Method does not act as expected

Question

When I go to check the following code, there is nothing in any of the ComboBoxes. For me, everything is normal, I see nothing wrong, what can be?

public FormExemplo()
{
    ExemploList = new List<string[]>;
    CmbB.Enabled = false;
}
private void AttCmbA(ComboBox A) 
{
    A.Items.Clear();
    for (i = 0; i < ExemploList.Count; i++)
    {
        A.Items.Add(ExemploList[i][0]);
    }
} 
private void AttCmbB(ComboBox A, ComboBox B) 
{
    B.Items.Clear();
    for (int i = 0; i < ExemploList.Count; i++)
    {
        if (A.SelectedItem.Equals(ExemploList[i][0]))
        {
            for (int j = 0; j < 3; j++)
            {
                CmbB.Add(ExemploList[i][j]);
            }
        }
    }
} 
private void CmbA_SelectedIndexChanged(object sender, EventArgs e)
{
    CmbB.Enabled = true;
    AttCmbB(CmbA, CmbB);
}
private List<string[]> ExemploList;
private void Btn_Click(object sender, EventArgs e)
{
    for (int i = 0; i < 10; i++)
    {
        ExemploList.Add(new string[3] { (2 * i).ToString(), (2 * (i++)).ToString(), (2 * (i * i).ToString() });
    }
    AttCmbA(CmbA);
}

Answer 1

The problem I found in your code was in the method AttCmbB(...). Accessing the Selecteditem property as it is, returns null because at this point the selection has not been made yet (we are inside the event SelectedIndexChanged).

One solution will be to change the event and the method to this form:

private void AttCmbB(ComboBox B, object selectedItem)
{
    B.Items.Clear();
    for (int i = 0; i < ExemploList.Count; i++)
    {
        if (selectedItem.Equals(ExemploList[i][0]))
        {
            for (int j = 0; j < 2; j++)
            {
                CmbB.Items.Add(ExemploList[i][j]);
            }
        }
    }
}

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    ComboBox comboBox = (ComboBox)sender;
    AttCmbB(CmbB, comboBox.SelectedItem);
}

This way, it uses the object that originated the event to fetch the value that was selected (which after the event completes is available in Cmba).