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How to disable / cancel undo button in xamarin

I am creating an application that has an entry. I am trying to restrict the entry to only allow for numeric input. I have already tried using Keyboard = "Numeric". For the iPad, however, the keyboard has more characters than just numbers. So I had to restrict what is entered. When I do this however, if the user types in a parenthesis, for example, it does stop the character from being entered. But then if the user presses undo, it crashes. I assume this is because the software keyboard is separate from the app, so it is looking for that parenthesis character, but it isn't there. Here is my code:
private void Entry_TextChanged(object sender, TextChangedEventArgs e)
{
Entry theEntry = (Entry)sender;
string entryText = theEntry.Text;
if(entryText != null && entryText.Length != 0)
{
char theMostRecentInput = entryText[entryText.Length - 1];
if(!Extension.IsNumeric(theMostRecentInput))
{
theEntry.TextChanged -= Entry_TextChanged;
theEntry.Text = e.OldTextValue;
theEntry.TextChanged += Entry_TextChanged;
}
}
}
Thanks for the help!

This issue will occur when Validation like special character, Max Limit, etc... for Input field are implemented.
By that time undo action will have extra character count than current input field text characters length in iOS ShouldChangeCharacters Delegate. This leads to app crash.
One of the solution is to return as false in such scenario instead of disabling undo buttons. Below solution worked for me.
public class ExtEntryRenderer : EntryRenderer
{
protected override void OnElementPropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
{
base.OnElementPropertyChanged(sender, e);
if (Control != null)
{
Control.ShouldChangeCharacters += (UITextField textField, NSRange range, string replacementString) =>
{
if (range.Location + range.Length > ((UITextField)textField).Text.Length)
return false;
return true;
};
}
}
}

I would do this in a Entry custom renderer that way you you can control the input via ShouldChangeCharacters and not have to kludge it by allowing the input and then having to remove the handler and change the text back to the old value...
Here is a quick example that allows numeric, It also automatically handles clipboard pasting non-numeric strings as those would be disallowed. I am using the NSCharacterSet.DecimalDigits character set as that would be internationalized by the OS, but you could allow/disallow any chars of your choosing.
You could also include haptic, visual or audio feedback on the disallowed/rejected entries...
[assembly: ExportRenderer(typeof(NumericEntry), typeof(NumericEntryRenderer))]
namespace Forms_PCL_Tester.iOS
{
public class NumericEntryRenderer : EntryRenderer
{
protected override void OnElementChanged(ElementChangedEventArgs<Xamarin.Forms.Entry> e)
{
base.OnElementChanged(e);
if (e.NewElement != e.OldElement)
if (Control != null)
{
Control.KeyboardType = UIKeyboardType.NumbersAndPunctuation;
Control.ShouldChangeCharacters += (UITextField textField, NSRange range, string replacementString) =>
{
foreach (var aChar in replacementString)
if (!NSCharacterSet.DecimalDigits.Contains(aChar))
return false;
return true;
};
}
}
}
}

Clearing out a c# form text box

I am making a simple hangman style game in a C# form. I have it set as textBox1_TextChanged. Except everytime the user backspaces their guess letter it takes in blank space. How can I make it so after the message saying right/wrong it clears the space. I am getting annoyed at it telling the user they made a wrong guess after they backspace. This is my first post on this forum so sorry if the code text is weird. I just want the program to clear the text in the textBox after they guess.
UPDATE: Added suggested information. Now it does everything it is supposed to do. Except it pops up a windows saying " was found in the target word". This happens if guessLetter == null || guessLetter == correct || guessLetter == false.
private void textBox1_TextChanged(object sender, EventArgs e)
{
string guessLetter = textBox1.Text;
//textBox1.ReadOnly = true;
if (targetWord == null)
{
MessageBox.Show("Please start a new game.");
textBox1.Text = ("");
}
else
{
if (targetWord.Contains(guessLetter))
{
MessageBox.Show(guessLetter + " was found in the word");
}
else
{
MessageBox.Show(guessLetter + " was not found in the word");
incorrectGuessCtr++;
textBox3.Text = incorrectGuessCtr.ToString();
}
textBox1.Text = ("");
}
}

Don't only check if the targetWord is null, but also the guessLetter. You'd better use string.IsNullOrEmpty too, since it also checks if the string is empty:
if (!string.IsNullOrEmpty(targetWord) && !string.IsNullOrEmpty(guessLetter))
{
...
}
I guess you should also check if there is exactly one letter entered. That would mean this additional check:
if (guessLetter.Length == 1)
{
...
}

You will enter this event when you write code that changes Text property in textbox. I mean this.
textBox3.Text = incorrectGuessCtr.ToString();
Put something in function arguments or set some flags so that you can identify whether the event is called from user input or your clearing the text.
Just check how many times this function is called when user press backspace. You will get the idea.

Make only 1 instance of a character allowed to be entered C#

I would like to allow only one instance of a character to appear in sequence in a textbox. In other words, I would like to prevent the user from entering sequential duplicate characters in my textbox. For example, if I were to make a calculator program, I would like to prevent the user from entering (( by accident instead of (.

You can detect the textbox's keydown event. And in the event, you check the last and second last character to see if it's the same. If it's the same, you can just remove the last character of the string.
But keep in mind that the above method has problem when your user pastes the entire string to the textbox.

The function will look the same for any textbox-like object, but of course the event to attach to will differ. Assuming Winforms TextBox, you will attach to the TextChanged event (i.e. mytextbox.TextChanged = mytextbox_TextChanged), and the function will look similar to the one below:
private void mytextbox_TextChanged(object sender, EventArgs e)
{
if (mytextbox.Text.Length < 2) { return; }
var stringToCheck = mytextbox.Text;
for (var i = 1; i <= stringToCheck.Length-1; i += 1)
{
if (stringToCheck[i].Equals(stringToCheck[i-1])
{
mytextbox.Text = stringToCheck.Remove(i, 1);
// Next two lines put cursor at end of textbox instead of beginning
mytextbox.SelectionStart = mytextbox.Text.Length;
mytextbox.SelectionLength = 0;
}
}
}

WPF Textbox - Append only

I need a textbox which can contain a maximum of 8 chars which can be achieved by setting
this.textBox1.MaxLength = 8;
However, sometimes my program will pre-enter a few characters (i.e. 6). When presented to the user, I want him to be able to append the missing 2, but not be able to delete the pre-entered 6 characters.
I do not want to have something like
if(!textBox1.Text.Equals(strPreEntered)) throw new Exception(); // Or show a messagebox, or ..
Ideally I would like the Textbox to not accept any backspaces in case textBox1.Length == strPreEntered.Length

What you really want is a flag indicating whether the TextBox has had pre-entered characters sent to it or not. Set it to true whenever you send the pre-entered characters to the TextBox.
Then in the TextBox's OnKeyPress event, ignore the keypress if the KeyCode is a backspace if the flag is set.
public class FormTest : Form
{
public FormTest() : base()
{
LimitedTextBox tb = new LimitedTextBox();
this.Controls.Add(tb);
tb.Text = "123456";
tb.MaxLength = 8;
tb.HasPreenteredText = true;
}
}
public class LimitedTextBox : TextBox
{
private int preenteredTextLength = -1;
private bool hasPreenteredText = false;
public bool HasPreenteredText
{
get { return hasPreenteredText; }
set
{
if (value == true)
{ preenteredTextLength = this.TextLength; }
else
{ preenteredTextLength = -1; }
hasPreenteredText = value;
}
}
protected override void OnKeyPress(KeyPressEventArgs e)
{
if (this.TextLength <= preenteredTextLength && e.KeyChar == '\b')
{ e.Handled = true; } // Causes the KeyPress to be skipped as it was already 'handled'
if (this.SelectionStart < preenteredTextLength) // Prevent user from overwriting/deleting selected text beyond the minimum text length
{ e.Handled = true; }
base.OnKeyPress(e);
}
}
The above code's a little sloppy but you would of course want to clean it up. Maybe have a SetPreenteredText() method that handles setting the boolean value within the LimitedTextBox control and make the boolean private, you don't want to trouble the caller with that sort of thing. The code works as expected though and accomplishes what you seem to want.

Is there any C# implementation or way to have a textbox that takes a key sequence as input?

I have an implementation which hooks into the keydown event, which suppresses the keypress event and does some magic. Now I want to show something more friendly than "ControlKey" etc, so there's a switch you can see in the code below. Except I've realised things like the number keys along the top end up as D1, D2, etc., and then there's things like Add showing up for the numpad +. In addition, Print Screen doesn't seem to be recognised.
Is there something I'm missing?
This is a hard question for me to describe fluently, but hopefully you get what I mean. If not I guess I'll be replying to comments and improving this as I go.
private int numKeys = 0;
private List<int> shortcutKeys = new List<int>();
private void textBoxRegionKeys_Click(object sender, EventArgs e)
{
textBoxRegionKeys.SelectAll();
}
private void textBoxRegionKeys_KeyDown(object sender, KeyEventArgs e)
{
// There are certain keys we want to ignore...
if (e.KeyCode != Keys.Delete && e.KeyCode != Keys.Back)
{
// We can handle this ourselves, thanks
e.SuppressKeyPress = true;
// Shortern what we show
string ret = e.KeyCode.ToString();
switch (ret)
{
case "ControlKey": ret = "Ctrl"; break;
case "ShiftKey": ret = "Shift"; break;
case "Menu": ret = "Alt"; break;
}
// If we haven't selected anything, we should be appending
if (textBoxRegionKeys.SelectionLength == 0)
{
if (numKeys > 0)
{
// Does our key already exist in the list?
if (shortcutKeys.Exists(x => x == e.KeyValue))
{
return;
}
textBoxRegionKeys.Text += " + ";
}
textBoxRegionKeys.Text += ret;
shortcutKeys.Add(e.KeyValue);
numKeys++;
}
else
{
textBoxRegionKeys.Text = ret;
shortcutKeys.Clear();
shortcutKeys.Add(e.KeyValue);
numKeys = 1;
}
}
}

The TextBox KeyDown/KeyPress etc will only be raised for keys that may be accepted as input in to the text box (and associated modifiers). As such, you will not see keys handled such as Print Screen etc. The best option I can think of is not ideal, but you could override the ProcessKeyPreview or some other Form level Message interceptor to get notified of ANY key press. Something like...
protected override bool ProcessKeyPreview(ref Message m)
{
var keyCode = (Keys)Enum.ToObject(typeof (Keys), m.WParam);
//Insert some logic
return base.ProcessKeyPreview(ref m);
}
Of course, this method will be invoked whenever the FORM has focus, and a key is pressed, so you would have to filter down by doing some form of check (which again is not ideal)
if(ReferenceEquals(ActiveControl, textBoxRegionKeys)) {}
Which if your dealing with things like UserControls will be very unreliable.
As for formatting in to nice friendly messages, I think you basically you will need your own map of special characters... I am not aware of any Globalized lookup for Keys. I will dig a little and update the answer if I find something.
Edit
Did a little digging and couldn't find anything obvious for nice key mappings. I would just create a map of "friendly" key names:
private static readonly Dictionary<Keys, String> KeysMap = new Dictionary<Keys, String>
{
{ Keys.D1, "1"},
{ Keys.D9, "9"}
};
And do something like:
String friendlyKeyCode;
if (!KeysMap.TryGetValue(keyCode, out friendlyKeyCode))
friendlyKeyCode = keyCode.ToString();
Personally, I find this approach better than a massive switch... but that works too.