Create a Winforms ComboBox with a default text when nothing is selected.

Follow these easy steps to create a combox that shows a text instead of an empty line when nothing is selected

• Create a new UserControl


• in the code view make sure the Class Enherits from ComboBox instead of UserControl


• In the default constructor set the DrawMode to OwnerDrawn.


• Add two properties UnSelectedColor of type Color and UnSelectedText of type string


• Override the OnDrawItem method

I will put a more detailed article on my personal homepage. Link will follow later.

Thanks to Richard Everett who provided me the example code

I came across an annoying little thing trying to trigger an event that updates the UI...

{"Cross-thread operation not valid: Control 'label1' accessed from a thread other than the thread it was created on."}


This happens because the UI runs in a specific thread. Now we create another thread and try to access the UI thread which throws the above exception. There is a solution in msdn but imo its not a good one.

Thus we need to marshal the event from one thread to the other thread as shown below.

the source (just create a new form called form1 and drag a label and a button on it):

ThreadComponent.cs

using System;
using System.Collections.Generic;
using System.Text;
using System.Threading;
using System.ComponentModel;


namespace WindowsApplication3 {
    public delegate void ShowSomethingHandler(string message);
  

    public class ThreadedComponent {
        private Thread _Thread;      

        public ThreadedComponent() {
            _Thread = new Thread(Poll);
            _Thread.Start();

        }

        private void Poll() {
            int i = 0;
            ISynchronizeInvoke invoker = ShowSomething.Target as ISynchronizeInvoke;
            while(true) {
                if(ShowSomething != null) {
                  
                   //ShowSomething("Message N°:" + (++i));
                    if(!invoker.InvokeRequired) {
                        object[] args = {"Message N°:" + (++i)};
                        invoker.Invoke(ShowSomething, args);
                    } else {
                        ShowSomething("Message N°:" + (++i));
                    }
                  
                  
                }
                Thread.Sleep(1000);
            }
        }

        public event ShowSomethingHandler ShowSomething;
    }
}

Form1.cs

namespace WindowsApplication3 {
    public partial class Form1 : Form {
        public Form1() {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e) {
            ThreadedComponent c = new ThreadedComponent();
            c.ShowSomething += new ShowSomethingHandler(c_ShowSomething);
        }

        void c_ShowSomething(string message) {
            label1.Text = message;
        }
    }
}