winform textBox没有更新

Question

我有两个winform form1和form2如下：

public partial class Form1 : Form
{
    Form2 frm2;
    int count = 0;
    public bool fromForm2;

    public Form1(bool fromForm2 = false)
    {
        InitializeComponent();

        this.fromForm2 = fromForm2;

        MessageBox.Show(fromForm2.ToString());
        if (fromForm2 == true) { 
            test();
        }

    }

    private void button1_Click(object sender, EventArgs e)
    {


        if (frm2 == null)
        {
            frm2 = new Form2();   //Create form if not created
            frm2.FormClosed += frm2_FormClosed;  //Add eventhandler to cleanup after form closes
        }

        frm2.Show(this);  //Show Form assigning this form as the forms owner

    }

    void frm2_FormClosed(object sender, FormClosedEventArgs e)
    {
        frm2 = null;  //If form is closed make sure reference is set to null
        Show();
    }



    public void test ()
    {

        textBox1.Text = "ABCDFGHIJKLM";
        MessageBox.Show(textBox1.Text);
    }
}

表格2

public partial class Form2 : Form
{
    Form1 f1;

    public Form2()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {

        Form1 objj = new Form1(true);

        objj = (Form1)Application.OpenForms["Form1"];


        objj.fromForm2 = true;


        objj = null;
    }


}

我想点击form2中的按钮，将运行test（），但我发现，如果我使用if（fromForm2 == true）{} if语句，test（）函数将不会更新（重绘？）要显示到textBox1的文本值，有谁知道这是什么原因？