[問題] 文字方塊控制

[ckkt (!@#$)]

namespace box
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            this.Text = "sell drinks";
            this.BackColor = Color.Yellow;
            label1.Text = "name";
            label2.Text = "how much";
            label3.Text = "how many";
            label4.Text = "total money";
            button1.Text = "蝣箏꺚";
            button2.Text = "皜雎妚";
            textBox1.BackColor = Color.Yellow;
            textBox1.Text = "cola";
            textBox1.ReadOnly = true;
            textBox2.BackColor = Color.Yellow;
            textBox2.Text = "25";
            textBox2.ReadOnly = true;
            textBox4.BackColor = Color.Yellow;
            textBox4.ReadOnly = true;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (??)
            {
                textBox3.Focus();
            }
            else
            {
                int a = int.Parse(textBox3.Text) * int.Parse(textBox2.Text);
                textBox4.Text = a.ToString();
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            textBox3.Clear();
            textBox4.Clear();
            textBox3.Focus();
        }
    }
}
請問??裡要怎麼設
才有辦法讓我在按了button1後
可以執行textBox3.Focus();
因為如果textBox3裡我沒有打任何數字
就去按button1就會當掉

還是有其他方法可以讓textBox3不輸入任何數字時
按button1也不會當機的

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◆ From: 122.125.196.27
推 wesley412:(?)..fill textBox3.text=="" try it!!
→ leo60037:在可能產生例外的地方加入 try catch
推 iamso1:if (textBox3.Text ==String.Empty)
→ iamso1:就是說 如果你的textbox3.text是空白的話
→ iamso1:嗚..剛剛看到才發現我跟一樓是同樣意思