Question

My Winform application has 3 form: MainForm, Form1 and Form2. MainForm has an UserControl; when application starts, it calls MainForm, MainForm will be loaded and added Form1 onto UserControl:

private void MainForm_Load(object sender, EventArgs e)
{
   Form1 frm = new Form1() { Dock = DockStyle.Fill, TopLevel = false, Visible = true };
   xtraUserControl1.Controls.Add(frm);
}

On Form1, I use delegate:

    public delegate void Tranferdata(string txt);
    public Tranferdata _tranfer;
    private void Gettxt(string txt)
    {
        tbx_Recieve.Text = txt;
    }

Form1 has a button to call Form2:

private void button1_Click(object sender, EventArgs e)
{
    Form2 frm2 = new Form2();
    frm2.ShowDialog(this);
}

Form2 will send value to Form1 after closed, Code on Form2:

private void Form2_FormClosed(object sender, FormClosedEventArgs e)
{
    using (Form1 frm1 = (Form1)this.Owner)
    {
       frm1._tranfer(tbx_Numb.Text);
    }
}

private void button1_Click_1(object sender, EventArgs e)
{
   this.Close();
}

But my code doesn't work correctly. It crashed at 'using (Form1 frm1 = (Form1)this.Owner)' and show the System.InvalidCastException. How can I fix this?

Answer 1

它在该行崩溃的原因是因为您没有将Form1设置为Form2的所有者。

要修复，首先将Form1作为类变量而不是局部变量：

Form1 form1;
private void MainForm_Load(object sender, EventArgs e)
{
   form1 = new Form1() { Dock = DockStyle.Fill, TopLevel = false, Visible = true };
   xtraUserControl1.Controls.Add(form1);
}

然后您可以在创建时使用它来分配Form2的所有者：

private void button1_Click(object sender, EventArgs e)
{
    Form2 frm2 = new Form2();
    frm2.Owner = form1;
    frm2.ShowDialog(this);
}

而且，就像Ron Beyer在评论中提到的那样，考虑删除使用声明，它会关闭Form1，听起来不像你想做的那样。

现在，虽然这种方法有效，但您还应该考虑一些问题：为什么你首先选择使用“所有者”属性？你真的需要它吗？将Form1作为Form2的所有者意味着Form2将在Form1关闭时关闭。由于您在Form2上使用ShowDialog，它将阻止用户在Form2显示时关闭Form1，因此它似乎不需要。如果使用Owner属性的原因只是为了使用委托，那么你可能只是将Form2的属性添加到Form1类型中，这样就可以为你输入类型安全和优质代码。

但是有更好的方法：MainForm可以注册到Form2的Closed事件并在Form1上调用该方法。这将删除不需要的依赖项（Form2和Form1根本不会彼此了解）使代码更健康。

Answer 2

我找到了解决这个问题的方法：在表格2中：

str = String.fromCString(UnsafePointer(chars)) // UTF-8 is implicit
      // or:
str = String(CString: UnsafePointer(chars), encoding: NSUTF8StringEncoding)

并在事件调用form2中以表单1调用它：

public void SetParent(Form1 frm)
{
    frm1 = frm;
}

How to get the value from active form to another form on UserControl using delegate

2 个答案: