In the Windows operating system, a user can only work with one control at a time. The control that is currently receiving the user's key presses is the control that has focus. Sometimes this control is drawn slightly differently. For example, the button control uses a dotted line around its caption to show that it has the focus. Figure 3-6 shows focused and unfocused buttons with both the classic Windows look and Windows XP visual styles.
Figure 3-6: Focused buttons
To move the focus, the user can click the mouse or use the tab key. The developer has to take some care to make sure that the tab key moves focus in a logical manner (generally from left to right and then down the form). The developer also has to choose the control that should receive the focus when the window is first presented.
All controls that support focusing provide a Boolean TabStop property. When set to true, the control can receive focus. When set to false, the control is left out of the tab sequence and can only be reached using a mouse click.
You should set the TabStop property to false for controls that can accept key presses but are not directly accessed by the user in your application. For example, you might provide a DataGrid control, but use it to display static information. Of course, setting the TabStop to false also means the user will need to use the mouse to scroll the control, if its contents extend beyond the bounds of its display region.
To set the tab order, you configure a control's TabIndex property. The control with a TabIndex of 0 gets the focus first. When the user presses the tab key, the focus moves to the next control in the tab order, as long as it can accept focus. Visual Studio .NET provides a special tool, shown in Figure 3-7, that allows you to quickly set tab order. Just select View → Tab Order for the menu. You can then assign TabIndex values by clicking controls in the desired order.
Figure 3-7: The Visual Studio .NET tab order tool
Label controls have a TabIndex setting even though they cannot receive focus. This allows you to use a label with an access key. When the user triggers the label's access key, the focus is automatically forwarded to the next control in the tab order. For that reason, you should give your labels an appropriate place in the tab order, especially if they use access keys. (You create an access key by placing an ampersand character before a letter in the label's text.)
Controls that are invisible or disabled (commonly known as "greyed out") are generally skipped in the tab order, and are not activated regardless of the TabIndex and TabStop settings. To hide or disable a control, you set the Visible and Enabled properties, respectively. Note that if you hide or disable a control at design time, the appearance is not modified. This is a deliberate idiosyncrasy designed to make it easier to work with controls at design time, and it is recommended that custom controls also follow this pattern.
Some other properties and methods for managing the focus programmatically are described in Table 3-3.
Table 3-3: Members for Dealing with Focus at Runtime MemberDescription
Focused
Returns true if the control currently has the focus.
ContainsFocus
Returns true if the control or one of its children currently has the focus.
Focus()
Sets the focus to the current control.
SelectNextControl()
Sets the focus to the next control in tab order. This is the programmatic equivalent of pressing the tab key.
GetNextControl()
Returns a reference to the next control in the tab order, without moving the focus.
LostFocus and GotFocus events
These events fire after the focus has moved. They do not give you the chance to stop the focus change, and are thus poor choices for validation routines. If you insist on programmatically resetting the focus in an event handler for one of these events, you may trigger a never-ending loop of focus events. Instead, use the validation events or the ErrorProvider control, which are described in Chapter 4.
The GetNextControl() and SelectNextControl() methods are particularly useful when you are combining some type of interactive wizard or application help, as it can direct the user to an important control or part of the screen.