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Allow only specific keys on TextBox

I have a question. Examples that i find is on a "KeyPress" and they are not longer working on WPF
Can you tell me, how to allow only specified keys from keybord to be write on WPF textbox? I know about keyUp and Down functions, but how to define letters that i want to by type into it?
I think it will be easier, if i will post my code and i tell u what i want to do. What to change here?
private void textBox_KeyDown(object sender, KeyEventArgs e)
{
//something here to only allow "A" key to be pressed and displeyed into textbox
if (e.Key == Key.A)
{
stoper.Start();
}
}
private void textBox_KeyUp(object sender, KeyEventArgs e)
{
if (e.Key == Key.A)
{
//here i stop the stopwatch to count time of pressing the key
stoper.Stop();
string aS = stoper.ElapsedMilliseconds.ToString();
int aI = Convert.ToInt32(aS);
stoper.Reset();
}
}

You can use PreviewKeyDown and use e.Key to filter out what you need.
Or, in any place of your code you can use Keyboard class:
if (Keyboard.IsKeyDown(Key.E)) { /* your code */ }
UPDATE:
To forbid a key, you need to set event as handled:
if (e.Key == Key.E)
{
e.Handled = true;
MessageBox.Show($"{e.Key.ToString()} is forbidden");
}

That thing works pretty fine to me (thanks for #JohnyL):
private void textBox_KeyDown(object sender, KeyEventArgs e)
{
//something here to only allow "A" key to be pressed and displeyed into textbox
if (e.Key == Key.A)
{
stoper.Start();
}
else
e.Handled = true;
}
private void textBox_KeyUp(object sender, KeyEventArgs e)
{
if (e.Key == Key.A)
{
//here i stop the stopwatch to count time of pressing the key
stoper.Stop();
string aS = stoper.ElapsedMilliseconds.ToString();
int aI = Convert.ToInt32(aS);
stoper.Reset();
}
}

How to stop the first character in a text box from being '.'?

This is the code I currently have:
private void textBox_KeyPress(object sender, KeyPressEventArgs e)
{
e.Handled = !char.IsDigit(e.KeyChar) && !char.IsControl(e.KeyChar) && e.KeyChar != '.';
if (e.KeyChar == '.' && (sender as TextBox).Text.IndexOf('.') > -1) e.Handled = true;
}

KeyPress isn't good enough to do this kind of validation. A simple way to bypass it is to paste text into the text box with Ctrl+V. Or the context menu, no key event at all.
In this specific case, the TextChanged event will get the job done:
private void textBox_TextChanged(object sender, EventArgs e) {
var box = (TextBox)sender;
if (box.Text.StartsWith(".")) box.Text = "";
}
But there's a lot more to validating numeric values. You also need to reject stuff like 1.1.1 or 1.-2 etcetera. Use the Validating event instead. Drop an ErrorProvider on the form and implement the event like this:
private void textBox_Validating(object sender, CancelEventArgs e) {
var box = (TextBox)sender;
decimal value;
if (decimal.TryParse(box.Text, out value)) errorProvider1.SetError(box, "");
else {
e.Cancel = true;
box.SelectAll();
errorProvider1.SetError(box, "Invalid number");
}
}

You probably want to use the TextChanged event, since the user could paste in values. For the best experience given the requirements, I'd suggest simply removing any leading . characters.
void textBox1_TextChanged(object sender, EventArgs e)
{
if (textBox1.Text.StartsWith("."))
{
textBox1.Text = new string(textBox1.Text.SkipWhile(c => c == '.').ToArray());
}
}
This does not address a requirement to use only digits -- wasn't clear in the question if that is the case.

This works for copy and pasting too.
private void textBox1_KeyUp(object sender, KeyEventArgs e)
{
int decimalCount=0;
string rebuildText="";
for(int i=0; i<textBox1.Text.Length; i++)
{
if (textBox1.Text[i] == '.')
{
if (i == 0) break;
if (decimalCount == 0)
rebuildText += textBox1.Text[i];
decimalCount++;
}
else if ("0123456789".Contains(textBox1.Text[i]))
rebuildText += textBox1.Text[i];
}
textBox1.Text = rebuildText;
textBox1.SelectionStart = textBox1.Text.Length;
}

You can try this:
private void TextBox_TextChanged(object sender, EventArgs e)
{
TextBox.Text = TextBox.Text.TrimStart('.');
}

Convert KeyDown keys to one string C#

I have magnetic card reader, It emulates Keyboard typing when user swipes card. I need to handle this keyboard typing to one string, when my WPF window is Focused. I can get this typed Key list, but I don't know how to convert them to one string.
private void Window_KeyDown(object sender, KeyEventArgs e)
{
list.Add(e.Key);
}
EDIT: Simple .ToString() method not helps. I've tried this already.

Rather than adding to a list why not build up the string:
private string input;
private bool shiftPressed;
private void Window_KeyDown(object sender, KeyEventArgs e)
{
if (e.Key == Key.LeftShift || e.Key == Key.RightShift)
{
shiftPressed = true;
}
else
{
if (e.Key >= Key.D0 && e.Key <= Key.D9)
{
// Number keys pressed so need to so special processing
// also check if shift pressed
}
else
{
input += e.Key.ToString();
}
}
}
private void Window_KeyUp(object sender, KeyEventArgs e)
{
if (e.Key == Key.LeftShift || e.Key == Key.RightShift)
{
shiftPressed = false;
}
}
Obviously you need to reset input to string.Empty when you start the next transaction.

...or you can try this:
string stringResult = "";
list.ForEach(x=>stringResult+=x.ToString());
EDIT:
After good Timur comment I decided to suggest this:
you can use keyPress event to everything like this:
string stringResult = "";
private void Window_KeyPress(object sender, KeyPressEventArgs e)
{
stringResult += e.KeyChar;
}

Listen To PreviewTextInput Event Instead ...
the TextCompositionEventArgs has a property called "Text" which give you the text representation for key
for example Key.D2 will be just "2" ...i think it will
do the purpose
private void MainWindow_OnPreviewTextInput(object sender, TextCompositionEventArgs e)
{
list.Add(e.Text);
}

You could have a member variable which is a StringBuilder.
something like
class A
{
StringBuilder _contents = new StringBuilder();
private void Window_KeyDown(object sender, KeyEventArgs e)
{
_contents.Append(e.Key.ToString());
}
}
You would have to create a new StringBuilder each time a new card was swiped and then to get the string you would use _contents.ToString();

String combined = String.Empty;
private void Window_KeyDown(object sender, KeyEventArgs e)
{
combined = combined + e.Key.ToString();
}

Keypress To Simulate A Button Click in C#

Ok, so I'm in the process of making a Tic-Tac-Toe game to help me learn C#.
I'm attempting to add a bit of functionality to it, so I want people to be able to use the NumPad on a computer to simulate clicking the buttons.
Here is what I have but when I use the NumPad the buttons don't click.
Can any of you see a reason as to why?
//===============================
// start NumPad Simulate Clicks
// NumPad MyButtons
// 7 8 9 1 2 3
// 4 5 6 4 5 6
// 1 2 3 7 8 9
//===============================
public void myControl_NumPad7(object sender, KeyPressEventArgs e)
{
if (e.KeyChar == (char)Keys.NumPad7)
{
button1_Click(null, null);
}
}
public void myControl_NumPad8(object sender, KeyPressEventArgs e)
{
if (e.KeyChar == (char)Keys.NumPad8)
{
button2_Click(null, null);
}
}
public void myControl_NumPad9(object sender, KeyPressEventArgs e)
{
if (e.KeyChar == (char)Keys.NumPad9)
{
button3_Click(null, null);
}
}
public void myControl_NumPad4(object sender, KeyPressEventArgs e)
{
if (e.KeyChar == (char)Keys.NumPad4)
{
button4_Click(null, null);
}
}
public void myControl_NumPad5(object sender, KeyPressEventArgs e)
{
if (e.KeyChar == (char)Keys.NumPad5)
{
button5_Click(null, null);
}
}
public void myControl_NumPad6(object sender, KeyPressEventArgs e)
{
if (e.KeyChar == (char)Keys.NumPad6)
{
button6_Click(null, null);
}
}
public void myControl_NumPad1(object sender, KeyPressEventArgs e)
{
if (e.KeyChar == (char)Keys.NumPad1)
{
button7_Click(null, null);
}
}
public void myControl_NumPad2(object sender, KeyPressEventArgs e)
{
if (e.KeyChar == (char)Keys.NumPad2)
{
button8_Click(null, null);
}
}
public void myControl_NumPad3(object sender, KeyPressEventArgs e)
{
if (e.KeyChar == (char)Keys.NumPad3)
{
button9_Click(null, null);
}
}

EDIT
Noticed that I have to be clearer about what I mean...
From the code you posted I suspect you have 9 controls you added your key-events to. These controls will only receive key-events when they are focused.
To handle keys globally for the form, you have to set Form.KeyPreview to true. Also, I'd not handle the keys the way you do, but add a Form.KeyDown event and write something like:
switch (e.KeyCode)
{
case Keys.NumPad9:
e.Handled = true;
button3.PerformClick();
break;
case Keys.NumPad8:
e.Handled = true;
button2.PerformClick();
break;
// And so on
}
This will handle the NumPad-Keys within the form - you can then remove all the event handlers you posted in your question.
To programatically "click" a button, you should use the Button.PerformClick() method, as more than one event handler may be added to the click event, which would not be called otherwise.
EDIT 2
Syntax for the switch-statement was invalid. Of course every "case" must start with the case keyword... Now it should work.

You need to use button1.PerformClick(); for every button to invoke event correctly, here's info.

Disable warning/error beep when changing focus

Whenever I change the focus from one textbox to another it plays an irritating warning/error beep.
Example:
public void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
if (e.KeyChar == (char)Keys.Return)
textBox2.Focus();
}
whenever I press Enter it changes the focus to textBox2 and gives the warning beep.
Any help to disable this would be appreciated.
Thank you.

I think you want to add e.Handled = true to the event handler:
public void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
if (e.KeyChar == (char)Keys.Return)
{
textBox2.Focus();
e.Handled = true;
}
}
A side node: you should be able to use the KeyCode instead of the KeyChar property, avoiding the cast:
public void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
if (e.KeyCode == Keys.Return)
{
textBox2.Focus();
e.Handled = true;
}
}

e.SuppressKeyPress = true;