Please suggest how to stuff a space character at the deleted position in a textbox

Here is a complete code of your problem.

public partial class TrapDeleteBackSpaceTextBox : TextBox
{
    string m_TextBeforeTheChange;
    int m_CursorPosition = 0;
    bool m_BackPressed = false;
    bool m_DeletePressed = false;

    public TrapDeleteBackSpaceTextBox():base()
    {

    }

    protected override void OnKeyDown(KeyEventArgs e)
    {
        m_TextBeforeTheChange = this.Text;
        m_BackPressed = (e.KeyData == Keys.Back) ? true : false;
        m_DeletePressed = (e.KeyData == Keys.Delete) ? true : false;
        m_CursorPosition++;
        base.OnKeyDown(e);
    }
    protected override void OnTextChanged(EventArgs e)
    {
        this.SelectionStart = m_CursorPosition;
        base.OnTextChanged(e);
    }
    protected override void OnKeyUp(KeyEventArgs e)
    {
        if (m_BackPressed || m_DeletePressed)
        {
            this.Text = m_TextBeforeTheChange;
            this.Text = this.Text + " "; //I added Space you an add any charecter
        }
        base.OnKeyUp(e);
    }
}

public partial class NumberBox : MaskedTextBox
{

    public NumberBox()
    {
        this.TextAlign = HorizontalAlignment.Right;
        this.Mask = "#####.##";
        this.PromptChar = ' ';
        this.KeyPress += NumberBox_KeyPress;
        this.KeyDown += NumberBox_KeyDown;
    }

    void NumberBox_KeyDown(object sender, KeyEventArgs e)
    {
        MaskedTextBox SenderTextBox = (MaskedTextBox)sender;
        if (e.KeyCode == Keys.Enter)
        {
            String NumberBoxMask = SenderTextBox.Mask;
            String NumberBoxText = SenderTextBox.Text;
            Int32 DecimalPointLocation = NumberBoxMask.IndexOf('.');

            /* Storing the Length of the mask
               before the decimal point location */
            Int32 IntegerLength = StrLeft(NumberBoxMask, DecimalPointLocation).Length;

            /* Storing the Length of the mask
               after the decimal point location */
            Int32 DecimalLength = StrRight(NumberBoxMask, NumberBoxMask.Length -  (DecimalPointLocation + 1)).Length;

            Double aDoubleNum = 0.00;

            /* Storing the integer part in a string.
               Even if all numbers are deleted in the
               integer part, we still get spaces
               to the extent of the integer part length atleast */
            String strInteger = NumberBoxText.Substring(0, IntegerLength);

            /* Storing the decimal part in a string.  If numbers are deleted from the
               decimal portion then an error occurs, hence applied this logic,
               though looks absurd, I accept ...
               (Better logic to handle this, Welcome) */

            String strDecimal = "".PadRight(DecimalLength, '0');
            for (int mi = DecimalLength; mi > 0; mi--)
            {
                try
                {
                    strDecimal = NumberBoxText.Substring(NumberBoxMask.Length - DecimalLength, mi);
                    break;
                }
                catch { }
            }

            /* Removing the spaces if any in
               between numbers in the integer part */
            for (int mi = 0; mi < strInteger.Length; mi++)
            {
                if (strInteger[mi] == ' ')
                    strInteger = strInteger.Remove(mi, 1);
            }

            /* Removing the spaces if any in
               between numbers in the decimal part */
            for (int mi = 0; mi < strDecimal.Length; mi++)
            {
                if (strDecimal[mi] == ' ')
                    strDecimal = strDecimal.Remove(mi, 1);
            }

            /* Arriving the finally formatted Double number. */

            String FormattedNum = "";
            FormattedNum = strInteger.Trim().PadLeft(IntegerLength, ' ') + "." + strDecimal.Trim().PadRight(DecimalLength, '0');
            SenderTextBox.Text = FormattedNum;

            // Just check if it is valid Double type digit
            try
            {
                aDoubleNum = Double.Parse(FormattedNum);
                e.Handled = true;
            }
            catch
            {
                MessageBox.Show("Invalid Value);
                e.Handled = false;
            }
        }
    }

    public void NumberBox_KeyPress(object sender, KeyPressEventArgs e)
    {
     /* This logic is as per suggestion from */ <a href="http://www.codeproject.com/Questions/869033/Please-guide-me-how-to-format-the-textbox-to-take?arn=0">Please guide me how to format the textbox to take input of double values.</a>[<a href="http://www.codeproject.com/Questions/869033/Please-guide-me-how-to-format-the-textbox-to-take?arn=0" target="_blank" title="New Window">^</a>]

        MaskedTextBox SenderTextBox = (MaskedTextBox)sender;
        if (e.KeyChar == '.')
        {
            Int32 PositionOfDecimalPoint = SenderTextBox.Text.IndexOf('.');
            PositionOfDecimalPoint++;
            SenderTextBox.SelectionStart = PositionOfDecimalPoint;
            SenderTextBox.SelectionLength = 0;
            e.Handled = true;
        }
    }

    private String StrLeft(String mstr, int mlen)
    {
        return mstr.Substring(0, mlen);
    }


    private String StrRight(String mstr, int mlen)
    {
        return mstr.Substring((mstr.Length - mlen), mlen);
    }
}