Redirect the Enter key from a Windows Forms TextBox

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• Download demo project - 4.48 Kb

Introduction

Would you like to prevent the Enter key in a TextBox from automatically pressing the OK button of a Form, assuming you attached a OK button to the AcceptButton property of the Form?

The common way to redirect keys is to intercept their interpretation by attaching a method to the event handlers KeyPress or KeyDown of the TextBox control, or to derive from the class TextBox in order to override the methods OnKeyPress or OnKeyDown. Unfortunately, in this case, it fails: the redirection code is not interpreted and the OK button is automatically activated by the form itself. It happens because the parent form intercepts and stops the KeyPress and KeyDown signals. You don't get better results by overriding ProcessKeyPreview.

Instead, you have to inherit from TextBox and override the ProcessCmdKey virtual method.

The following C++ and C# code snippets show how to apply a tabulation when the user presses Enter, so that it switches to the next control.

Using the code

If you would like to use this code in your program, simply paste the class declaration in your code, and use CustomizedTextBox instead of System.Windows.Forms.TextBox in your code. If you want to enable the use of this control in the Forms Editor, then you should make a separate library project and import the contents of the DLL into the Visual Studio ToolBox.

The code in C#:

//! Use instances of this class instead of TextBox

// in order to Redirect the process of the Enter Key,

// before the Form does it for you

public class CustomizedTextBox : System.Windows.Forms.TextBox 
{    
    // This method intercepts the Enter Key

    // signal before the containing Form does

    protected override bool ProcessCmdKey(ref 
              System.Windows.Forms.Message m, 
              System.Windows.Forms.Keys k) 
    {
        // detect the pushing (Msg) of Enter Key (k)

        if(m.Msg == 256 && k == 
               System.Windows.Forms.Keys.Enter) 
        {
            // Execute an alternative action: here we

            // tabulate in order to focus

            // on the next control in the formular

            System.Windows.Forms.SendKeys.Send("\t");
            // return true to stop any further

            // interpretation of this key action

            return true; 
        }
        // if not pushing Enter Key,

        // then process the signal as usual

        return base.ProcessCmdKey(ref m,k);
    }
}

The code in managed C++:

 //! Use instances of this class instead

// of TextBox in order to Redirect the process

// of the Enter Key, before the Form does it for you

public __gc class CustomizedTextBox : 
       public System::Windows::Forms::TextBox 
{
protected:
    // This method intercepts the Enter Key

    // signal before the containing Form does

    virtual bool ProcessCmdKey(System::Windows::Forms::Message 
                           * m, System::Windows::Forms::Keys k) 
    {
        // detect the pushing (Msg) of Enter Key (k)

        if(m->Msg == 256 && k == 
               System::Windows::Forms::Keys::Enter) 
        {
            // Execute an alternative action: here we

            // tabulate in order to focus

            // on the next control in the formular

            System::Windows::Forms::SendKeys::Send(S"\t");
            // return true to stop any further

            // interpretation of this key action

            return true; 
        }
        // if not pushing Enter Key,

        // then process the signal as usual

        return __super::ProcessCmdKey(m,k);
    }
};

Points of Interest

I hope this code will prevent some of you from wasting 3 or 4 hours looking for a trick that is not really (conceptually) interesting, but useful.