Only numbers are allowed in the text box, and other characters are not processed

Today, I replied to a similar post. This is also a problem that I wanted to solve when I was just learning C #. I wanted to write a "small family ledger". However, the keydown () event always determines whether the input is a non-numeric character, but cannot prevent the text box from accepting the input. The non-numeric characters are displayed.

First, I repeated the problem in vs2003.

1. added a Windows form form1 and a text box textbox1 in form1.
2. Add the following code to the keydown () event of textbox1:
Private void textbox1_keydown (Object sender, system. Windows. Forms. keyeventargs E)
{
If (E. keyValue <48 & E. keyValue> 57) // If the input character is from '0' to '9'
{
// Do nothing
}
Else
{
E. Handled = true; // If the input is a non-numeric character, the event is terminated before being added.
MessageBox. Show (E. Handled. tostring ());
}
}

Not finished. To be continued...

The example http://msdn.microsoft.com/library/chs/default.asp on msdn? Url =/library/CHS/cpref/html/frlrfsystemwindowsformscontrolclasskeydowntopic. asp

[C #]

// Boolean flag used to determine when a character other than a number is entered.

Private bool nonnumberentered = false;

 

// Handle the keydown event to determine the type of character entered into the control.

Private void textbox1_keydown (Object sender, system. Windows. Forms. keyeventargs E)

{

// Initialize the flag to false.

Nonnumberentered = false;

 

// Determine whether the keystroke is a number from the top of the keyboard.

If (E. keycode <keys. D0 | E. keycode> keys. D9)

{

// Determine whether the keystroke is a number from the keypad.

If (E. keycode <keys. numpad0 | E. keycode> keys. numpad9)

{

// Determine whether the keystroke is a backspace.

If (E. keycode! = Keys. Back)

{

// A non-numerical keystroke was pressed.

// Set the flag to true and evaluate in keypress event.

Nonnumberentered = true;

}

}

}

}

 

// This event occurs after the keydown event and can be used to prevent

// Characters from entering the control.

Private void textbox1_keypress (Object sender, system. Windows. Forms. keypresseventargs E)

{

// Check for the flag being set in the keydown event.

If (nonnumberentered = true)

{

// Stop the character from being entered into the control since it is non-numerical.

E. Handled = true;

}

}