
     if(comboBox1.SelectedIndex == -1 || comboBox2.SelectedIndex == -1)
        return;
     label1.Text = arr[comboBox1.SelectedIndex, comboBox2.SelectedIndex].ToString();
}

You can even use this event for both comboboxes

Answer 3

When there's nothing selected in a ComboBox , its SelectedIndex may become -1. I suggest you first check if SelectedIndex is not -1 first, or normalize it so that if it is -1 treat it as 0.