Question

I’m creating a custom C# control (form’s title bar). One form can have only one title bar, and that’s why i’m wondering something: When user (programmer) adds my title bar to his form, is there ANY way i can check if ParentForm already contains my title bar, and if so can i cancel adding another instance of my control?

I know how to perform check to see types of controls ParentForm contains, but what event is raised when my control is dropped from toolbox to form, and how to “cancel” layout of my control if necessary?

Answer 1

You should read in-depth about the designer technologies available in .NET, because these are not what I would call production-ready examples. However, this gives you a solid start and both code snippets do what you are asking.

For design time, you can override the designer site in your control and do the following:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Data;
using System.Text;
using System.Windows.Forms;
using System.ComponentModel.Design;
using System.Diagnostics;

namespace WindowsFormsControlLibrary1 {
    public partial class DebugControl : UserControl {
        public DebugControl() {
            InitializeComponent();
        }

        public override ISite Site
        {
            get
            {
                return base.Site;
            }
            set
            {
                base.Site = value;
                IComponentChangeService service = (IComponentChangeService)GetService(typeof(IComponentChangeService));
                service.ComponentAdding += (sender, e) => {
                    IDesignerHost host = (IDesignerHost)value.Container;
                    Component component = (Component)host.RootComponent;
                    if (component as Form != null)
                    {
                        Form form = (Form)component;
                        foreach (Control c in form.Controls)
                        {
                            if (c.GetType() == this.GetType())
                            {
                                throw new System.ArgumentException("You cannot add two of these controls to a form!");
                            }
                        }
                    }
                };
            }
        }
    }
}

For runtime in your form you can override OnControlAdded and do the following:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Diagnostics;
using WindowsFormsControlLibrary1;

namespace WindowsFormsApplication1 {
    public partial class Form1 : Form {
        public Form1() {
            InitializeComponent();
        }

        protected override void OnShown(EventArgs e)
        {
            Controls.Add(new DebugControl());
        }

        protected override void OnControlAdded(ControlEventArgs e)
        {
            base.OnControlAdded(e);
            if (e.Control.GetType() == typeof(DebugControl))
            {
                int count = 0;
                foreach (Control c in Controls)
                {
                    if (c is DebugControl)
                    {
                        count++;
                    }
                }
                if (count > 1)
                {
                    Controls.Remove(e.Control);
                    Debug.Assert(false, "Cannot add two of these controls!");
                }
            }
        }
    }
}

There are more than one way to do this, but these are rough examples, mind you. Read up on design-time support that the .NET framework, it is very rich and there are extensive documentation on it. Another method is to implement custom designers and implement CanBeParentedTo and CanParent, but mind you CanBeParentedTo is not called when your control is drug from the ToolBox to your form.