Add $ sign in starting and end of textbox as well as format value as comma separated(thousand separated) values in textbox WPF C#

Question

What I have tried : I am able to achieve both scenarios separately but only one works if adding the code together on the TextChanged event.

1)Add $sign in starting and ending of text as user types in Textbox code:

private void TextBox_TextChanged(object sender, TextChangedEventArgs e)
        {
            string symbol = "$"; 
            if (TB.Text != "" && !TB.Text.StartsWith("$"))
            {
                var selectionIndex = TB.SelectionStart;
                TB.Text = TB.Text.Insert(selectionIndex, symbol);
                TB.SelectionStart = selectionIndex + symbol.Length;
            }
        }

2)Formatting comma-separated(thousand separators)values as user types in textbox code:

 private void TextBox_TextChanged(object sender, TextChangedEventArgs e)
        {
            var TB = sender as TextBox;
            string textValue = TB.Text.Replace(",", "");
            if (double.TryParse(textValue, out result))
            {
                TB.TextChanged -= TextBox_TextChanged;
                TB.Text = string.Format("{0:#,#}", result);
                TB.SelectionStart = TB.Text.Length;
                TB.TextChanged += TextBox_TextChanged;
            }
      }

Desired Output - $3,333.55$ Decimal point getting dynamically 2, 3, 4 decimals like $4,566.444$, $3,3,456.33$ and so on

Answer 1

This is a simple string handling task:

private int DecimalPlaces { get; set; } = 3;
private char Delimiter { get; set; } = '$';

private void TextBox_TextInput(object sender, EventArgs e)
{
  var textBox = sender as TextBox;
  var plainInput = textBox.Text.Trim(this.Delimiter);  
  if (plainInput.EndsWith(CultureInfo.CurrentCulture.NumberFormat.NumberDecimalSeparator))
  {
    return;
  }

  if (double.TryParse(plainInput, NumberStyles.Number, CultureInfo.CurrentCulture, out double number))
  {
    var decimalPlacesFormatSpecifier = GetNumberFormatSpecifier(this.DecimalPlaces);
    textBox.Text = $"{this.Delimiter}{number.ToString(decimalPlacesFormatSpecifier, CultureInfo.CurrentCulture)}{this.Delimiter}";
  }
}

private string GetNumberFormatSpecifier(int decimalPlaces)
{
  var specifierBuilder = new StringBuilder("#,#");
  if (decimalPlaces < 1)
  {
    return specifierBuilder.ToString();
  }

  specifierBuilder.Append(".");
  for (int count = 0; count < decimalPlaces; count++)
  {
    specifierBuilder.Append("#");
  }
  return specifierBuilder.ToString();
}

To make the above solution robust, you must validate the input:

• ensure that the user can only input numbers and
• ensure that the caret can't be moved before the leading '$' and after the trailing '$'. To accomplish this you have to track the caret position.