Custom User Control Not Initialized in Auto-Generated Code

This has happened many times before, but I never bothered to figure out why, and now I am tired of it:

For instance, I derive a class from RichTextBox or Panel, I rebuild my project to have the class added to the VS designer toolbox, and then I drag & drop the custom user control to a Form. Everything works fine, and I can run my project...

The problem comes when I edit properties of the Form or the custom user control through the designer. Sometimes, the designer removes the initialization line from its code-behind, causing an exception in the designer and the executable because the control remains uninitialized.

In other words, the following line is removed from say, Form1.Designer.cs:

this.customRichTextBox1=new CustomRichTextBox();

No other line is removed from the code-behind, so the attributes of the custom control are still set, although the variable stays uninitialized.

My solution has always been to manually initialize my user control in the designer code-behind, but the designer eventually removes it again.

I believe that this does not happen when I build a Custom UserControl through the designer (but I am not completely sure of this). It only happens when I define something like the following manually:

class CustomRichTextBox:RichTextBox{}

This is so annoying. What am I doing wrong?

As @Cody requested, here are the steps to reproduce the problem. I am using VS2010, but I've had this problem since 2005, I think.

Step 1. Create new Windows Forms Application, any Framework

Step 2. Add the following class below your main Form class: (It just happens that this is the control that is causing me this problem this t开发者_开发百科ime.)

class CustomRichTextBox : RichTextBox
{
    Timer tt = new Timer();

    internal CustomRichTextBox()
    {
        tt.Tick += new EventHandler(tt_Tick);
        tt.Interval = 200;
    }


    protected override void OnTextChanged(EventArgs e)
    {
        tt.Stop();
        tt.Start();
    }

    void tt_Tick(object sender, EventArgs e)
    {
        System.Diagnostics.Trace.WriteLine("Hello world!");
    }
}

Step 3. Press F6 to rebuild.

Step 4. Add the CustomRichTextBox control to your Form by dragging and dropping from the Toolbox.

Step 5. If you wish, you may press F5 to test the application, but it should work. Close the running application.

Step 6. Press F6 to rebuild, and at this point, the designer should crash with the following message: "The variable 'customRichTextBox1' is either undeclared or was never assigned." (In one case, the whole VS completely crashed, but the error is usually contained within the designer.)

Step 7. To correct the issue, go into the code-behind and initialize the variable, but next time you rebuild, the initialization line will be gone.

Thanks to everyone who tried answering my question and who posted comments that helped me diagnose and solve the problem.

The problem occurs when using an "internal" keyword with the control's constructor. Changing it to "public" fixes the problem. The reason for this behavior might be that the Designer's own classes cannot see the constructor because they are not within the namespace of my class unless it is marked public. This all makes sense, and I will use the public keyword from now on.

The class does not need to be in its own individual file or be the first declared class in the file as other answers suggested.

The following class works well because the constructor's keyword was changed to public.

class CustomRichTextBox : RichTextBox
{
    Timer tt = new Timer();

    public CustomRichTextBox()
    {
        tt.Tick += new EventHandler(tt_Tick);
        tt.Interval = 200;
    }


    protected override void OnTextChanged(EventArgs e)
    {
        tt.Stop();
        tt.Start();
    }

    void tt_Tick(object sender, EventArgs e)
    {
        System.Diagnostics.Trace.WriteLine("Hello world!");
    }
}

Is your build set to Debug or it is Release? I suppose that it is release as I think compiler optimizes the code and remove designer generated line.

Have you tried putting the control code in its own file? I've had problems even with the form designer in the past when the designer code was not int he first class in the file.

I had a similar problem that this posted helped me solve. I have a CustomControl that extends ComboBox, that class contained an internal private class YearItem. I've tried to highlight only the code needed to understand the problem and the solution.

public class YearsCbo : ComboBox //Inherits ComboBox
{
    public YearsCbo() { 
        fill();
    }
    private void fill() { // <<<=== THIS METHOD ADDED ITEMS TO THE COMBOBOX
        for(int idx = 0; idx < 25; idx++) {
            this.Items.Add(new YearItem());
        }
    }
    // Other code not shown
    private class YearItem {} // <<<=== The VS designer can't access this class and yet 
        // it generated code to try to do so.  That code then fails to compile.
        // The compiler error rightfully says it is unable to access 
        // the private class YearItem
}

I could drag/drop that control YearsCbo onto a form and it worked correctly, but after I returned and edited the form the VS designer generated code that would not compile. The offending code something like this:

Dim YearItem1 As my.ns.YearsCbo.YearItem = New my.ns.YearsCbo.YearItem()
Dim YearItem2 As my.ns.YearsCbo.YearItem = New my.ns.YearsCbo.YearItem()
// This was repeated 25 times because in my constructor I created 25 of these
Me.YearsCbo1.Items.AddRange(New Object() {YearItem1, 2, 3, ..., YearItem25 });

Notice that the designer generated code which tried to access the private class. It didn't need to do that but for some reason it did.

Through trial and error, and this post: How to tell if .NET code is being run by Visual Studio designer came up with a solution:

I added a property to tell if I am running in the designer.

public bool HostedDesignMode
{
    get
    {
        if (System.ComponentModel.LicenseManager.UsageMode == System.ComponentModel.LicenseUsageMode.Designtime)
            return true;
        else
            return false;
    }
}

I also changed the constructor so that it doesn't call fill() so when the designer runs, there are no items in the ComboBox so the designer doesn't feel the need to manually create those items.

The "fixed" code is shown below:

public class YearsCbo : ComboBox //Inherits ComboBox
{
    public YearsCbo() { 
        if ( ! HostedDesignMode ) {
            fill();
        }
    }
    private class YearItem {} // <<<=== Now the VS Designer does not try to access this
}

This code was written using VS2012 Premium on Win7x64 OS (in case it matters).