Implementing strings and the OR function (||) in an if statement

You're almost there:

if (justThePlus == "+" || justThePlus =="1" || justThePlus =="2")

Other issues:

1. Double == sign
2. remove ; at the end of the if statement

To improve readability:

string[] allowedValues = { "+", "1", "2" };
if (allowedValues.Contains(justThePlus)) {

OR statements need a condition either side:

if (justThePlus == "+" || justThePlus == "1" || ....)

Also:

• Use == instead of = for string comparison
• Remove semi-colon from the end of your if statement.

You can use an array contains:

if (new[] { "+" ,  "1" ,  "2" ,  "3" ,  "4" ,  "5" ,  "6" , "7" ,  "8" ,  "9" , "0" }.Contains(justThePlus));
{

}

This is a different logic to achieve the same.

var validChars= "+1234567890" ;
if(justThePlus.Length==1 && validChars.IndexOf(justThePlus)>=0)
{
}

I think you are making this way too hard.

// Ensures the string has 9 digits and optionally starts with a "+"
Regex regex = new Regex(@"^(\+)?([0-9]{9})");
string input;

while (!regex.IsMatch(input))
{
    Console.Write("Please enter your phone number: ");
    input = Console.ReadLine();
}

|| operator expects both operand as a bool value. This operand performs logical-OR between both operands.

As you can see, using this operator with string operands is meaningless.

You can put your all numbers and + sign in a List<string> and you can check it with Contains and Any methods like;

var listOfNumbersAndPlus = new List<string>(){"+", "1", "2", "3", "4", "5", "6", "7", "8", "9", "0"};
if(listOfNumbersAndPlus.Any(justThePlus.Contains))

You can use Regex to validate input.

var input = "+12345678";
var pattern = @"\+\d{8}"; // matches number, which is at leas 8-digit long and starts with '+' sign
var isValidNumber = Regex.IsMatch(input, pattern);

Phone numbers can be represented by user in many different formats, e.g.:

 +1(555)123-45-67
  8 555 123 45 67
     +75551234567 

So I strongly suggest you using regular expressions; in your case it could be

https://www.safaribooksonline.com/library/view/regular-expressions-cookbook/9781449327453/ch04s03.html

For instance

  Console.Write("Please enter your phone number: ");
  string input = Console.ReadLine();

  //TODO: you may want to update/change this pattern
  String pattern = "^\+(?:[0-9] ?){6,14}[0-9]$";

  Boolean isNumberValid = Regex.IsMatch(input, pattern); 

Then, if input string is a valid one, you can just collect all digits within it with a help of Linq:

  // Standard phone number representation: e.g. 155512345678
  String stdNumber = new String(input.Where(ch => ch >= '0' && ch <= '9').ToArray());

Your question has been answered directly (justThePlus == "+" || justThePlus =="1"...). Here's a more efficient way to do the same thing:

    switch (justThePlus)
    {
        case "+" :
        case "0" :
        case "1" :
        case "2" :
        case "3" :
        case "4" :
        case "5" :
        case "6" :
        case "7" :
        case "8" :
        case "9" :
            // ???
            break;

        default :
            // ???
            break;

    }