Can't remove text from string using button

Question

I'm trying to make a super simple program where you use 10 different buttons to write in a code (kind of like the code to the door of an appartment complex). All the buttons have this click event:

    private void number_click(object sender, EventArgs e)
    {
        var button = (sender as Button);

        if (code.Length == 4)
        {
            code.Remove(0, 1);
        }

        switch (button.Name)
        {
            case "button_1":
                code += "1";
                break;

            case "button_2":
                code += "2";
                break;

            case "button_3":
                code += "3";
                break;

            case "button_4":
                code += "4";
                break;

            case "button_5":
                code += "5";
                break;

            case "button_6":
                code += "6";
                break;

            case "button_7":
                code += "7";
                break;

            case "button_8":
                code += "8";
                break;

            case "button_9":
                code += "9";
                break;

            case "button_0":
                code += "0";
                break;
        }

        label1.Text = code;
    }

I'm simply trying to make so the number the user presses get added to the code string. When there length of the string reaches 4 it is supposed to remove the first character so that there is never more than 4 characters in the string. For some reason this doesn't seem to work. What am I doing wrong?

`code = code.Remove(0, 1);` - you need to store the return value of from Remove method — Anu Viswan, Jul 31 '21 at 14:36
Because [strings in C# are immutable](https://stackoverflow.com/a/3428743/107625). — Uwe Keim, Jul 31 '21 at 14:41

score 0 · Answer 1 · answered Jul 31 '21 at 14:37

0

If you want to update the code variable you can use Substring method.

if (code.Length == 4)
{
    code = code.Substring(1);
}

answered Jul 31 '21 at 14:37

Misha Zaslavsky

8,414
11
70
116

Because [strings in C# are immutable](https://stackoverflow.com/a/3428743/107625). – Uwe Keim Jul 31 '21 at 14:41

score 0 · Answer 2 · answered Jul 31 '21 at 14:42

Your problem is caused by the fact that System.String instances are immutable. This means you can't modify the string code points to, but you can make it point to another string. The Remove method of System.String doesn't actually remove anything, it just creates a new string without the unwanted characters and returns a reference to it. This means that instead of

code.Remove(0, 1);

You want:

code = code.Remove(0, 1);

score 0 · Accepted Answer · answered Jul 31 '21 at 15:13

0

The immediate cause of the error is that string is immutable and so

code.Remove(0, 1);

computes result string and throws it away. You should assign the result string back:

...
code = code.Remove(0, 1);
...

You can get rid of long switch case and obtain digit from the button's name:

private void number_click(object sender, EventArgs e) {
  var button = (sender as Button);

  // Add new digit to the right
  code += button.Name[button.Name.Length - 1].ToString();

  // Ensure code is at most 5 digits long by removing digits from the left
  code = code.Substring(Math.Clamp(code.Length - 5, 0, code.Length)); 
}

answered Jul 31 '21 at 15:13

Dmitry Bychenko

180,369
20
160
215

Ah okay I understand. Thank you very much! And using the name of the button to get the number was very clever. I did however not understand that last part with substring, math.clamp etc. Whould you mind explaining further how that workls? – MacA Jul 31 '21 at 15:31
@MacA: `Math.Clamp(x, min, max)` returns `x` when it within `min..max`, `min` when `x` is too small (x < min) and `max` when `x` is too large (x > max). Here I put `Math.Clamp` to ensure that `Substring` has argument in valid range – Dmitry Bychenko Jul 31 '21 at 15:37

Can't remove text from string using button

3 Answers3