Windows Forms Controls

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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.

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:
This is wrong:

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

We have a small sample that shows how to do this. Download PropTab.zip. After you download and unzip the sample, build the project. Both the control assembly and a small driver assembly get built. After that add the control to your toolbox using 'Customise toolbox...'. Then drag and drop an instance of the added control onto the form in the driver winforms sub-project. When you select this control the properties window should have a tab with a bitmap 'T' as shown below.
These are the steps involved. Before following these steps please download and take a look at the sample. That will greatly help when following these steps.

Control related steps Assume that you have a control that has two sets of properties, one set that you wish to have displayed in the main property tab and another set that you wish to have displayed in the second tab.

* Mark those properties that you wish to display in the first tab with the BrowsableAttribute to true.
* Mark those properties that you wish to display in the second tab with the BrowsableAttribute set to false. Create another attribute. Name it anything you want and give it a single boolean property. Initialize this property to true.
Other steps

* Derive a class from System.Windows.Forms.PropertyGridInternal.PropertiesTab. You have to override a few methods. The most important of these is GetProperties. We override GetProperties as shown below to use our CustomAttribute as the filtering attribute instead of the BrowsableAttribute that the PropertyGrid uses by default.

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:
When you include a scrollbar in your control, the scrollbar will appear inside the border, rather than to the outside if you drew the border in the client area.
When you allow custom painting of the control, your user will not draw over the NC border.
Your own client painting code will be simplified in that you will not have to bother about taking the border into account while painting the client area. The next faq will tell you how to include a non-client border.

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?

Normally, the context menu will be shown at the center of the control when the keyboard is used to invoke the context menu of a control (Shift+F10, for example). You can customize the location as follows.
1. Override WndProc in the grid and check if the Message.Msg is WM_CONTEXTMENU (0x007b)
2. If so, check if the Message.LParam is -1, which means this was due to a keyboard message as opposed to the user right-clicking the mouse.
3. Now, call the associated ContextMenu's Show method explicitly at the selected row position.
4. Make sure NOT to call the base class after showing the menu explicitly.

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.

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.


Technique 1: Use a bitmap (not an icon, in the embedded resource) file implicitly without specifying the ToolboxBitmapAttribute for the type:
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 definition.

Technique 2: Use ToolboxBitmap attribute. Example 1:
Use a bitmap (not icon) in the embedded resource with the same name as the type. Default Assembly Namespace: "MyAssemblyNamespace"

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 cancelButton 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 retrieve 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:

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 selected 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.

2.26 What is the best method to override in custom Controls to perform custom initialization during runtime?

When custom initialization is to be done during runtime on certain Controls, the best way is to implement the ISupportInitialize interface in that Control. Then the BeginInit method will be called as soon as the Control gets created and EndInit will be called after the design-time initialization of that control.

2.27 set a Control's Visible property to true and in the immediate next statement, it returns false. Why doesn't setting the Visible property 'take'?

A control's Visible property will also depend on it's parent control's (if any) visible property. If the parent control is not visible, the control will also return false.

2.28 I'm trying to make the background of my linklabel transparent so a picturebox will show through it. However, if I set the link label's BackColor property to Transparent the label still has a white background. Why?

Controls with a "Transparent" color actually render their parent's background, so you're seeing the White background of the Form, not the PictureBox. Three easy ways to deal with this:
* Use a Panel with it's "BackgroundImage" property set instead of a PictureBox, and parent the LinkLabels to the panel (PictureBoxes generally don't have children)
* Set the BackgroundImage of the Form to the image (basically the same as 1 above, but avoids the extra control)
* In code, set the Parent of the LinkLabel to be the PictureBox. You'll need to update the LinkLabel's position to match the new origin of the parent if the PictureBox isn't at (0,0)

2.29 How do I dynamically load a control from a DLL?

You use System Reflection to dynamically load a control. If the DLL is named "SpecControls.DLL" and the class you want is "SpecControls.ColorControl", then use this code.

2.30 What is the (DynamicProperties) item listed on a control's property page in VS.NET?

Clicking Advanced... under this DynamicProperties option in the control properties displays certain control properties that can be set through an XML app.config file that is added to your project. This file stores the dynamic property values and is automatically read during the initialization process on the form at design time. You can manually edit this file to change the properties and next when you run the application, controls will pick up the new values. This file is used strictly for design time initializations.

2.31 How can I make a Panel or Label semi-transparent on a Windows Form?

Type conversion is usually possible in one of the following ways:
1) Using explicit methods in source type like:B

2.32 How can I add a control to a Window Form at runtime?

To add a control at runtime, you do three steps:
* 1. Create the control
* 2. Set control properties
* 3. Add the control to the Form's Controls collection
In general, if you need help on exactly what code you need to add, just look at the code generated by the designer when you add the control at design time. You can generally use the same code at runtime.
Here are code snippets that create a textBox at runtime.

2.33 How do I make the arrow keys be accepted by a control (such as a button) and not handled automatically by the framework's focus management?

By default, the arrow keys are not handled by a control's key processing code, but instead are filtered out for focus management. Hence, the control's KeyDown, KeyUp and KeyPressed events are not hit when you press an arrow. If you want your control to handle these keyboard events, you tell the framework by overriding your control's IsInputKey method.

2.34 In the property browser for a custom control, how do I disable a property initially, but enable it later based on some other property changing?

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.

2.35 How can I have the control designer create the custom editor by calling the constructor with the additional parameters rather than the default constructor?

You can do this by creating the editor yourself rather than allowing TypeDescriptor to do it:

2.36 How do I listen to the screen resolution change in my control?

You should override WndProc in your control/form and listen to the WM_DISPLAYCHANGE message. Refer to this faq a WndProc override sample: How do I listen to windows messages in my Control?

2.37 How do I determine which button in a Toolbar is clicked?

When you click on a button on a Toolbar the click is sent to the Toolbar, so you need to check the ToolBarButtonClickEventArgs's button property to determine the button on the Toolbar that was clicked.