C# Unable to cast object of type 'TCSCapstone.frmInventory' to type 'System.IConvertible'

Question

I'm trying to add up all the values in a Listbox which the Listbox is populated by the value of two other list boxes multiplied together. It's essentially a sum of a Listbox.

I keep getting an error and I looked it up and everyone else has similar issues with similar problems. It has something to do with the ToInt32 or the ToString, I just cannot fix it. Whenever I click my button to load the Listbox which executes that loop, it gives me the error.

Please help, this is all I need left.

         int i = 0, result = 0;
         while (i < lstTotalPrices.Items.Count)
         {
             result += Convert.ToInt32(lstTotalPrices.Items[i++]);
         }

         txtTotal.Text = Convert.ToString(result);

Answer 1

Based on comments and chat this should work:

int i = 0; 
decimal result = 0; 
while (i < lstTotalPrices.Items.Count) 
{ 
    result += ((frmInventory)lstTotalPrices.Items[i++]).TotalPrice; 
}

Although a tidier solution would be

int i = 0;
decimal result = 0;

foreach (var item in lstTotalPrices.Items)
{
    result += item.TotalPrice;
}