ReadOnly Combobox

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Introduction

Just like a lot of people, I was looking for a read-only combobox. Most of the code samples available are complex, have some faults or just aren't that kind of read-only we want. So I went looking for a simple solution using Windows messages to keep the drop-down closed. As you will see in this article, the code is pretty easy.

Using the code

Inherit a class from a combobox and override the events that can modify the value of it. Under events, you can check if the combobox is read-only, as well as block or ignore the input.

// =============================================
// Author: KC
// Create date: 03/05/07
// Description: ReadOonly ComboBox
// =============================================
    class RoComboBox : System.Windows.Forms.ComboBox
    {
        private bool readOnly;
        public bool ReadOnly
        {
            get { return readOnly;}
            set { readOnly = value;}
        }
        protected override void  
            OnKeyDown(System.Windows.Forms.KeyEventArgs e)
        {
            if (readOnly)
                e.Handled = true;
             base.OnKeyDown(e);
        }
        protected override void 
            OnKeyPress(System.Windows.Forms.KeyPressEventArgs e)
        {
            if (readOnly)
                e.Handled = true;
            base.OnKeyPress(e);
        }
        protected override void WndProc(ref System.Windows.Forms.Message m)
        {
            // WM_LBUTTONDOWN && WM_LBUTTONDBLCLK
            if ((m.Msg != 0x201 && m.Msg != 0x203) || !readOnly)
                base.WndProc(ref m);
        }
    }

So, what is this code doing?

It is adding a private field and a property called ReadOnly.

 private bool readOnly;
        public bool ReadOnly
        {
            get { return readOnly;}
            set { readOnly = value;}
        }

        protected override void  
            OnKeyDown(System.Windows.Forms.KeyEventArgs e)
        {
            if (readOnly)
                e.Handled = true;
             base.OnKeyDown(e);
        }

        protected override void 
            OnKeyPress(System.Windows.Forms.KeyPressEventArgs e)
        {
            if (readOnly)
                e.Handled = true;
            base.OnKeyPress(e);
        }

        protected override void WndProc(ref System.Windows.Forms.Message m)
        {
            // WM_LBUTTONDOWN && WM_LBUTTONDBLCLK
            if ((m.Msg != 0x201 && m.Msg != 0x203) || !readOnly)
                base.WndProc(ref m);
        }

Points of interest

History

Original version posted: May 7, 2007