Differences and connections of previewkeydown, Keydown, keypress and Keyup in C WinForm

1、 Key events occur in the following order ：

PreviewKeyDown

        KeyDown

        KeyPress

        KeyUp

2、KeyDown After triggering , Not necessarily triggered KeyUp, When KeyDown Press after , Drag the mouse , Then it won't trigger KeyUp event .

3、 Definition

​​ ​PreviewKeyDown​​​： With the focus on this control , Occurs when there is a key action （ stay KeyDown Before the event ）.

Note ：

Some keys , such as Tab、Return、Esc And arrow keys , It is usually ignored by some controls , Because they are not considered input keys . for example , By default ,Button Control ignores arrow keys . Usually pressing the arrow key will move the focus to the previous or next control . Arrow keys are considered navigation keys , Pressing these keys usually does not trigger Button Of KeyDown event . however , Press Button The arrow keys of will indeed trigger PreviewKeyDown event . By providing Button Handle PreviewKeyDown Event and handle IsInputKey Property is set to true, It can be raised when pressing the arrow key KeyDown event . however , If you handle the arrow keys , Then the focus will no longer move to the previous or next control .

        ​​ ​KeyDown​​​： Occurs when a key is pressed while the control has focus .

Note ：

To process keyboard events only at the form level and not allow other controls to receive keyboard events , Please put the KeyPress In the event handling method KeyPressEventArgs.Handled Property is set to true. Some key , Such as Tab、Return、Esc And arrow keys , Automatically processed by the control . To make these keys trigger KeyDown event , Must be overridden in every control on the form IsInputKey Method . Used to rewrite IsInputKey The code of needs to determine whether a special key is pressed , And you need to return a true value . No need to rewrite IsInputKey Method , You can deal with PreviewKeyDown Incident and will IsInputKey Property is set to true. For code examples , Please see the PreviewKeyDown event .

        ​​ ​KeyPress​​​： Occurs when a key is pressed while the control has focus .( The following will say and KeyDown The difference between )

Note ：

KeyPress Events cannot be raised by non character keys ; But non character keys can trigger KeyDown and KeyUp event .

Use KeyChar Attribute sampling run-time keystrokes , And use or modify a subset of commonly used keystrokes .

To process keyboard events only at the form level and not allow other controls to receive keyboard events , Please put the KeyPress In the event handling method KeyPressEventArgs.Handled Property is set to true.

        ​​ ​KeyUp​​​： Occurs when the key is released when the control has focus .

Note ：

To process keyboard events only at the form level and not allow other controls to receive keyboard events , Please put the KeyPress In the event handling method KeyPressEventArgs.Handled Property is set to true. Some key , Such as Tab、Return、Esc And arrow keys , Automatically processed by the control . To make these keys trigger KeyUp event , Must be overridden in every control on the form IsInputKey Method . Used to rewrite IsInputKey The code of needs to determine whether a special key is pressed , And you need to return a true value .

4、KeyPress and KeyDown 、KeyPress The difference between

        KeyPress It's mainly used to capture numbers ( Be careful ： Include Shift+ Symbols of numbers )、 Letter （ Be careful ： Include case ）、 Keyboards, etc. in addition to F1-12、SHIFT、Alt、Ctrl、Insert、Home、PgUp、Delete、End、    PgDn、ScrollLock、Pause、NumLock、{ Menu key }、{ Start button } And outside the arrow keys ANSI character

        KeyDown and KeyUp You can usually capture the keyboard except PrScrn All the buttons ( There are no special keys for special keyboards ）

        KeyPress Only a single character can be captured

        KeyDown and KeyUp You can capture key combinations .

        KeyPress You can capture the case of a single character

        KeyDown and KeyUp For single character capture KeyValue It's all a value , That is, you can't judge the case of a single character .

        KeyPress The keyboard doesn't distinguish between the main character and the keypad .

        KeyDown and KeyUp Distinguish between numeric characters on the keypad and on the main keyboard .

among PrScrn Key KeyPress、KeyDown and KeyUp Can't capture .

Note ：

        KeyDown Keyboard input cannot be captured , Pay attention to the window keypreview Property is set to true

​ ​ stay winform Capture the key events of control keys such as up, down, left and right keys in ​​