textbox不支持Ctrl+A

http://stackoverflow.com/questions/5885739/why-are-some-textboxes-not-accepting-control-a-shortcut-to-select-all-by-defau

You might be looking for the ShortcutsEnabled property. Setting it to true would allow your text boxes to implement the Ctrl+A shortcut (among others). From the documentation:

Use the ShortcutsEnabled property to enable or disable the following shortcut key combinations:

• CTRL+Z
• CTRL+E
• CTRL+C
• CTRL+Y
• CTRL+X
• CTRL+BACKSPACE
• CTRL+V
• CTRL+DELETE
• CTRL+A
• SHIFT+DELETE
• CTRL+L
• SHIFT+INSERT
• CTRL+R

EDIT:
However, the documentation states:

The TextBox control does not support the CTRL+A shortcut key when the Multiline property value is true.

You will probably have to use another subclass of TextBoxBase, such as RichTextBox, for that to work.

http://stackoverflow.com/questions/16197915/how-can-i-allow-ctrla-with-textbox-in-winform

Like other answers indicate, Application.EnableVisualStyles() should be called. Also the TextBox.ShortcutsEnabled should be set to true.

But if your TextBox.Multiline is enabled then Ctrl+A will not work (see MSDN documentation). Using RichTextBox instead will get around the problem.