这是我用窗体编的计算器代码，计算结果都不对，求解释,有附件（只编到计算的...)
namespace ex4
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Button btn = sender as Button;
            textBox1.Text = textBox1.Text + btn.Text;
            
        }
        string a ;
        string f;
        int num1;

        private void button14_Click(object sender, EventArgs e)
        {
            Button btn = sender as Button;
            a = textBox1.Text;
            num1 = Convert.ToInt32(a);
            textBox1.Text = "";
            f = btn.Text;
            


        }
        
        private void button18_Click(object sender, EventArgs e)
        {
            if (f == "+")
            {
                int c = num1 + Convert.ToInt32(a);
                textBox1.Text = Convert.ToString(c);
            }
            else if (f == "-")
            {
                int c = num1 - Convert.ToInt32(a);
                textBox1.Text = Convert.ToString(c);
            }
            else if (f == "*")
            {
                int c = num1 * Convert.ToInt32(a);
                textBox1.Text = Convert.ToString(c);
            }
            else if (f == "/")
            {
                int c = num1 / Convert.ToInt32(a);
                textBox1.Text = Convert.ToString(c);
            }

        }  
    }
}

木有办法啊，刚学个皮毛老师就叫做计算器了......

不行，也在为这个发愁呢.....