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How to make TextBox to receive only number key values using KeyDown Event in C#?

This code in my form updates the textBox1.Text twice whenever number keys are pressed.
private void textBox1_KeyDown( object sender, KeyEventArgs e ) {
//MessageBox.Show();
if( char.IsNumber((char)e.KeyCode) ) {
textBox1.Text += (char)e.KeyCode;
}
}
Explain why if you can?
Modify the code or provide me a better solution for this.
Input ( in textbox1 ):
54321
Output:
1234554321

When you press a key, a character is already appended to your TextBox. Then you run the following code and, if the key represents a number, you append it again:
if (char.IsNumber((char)e.KeyCode)) {
textBox1.Text += (char)e.KeyCode;
}
If you want to suppress any key that's not a number, you could use this instead:
e.SuppressKeyPress = !char.IsNumber((char)e.KeyCode);

From the syntax I assume you are using WinForms for the following answer.
The key pressed event is not suppressed, so it still works like a normal key pressed event and adds the character to the text of the box. Additionally you add the character to the text yourself once again.
Try to suppress the key pressed event in case a key is pressed, you do not want to allow.
private void textBox1_KeyDown(object sender, KeyEventArgs e)
{
if (!char.IsNumber((char)e.KeyCode))
{
e.SuppressKeyPress = true;
}
}

You can try like this:
private void textBox1_KeyDown(object sender, KeyEventArgs e)
{
e.SuppressKeyPress = !(e.KeyValue >= 48 && e.KeyValue <= 57);
}
Check New keyboard APIs: KeyEventArgs.SuppressKeyPress
The problem is that "Handled" doesn't take care of pending WM_CHAR
messages already built up in the message queue - so setting Handled =
true does not prevent a KeyPress from occurring.
In order not to break anyone who has currently got e.Handled =
true, we needed to add a new property called SuppressKeyChar. If we
went the other way, if "handling" a keydown suddenly started to
actually work, we might break folks who accidentally had this set to
true.

Try this code to accept numbers only
private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
if (!char.IsDigit(e.KeyChar) && !char.IsControl(e.KeyChar))
e.Handled = true;
}

How to execute a method in C# WinForms if a textbox value equals a predermined length

I have a form where a user enters a 4 digit pin number. When the pin number is entered, I would like to call my method automatically once the last number of the pin number is pressed. I am assuming this needs to be done in a keydown event on the textbox.
Here is what I tried so far.
private void txtPinNumber_KeyDown(object sender, KeyEventArgs e)
{
if (txtPinNumber.Text.Trim().Length == 4)
{
SendKeys.Send("{ENTER}");
if (e.KeyCode == Keys.Enter)
Verify_Pin();
}
}
It seems to work but the user has to press an addition key to execute the method. What am I missing?

There's no reason to press Enter programmatically and then check for it. Just call the other method.
Also, the KeyDown event fires before the Text property changes to reflect the most recently typed character, so you'll have to place that code in a different event.
Use TextChanged or KeyUp.
private void txtPinNumber_TextChanged(object sender, KeyEventArgs e)
{
if (txtPinNumber.Text.Trim().Length == 4)
Verify_Pin();
}

You said
When the pin number is entered, I would like to call my method automatically once the last number of the pin number is pressed.
So you can simply do this in the KeyUp event of your textbox
private void txtPinNumber_KeyUp(object sender, KeyEventArgs e)
{
if (txtPinNumber.Text.Trim().Length == 4)
Verify_Pin();
}
BTW, it is advisable to let user press enter and then you run the code. Because it can be that user accidentally presses the last number wrong.

Text length exceeding maximum allowed text

I am using textbox and here's my problem...
I already have a handler where program will not allow anymore inputs when defined Maximum text length is reached. Here's the code:
private void txtText_KeyPress(object sender, KeyPressEventArgs e)
{
if (txtText.Text.Length > MaxLengthAllowed - 1 && e.KeyChar != 8)
{
e.Handled = true;
Console.Beep(2000, 90);
return;
}
}
Now my problem is when I press and HOLD a key (for example the letter A), it exceeds the "MaxLengthAllowed" then beeps.
Is this a fault or something like a limitation of the textbox? Or am I missing something?
This code is working fine when you are not HOLDING a key. Hence, that's my problem.

Just set the MaxLength property of the textbox. It's as simple as
textboxVar.MaxLength = maxLengthAllowed;
Hope that helps.
You could always subscribe a PropertyChanged event, which you can rig to fire whenever the Text property of the textbox changes. Have a read of how to raise an event on Property Change

You could use the KeyUp event instead. That only fires once when you release the key. KeyPress will be fired multiple times for as long as you hold down the key.

The TextBox.Text is not updated when TextBox.KeyPress is fired. For example, if the Text before KeyPress is A, When user types B, the Text is still A (not AB) in the KeyPress event handler. So you have to define your MaxLengthAllowed to be 1 less than the desired value. The code should look like this:
private void txtText_KeyPress(object sender, KeyPressEventArgs e)
{
if ((txtText.Text.Length > MaxLengthAllowed - 1) && e.KeyChar != 8)
{
e.Handled = true;
Console.Beep(2000, 90);
}
}
I think you should use TextBox.MaxLength property instead, it also supports Beep, the Beep sound is even nicer than the Beep generated by Console.Beep().

How to make Textbox only accept alphabetic characters

I have a windows forms app with a maskedtextbox control that I want to only accept alphabetic values in.
Ideally, this would behave such that pressing any other keys than alphabetic keys would either produce no result or immediately provide the user with feedback about the invalid character.

This question has probably been asked and answered a million times on every conceivable programming forum. Every answer provided has the distinction of being unique to the stated requirements.
Since you are using a MaskedTextBox, you have additional validation features available to you and do not really need to handle keypresses. You can simply set the Mask property to something like "L" (character required) or "?" (optional characters). In order to show feedback to the user that the input is not acceptable, you can use the BeepOnError property or add a Tooltip to show the error message. This feedback mechanism should be implemented in the MaskedInputRejected event handler.
The MaskedTextBox control offers a ValidatingType property to check input that passes the requirements of the Mask, but may not be the correct datatype. The TypeValidationCompleted event is raised after this type validation and you can handle it to determine results.
If you still need to handle keypress events, then read on...!
The method I would recommend in your case is that instead of handling the KeyDown event (you ostensibly do not need advanced key handling capability) or using a Regex to match input (frankly, overkill), I would simply use the built-in properties of the Char structure.
private void maskedTextBox1_KeyPress(object sender, KeyPressEventArgs e)
{
Char pressedKey = e.KeyChar;
if (Char.IsLetter(pressedKey) || Char.IsSeparator(pressedKey) || Char.IsPunctuation(pressedKey))
{
// Allow input.
e.Handled = false
}
else
// Stop the character from being entered into the control since not a letter, nor punctuation, nor a space.
e.Handled = true;
}
}
Note that this snippet allows you to handle punctutation and separator keys as well.

From MSDN (This code shows how to handle the KeyDown event to check for the character that is entered. In this example it is checking for only numerical input. You could modify it so that it would work for alphabetical input instead of numerical):
// 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;
}
}
}
//If shift key was pressed, it's not a number.
if (Control.ModifierKeys == Keys.Shift) {
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;
}
}

This code will distinguish alphabetic character key presses from non alphabetic ones:
private void maskedTextBox1_KeyPress(object sender, KeyPressEventArgs e)
{
if (Regex.IsMatch(e.KeyChar.ToString(), #"\p{L}"))
{
// this is a letter
}
else
{
// this is NOT a letter
}
}
Update: note that the above regex pattern will match ONLY alphabetic characters, so it will not allow spaces, commas, dots and so on. In order to allow more kinds of characters, you will need to add those to the pattern:
// allow alphabetic characters, dots, commas, semicolon, colon
// and whitespace characters
if (Regex.IsMatch(e.KeyChar.ToString(), #"[\p{L}\.,;:\s]"))

// This is to allow only numbers.
// This Event Trigger, When key press event occures ,and it only allows the Number and Controls.,
private void txtEmpExp_KeyPress(object sender, KeyPressEventArgs e)
{
if(Char.IsControl(e.KeyChar)!=true&&Char.IsNumber(e.KeyChar)==false)
{
e.Handled = true;
}
}
//At key press event it will allows only the Characters and Controls.
private void txtEmpLocation_KeyPress(object sender, KeyPressEventArgs e)
{
if (Char.IsControl(e.KeyChar) != true && Char.IsNumber(e.KeyChar) == true)
{
e.Handled = true;
}
}

//Add a text box select it & goto Events & In the event list double click on "keypress" event.
if (!char.IsLetter(e.KeyChar))
{
MessageBox.Show("Enter only characters");
}
}

This works for me :)
private void txt_KeyPress(object sender, KeyPressEventArgs e)
{
e.Handled = !((e.KeyChar != 'ñ' && e.KeyChar != 'Ñ') && char.IsLetter(e.KeyChar));
}

Try this code
private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
e.Handled = !(char.IsLetter(e.KeyChar) || e.KeyChar == (char)Keys.Back || e.KeyChar == (char)Keys.Space);
}

Validate on text change in TextBox

I have implemented validation rules on a textBox in my WinForm and it works well. However it checks the validation only when I tab out of the field. I would like it to check as soon as anything is entered in the box and everytime the content changes. Also I'd like it to check validation as soon as the WinForm opens.
I remember doing this fairly recently by setting some events and whatnot, but I can't seem to remember how.

If you're using databinding, go to the Properties of the textbox. Open (DataBindings) at the top, click on the (Advanced) property, three dots will appear (...) Click on those. The advanced data binding screen appears. For each property of the TextBox that is bound, in your case Text, you can set when the databinding, and thus the validation, should "kick in" using the combobox Data Source Update mode. If you set it to OnPropertyChanged, it will re-evaluate as you type (the default is OnValidation which only updates as you tab).

TextChanged event
in the future you can find all of the events on the MSDN library, here's the TextBox class reference:
http://msdn.microsoft.com/en-us/library/system.windows.forms.textbox(VS.80).aspx

How will your data be valid if it isn't finished? i.e. a user types a number and you try and validate it as a date?

When binding your textbox to a bindingSource go to Advanced and select validation type
"On Property Changed". This will propagate your data to your entity on each key press.
Here is the screen shot

You should be checking on KeyPress or KeyDown events and not just your TextChanged event.
Here is a C# Example direct from the MSDN documentation:
// 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;
}
}
}
//If shift key was pressed, it's not a number.
if (Control.ModifierKeys == Keys.Shift) {
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;
}
}