2 .表单通信c#

嗨，我在一个应用程序上有2个表单，并想使用一个表单在另一个表单中设置一个标签，我四处寻找答案，但我没有得到它的工作。代码:

form 1

public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void uit_Click(object sender, EventArgs e)
        {
            Form2 frm = new Form2(this);
            frm.Show();
        }
        public string LabelText
        {
            get { return uit.Text; }
            set { uit.Text = value; }
        }
    }

形式2:

public partial class Form2 : Form
{
    private Form1 mainForm = null;
    public Form2(Form callingForm)
    {
        mainForm = callingForm as Form1;
        InitializeComponent();
    }
    private void Form2_Load(object sender, EventArgs e)
    {
        txtmessage.Text = this.mainForm.LabelText;
    }
    public void button1_Click(object sender, EventArgs e)
    {
        this.mainForm.LabelText = txtmessage.Text;
    }
}

有人能解释一下为什么这不起作用吗?