//方法1
     private void button1_Click(object sender, EventArgs e)
        {
            float a;            
            a = float.Parse(textBox1.Text);
            int b;
            b = float.Parse(textBox2.Text);
            int c;
            c = float.Parse(textBox3.Text);
            float d;
            d = (a + b) * c / 2;
            textBox4.Text = d.ToString();
 
        }


//方法2
      private void button1_Click(object sender, EventArgs e)
        {
            int a;            
            a = int.Parse(textBox1.Text);
            int b;
            b = int.Parse(textBox2.Text);
            int c;
            c = int.Parse(textBox3.Text);
            float d;
            d = (float)(a + b) * c / 2;
            textBox4.Text = d.ToString();
 
        }