Refactor multiple if-else conditions when condition is a minor change

You really need to store the original values into an int array or collection. The reason behind that is you don't need to recasting the values every now and then. Keep the original values in their correct form, and then cast them to string or any other type whenever needed. If you just keep going doing the same concept (using strings on everything) you'll end up having many parts that need to be refactored, and probably it will impact the overall performance.

Your Pin Code can be simplified by using string[] for the code mask, and List<int> for the actual integers.

public string PinCode { get; set; } = string.Empty;

private string[] _codeMask = new string[] { "_", "_", "_", "_" };

private List<int> _pinNumber = new List<int>(4); // limit list size to 4 elements, if you remove it would dynamic size. 

private async Task<bool> SelectedButton(Button button)
{
    if(PinCode == 4) { return true; }
        
    // validate input
    if(int.TryParse(button.Text, out int number) && (number < 10 && number > 0))
    {
       if(_pinNumber.Count == 4) { return; } 
        
        _pinNumber.Add(number);
        
        var maskPosition = _pinNumber.Count - 2;
        
        var numberPosition = _pinNumber.Count - 1;
     
         _codeMask[numberPosition] = number.ToString();                                        
            
        if(maskPosition != -1)
        { 
          _codeMask[maskPosition] = "*";
        }
        
        PinCode = String.Join(' ', _codeMask);              
    }
    
    return false;
}

Then just use the _codeMask or _pinNumber to assign their values to any releated variable. But I would suggest you get rid of PinNumberOne , PinNumberX variables, and use _pinNumber[x] directly. would be more appropriate and much maintainable.

This is a common problem. Sometimes it's tricky to generalise a process even when its iterative in nature.

By keeping track of both:

• the location of the user's cursor (cursor_position), and
• an iterable data structure storing which characters are shown (pinNumbers),

it is possible to implement this behaviour without so much repeated code. This gave the desired behaviour after some thorough testing (in my imagination).

public string PinCode { get; set; } = string.Empty;

private int cursor_position = -1;
private string[] pinNumbers = new string[] {"_", "_", "_", "_"};

    private async Task<bool> SelectedButton(Button button)
    {
        //If button is a number then
        if (button.Text != null)
        {
            if ( cursor_position < 3 )
            {
                PinCode = PinCode + button.Text;

                if ( cursor_position > 0 )
                {
                    pinNumbers[cursor_position] = "*";
                }

                cursor_position += 1;
                pinNumbers[cursor_position] = button.Text;
            }
        } else { // apparently backspace?
            if ( cursor_position >= 0 )
            {
                PinCode = PinCode.Remove(PinCode.Length - 1);
                pin_numbers[cursor_position] = "_";
                cursor_position -= 1;
            }
        }

        pinNumberOne   = pinNumbers[0]; // update actual shown values with array
        pinNumberTwo   = pinNumbers[1]; // you could probably take out the "pinNumberX" middleman
        pinNumberThree = pinNumbers[2]; // but I dont know what relation these variables have to
        pinNumberFour  = pinNumbers[3]; // the rest of your code

        return true;
    }

Depending on how you are assigning values to the actual rendered characters you may be able to remove the first four of the last five lines. You likely could just refer the rendered components directly to the values in the pinNumbers array.