从子窗体访问主窗体中的控件

Question

I have a main form with 3 main panels:

• Left one is the menu
• Top one is a simple media player
• Main one is the host for child panels

The song selection is done in the child panel (through a datagrid ) and I would like to send the song name to the top panel, in a label song_played to begin with (which is in the main form).

This is how I'm opening the child form (click on the menu):

private void openChildFormInPanel(Form childForm)
    {
        if (activeForm != null)
            activeForm.Close();
        activeForm = childForm;
        childForm.TopLevel = false;
        childForm.FormBorderStyle = FormBorderStyle.None;
        childForm.Dock = DockStyle.Fill;
        panelChildForm.Controls.Add(childForm);
        panelChildForm.Tag = childForm;
        panelChildForm.Parent = this;           
        childForm.BringToFront();
        childForm.Show();
    }

panelChildForm is the child panel located in the main form.

On the click event I fire:

openChildFormInPanel(new Stats_form());

In my child form, i tried several solutions but i have an exception System.NullReferenceException because the parent form is always null :

this.Owner.Controls["song_played"].Text = (string)dataGridView1.Rows[dataGridView1.CurrentRow.Index].Cells[2].Value;

I also tried :

((Form1)this.Owner).lablSetText((string)dataGridView1.Rows[dataGridView1.CurrentRow.Index].Cells[2].Value);

Form1 being the name of my main form and labSetText being a function to set the text of the label on the main form.

Last,

this.Parent.Controls["song_played"].Text = " dfgdfggdf";

fires the same exception

Any help is appreciated :)

Answer 1

You could keep looking around in the debugger - Quick Watch will help - and eventually you will find the properties of this on the childForm instance that allow you to go back "up" to the main for and back "down" to the top panel.

I'd like to suggest another approach, that is less fragile though.

Instead of just having a Form for the Child form with the data grid view, derive your own form and add an event:

public SongSelectorForm : Form
{
    public event EventHandler<String> SongChanged;

    private void OnDataGridViewClick(object sender, EventArgs e)
    {
        SongChanged?.Invoke(this, "New Song Name"); //Taken from the data grid view.
        //N.B. ?.Invoke is a safe way of calling the event handler in case no one is listened.
    }
}

Now in the MainForm when you add the ChildForm you can subscribe to the event:

private void openChildFormInPanel(SongSelectorForm childForm)
{
    //snip the above
    childForm.SongChanged += HandleSongChange;
}

//We are adding a new method to the main form now to specifically handle a song change event from somewhere.
private void HandleSongChange(object sender, String songName)
{
    this.TopForm.SongName = songName; //or whatever you use to update the songname in the top form.
}

As the Main Form is the "parent" it makes sense that it knows about its children. Using events is a way for the "parent" to orchestrate the behavior of the children without the children knowing about each other.

I added a TopForm.SongName , so that the "parent" had an easier way of talking to the TopForm without knowing what the underlying controls are:

eg

//in topform
public String SongName
{
    get { return someControl.Text; }
    set { someControl.Text = value; }
}

This allows you to change things in TopForm but still maintains a simple way for Mainform to change the SongName without having to change that too.

The more you learn about C# you will see that you could use Interfaces to define "contracts" between these components so they know less about each other, making it easier to change one part without worrying about breaking anything else.