2. Windows Forms Controls

2.1 How do you prevent serialization of certain child controls in your Composite Control?

One solution is to use a panel that has a picturebox placed on it with DockStyle.Fill. This will make the picturebox assume the size of the panel. In addition, set the DockPadding.All property to the width of the desired border. Then in the Panel's OnPaint method, call the baseclass and then paint the desired borders.

Here are both VB and C# projects that illustrate how you might go about this. The derived PicturePanel class has properties that allow you to set the bordersize and color as well as the image that is to be displayed. This sample retrieves the image from an embedded

2.2 How do I get hold of the currently focused Control?

The .Net framework libraries does not provide you an API to query for the focused Control. You have to invoke a windows API to do so:

2.3 Why do calling Focus() on a control not set focus on it?

Note that when you call this method the control should be visible, otherwise the focus will not be set. Hence calling this in say Form_Load on a control in the form will be ineffective. You should instead consider give that control an appropriate TabIndex, so that it will be the first focused control.

2.4 How do I listen to windows messages in my Control?

In a derived class you should override WndProc as follows (listening to the WM_KEYUP message, for example):

2.5 How do I programatically change the color of a control?

Use the properties BackColor and ForeColor.

2.6 How can I change the Border color of my control?

Override the OnPaint. Here is some code for a derived Button.

2.7 Why should I provide a Non-Client border to my Control derived class?

Providing a border in the non-client region of your control rather than in the ClientRectangle has very many advantages:

2.8 How do I provide a 2 pixel 3d border in the Non-Client area of my Control derived class?

You can do so as follows by overriding the CreateParams property in your Control. The advantage with this approach is that drawing is handled by the system as soon as you set the flag below.

2.9 How do I provide a 1 pixel border in the NonClient area of my Control?

You will have to first provide some space in the NC area by setting the WS_BORDER flag in CreateParams and then draw the border yourself by listening to the WM_NCPAINT message in your Control, as follows:

2.10 How do I invalidate a control including it's NonClient area?

You can do so as follows in your Control:

2.11 How can I implement a scrollable picture box?

See Mike Gold's article on C# Corner for a detailed discussion.

2.12 How can I put Controls, a ProgressBar for example, into a StatusBar?

You cannot place controls into a StatusBar control in the designer. However, you can add any no. of Controls to the StatusBar programatically through it's Controls property. After adding the Controls, set their Visible, Location and Bounds property appropriately.

You could then create a status bar that looks like this, for example:

2.13 How do I implement an ownerdrawn statusbar so I can put a progressbar in it?

Check out the code posted originally in VB by Jacob Grass, and translated to C# by Jason Lavigne on the dotnet.discussion newsgroup at develop.com.

2.14 How would I change the icon that appears on the toolbox for a custom control?

You can do this in different ways explained below. In all the cases the bitmap or icon should follow these rules:

• The bitmap or icon dimension should be 16X16 with 16 colors.
• The left-bottom pixel-color will be assumed to be the transparent color.

Say, you have a custom control MyControl in the namespace MyNamespace, create a bmp file MyControl.bmp following the above rules. Add this file to your project at the top-level and make it an embedded resource. The project's default namespace should be MyNamespace.

If the control's namespace and the project's default namespace don't match then move the bitmap to appropriate subfolders so that they match. If this is not possible, typically when the namespaces are not related at all then you cannot use this technique, use instead one of the techniques below using the ToolboxBitmap attribute.

Create the assembly and the next time you add it to the toolbox the custom image in MyControl.bmp should be available in the toolbox.

This is the easiest technique to implement as it doesn't require you to use the ToolboxBitmapAttribute in your type defenition.

Example 1:

Default Assembly Namespace: "MyAssemblyNamespace"

In the above scenario the runtime will look for a embedded bmp file of name MyCustomType.bmp in the project's root directory. Note that the default namespace and the type's namespace match.

Example 2:

or

where the bmp or ico file (yap, now, when you explicity specify the resource, you can use an ico file) is in a sub-directory called "ToolboxIcons".

Example 3:

Default namespace: "MyAssemblyNamespace"

In this case the runtime will look for the above resource at the top-most directory. If your icons were in a subdirectory named "ToolboxIcons" then the attribute would look like this:

2.15 A control's Validating event is hit even when the user clicks on the Close box. How can I avoid this behavior?

One way is to add code to your Validating handler and only execute the validation routine if the mouse is in the client area. This will avoid the click on the title bar and its system menu. You might also want to add special handling for the tab key so your validation is hit independent of the mouse location when you tab off the control.

2.16 I would like to prevent validation in my textbox when the user clicks on my Cancel button, how do I do this?

Say textBox1 and canelButton and the control names, then this is how you could do this:

2.17 Why does adding images to an ImageList in the designer cause them to lose their alpha channel?

Looks like the ImageList editor loses the transparency when it does some internal copy/clone of the images. However, it seems that it does work when you add the images in code to the ImageList.

One workaround (not so tidy) is to add the images to the ImageList in the design time (so that your design-time will be closer to the runtime) and then clear that ImageList and refill it with the images again, in code.

Take a look at this faq on how to add images to your project and retrive them programatically during runtime. Adding image files to a project as an embedded resource and retrieving them programatically.

2.18 Why do the XP Icons when drawn using Graphics.DrawImage not draw transparently?

Note that it's only the ImageList class that can draw an Icon with alpha channel properly. So, instead of using the Graphics.DrawImage method, first associate this icon to a ImageList and then use the ImageList.Draw method to draw the icon. Also, note that this is possible only in XP with XP Themes enabled for that application.

2.19 Why do the XP Icons that have alpha channel not draw properly when associated with controls like ListView?

Make sure that you include the manifest file that will enable XP themes support for you application. Then the icons with alpha channel will draw semi-transparently.

2.20 How do I prevent resizing of my Controls by the user, via Docking or anchoring or manual resizing during design-time?

The best place to ensure a particular height/width for you control is in the SetBoundsCore override of your Control, as follows:

2.21 What control do I use to insert Separator lines between Controls in my Dialog?

Use the Label Control with the BorderStyle set to Fixed3D and height set to 2.

2.22 How can I make my controls transparent so the form's background image can show through them?

By default, a control's background color will be the same as the container's background. In the picture below, the form's background was set to Color.Red. In design mode, you can see the four controls have the red background, but since the form's Image was set to a bitmap, you cannot see the form's red background. To make the controls' background transparent, you can set the alpha blend value of their background to zero. In code, you could use:

In design mode, you can set the alpha blend value to zero by typing the four component in the property grid. So, for each control's BackColor property, you would type 0,255,0,0 to make it a tranparent red.

Here is a VB project using this idea.

2.23 How do I create an editable listbox with an in-place TextBox and Button?

The attached EditableList UserControl implements an editable listbox with an in-place TextBox and Button allowing users to directly edit the contents of the list box.

When the user clicks on a selected item, a textbox and a button is shown over the selected item and the user can directly edit the selectected item text. The button can be programmed to show for example a OpenFileDialog to allow user to select a file (useful while implementing a Files list).

2.24 How do I determine the width/height of the Non-Client area (like the border in a textbox) of a Control?

One generic way for all Controls is to get the difference between the ClientRectangle's width and the Control.Bounds' width. That should give the border width (and in general the NC area width); similarly for height.

2.25 How can I programmatically manipulate Anchor styles?

You can do this using the bitwise operators &, | and ^ ( And, Or and Xor (or &, Or, ^) in VB.Net). Here is code that will toggle label1 being anchored on the left.