Q:

Why when adding items to a combobox the values are the same and not increasing by the loop?

ComboboxItem boxitem = new ComboboxItem();

        public Form1()
        {
            InitializeComponent();

            for (int i = 0; i < 100; i++)
            {

                boxitem.Text = i.ToString();
                comboBox1.Items.Add(boxitem);
            }
        }

And the class ComboboxItem :
public class ComboboxItem
        {
            public string Text { get; set; }
            public object Value { get; set; }

            public override string ToString()
            {
                return Text;
            }
        }

But for some reason all the items in the comboBox1 are 99
There are 100 items all of them the same 99
Instead 1 2 3 4 5 6 7 .... 99

A:

Here you are setting the text of the same ComboBox item over and over and adding it to your list:
for (int i = 0; i < 100; 
{
    boxitem.Text = i.ToString();
    comboBox1.Items.Add(boxitem);
}

Instead, create a new ComboBox inside the loop and add that to the list. Just move the 
ComboboxItem boxitem = new ComboboxItem();

inside your for-loop, and you're good.

