我有一个“MainForm”和一个“GraphicsForm”。单击主窗体上的“新建”，将创建一个“GraphicsForm”。
问题是，当我创建多个“GraphicsForm”，而当我想保存其中一个“GraphicsForm”的内容时，我需要在“MainForm”上点击“保存”，程序会将活动的“GraphicsForm”的内容保存到一个文件中，我不知道如何将这个“GraphicsForm”的内容传递给“MainForm”进行存储。
MainForm.cs

public partial class MainForm : Form
{
    public MainForm()
    {
        InitializeComponent();
    }

    private ToolStripMenuItem _winMenuItem = new ToolStripMenuItem();
    private GraphicsForm _graphicsForm;
    private int _counter = 1;

    private void New_Click(objec sender, EventArgs e)
    {
        _winMenuItem.Name = "Win";
        _winMenuItem.Text = "Windows";
        int item = MainMenuStrip.Items.IndexOf(_winMenuItem);
        if (item == -1)
        {
            MainMenuStrip.Items.Add(_winMenuItem);
            MainMenuStrip.MdiWindowListItem = _winMenuItem;
        }

        _graphicsForm = new GraphicsForm();
        _graphicsForm.Name = string.Concat("Win_", _counter.ToString());
        _graphicsForm.Text = _graphicsForm.Name;
        _graphicsForm.MdiParent = this;
        _graphicsForm.Show();
        _graphicsForm.WindowState = FormWindowState.Maximized;
        _counter++;
    }

    private void Save_Click(object sender, EventArgs e)
    {
        ... // Problem here
    }

    private void Open_Click(object sender, EventArgs e)
    {
        ... // Problem here
    }
}

GraphicsForm.cs

public partial class GraphicsForm : Form
{
    //StorageDoc is a class to manage all the graphics drawn by the user in the form.

    private StorageDoc _storageDoc = new StotageDoc();

    public GraphicsForm()
    {
        InitializeComponent();
    }

    private Canvas_MouseDown()
    {
    }
    
    private Canvas_Paint()
    {
    }
    
    ...