Delete Selected Text from Textbox and Enter New Char in C#.NET - c#-4.0

Question

I am trying to delete selected text from textbox and enter new character in place of it.
For example, if textbox consists of 123456 and I select 345, and press r on the keyboard, it should replace the selected text.
here is my code:
string _selectText = txtCal.SelectedText;
string _text = Convert.ToString(btn.Text);
if (_selectText.Length > 0) {
int SelectionLenght = txtCal.SelectionLength;
string SelectText = txtCal.Text.Substring(txtCal.SelectionStart, SelectionLenght);
txtCal.Text = ReplaceMethod(SelectText, _text);
}
//replace method function
public string ReplaceMethod(string replaceString, string replaceText) {
string newText = txtCal.Text.Replace(replaceString, replaceText);
return newText;
}
Can anyone show me where my mistake is?

Answer 1

The replace-based answer offered above may well replace the wrong instance of the selection, as noted in the comments. The following works off positions instead, and doesn't suffer that problem:
textbox1.Text = textbox1.Text.Substring(0, textbox1.SelectionStart) + textbox1.Text.Substring(textbox1.SelectionStart + textbox1.SelectionLength, textbox1.Text.Length - (textbox1.SelectionStart + textbox1.SelectedText.Length));

Answer 2

The following does what you want and then selects the replacing text :)
string _text = Convert.ToString(btn.Text);
int iSelectionStart = txtCal.SelectionStart;
string sBefore = txtCal.Text.Substring(0, iSelectionStart);
string sAfter = txtCal.Text.Substring(iSelectionStart + txtCal.SelectionLength);
txtCal.Text = sBefore + _text + sAfter;
txtCal.SelectionStart = iSelectionStart;
txtCal.SelectionLength = _text.Length;

Answer 3

Try this instead
if (textbox1.SelectedText.Length > 0)
{
textbox1.Text = textbox1.Text.Replace(text1.Text.Substring(textbox1.SelectionStart, textbox1.SelectionLength), btn.Text);
}

Answer 4

This is essentially the same as other answers, but formatted differently using C# 6.0.
// If there is selected text, it will be removed before inserting new text.
// If there is no selected text, the new text is inserted at the caret index.
string before = textBox.Text.Substring(0, textBox.SelectionStart);
string after = textBox.Text.Substring(textBox.SelectionStart + textBox.SelectedText.Length);
textBox.Text = $"{before}{insertText}{after}";
textBox.CaretIndex = $"{before}{insertText}".Length;
Note that I set the CaretIndex to a new position after changing the text. This may be useful since the caret index resets to zero when changing the text like this. You may also want to focus the textbox to draw the user's attention to the change and allow them to know where the caret currently is.