Caesar Cipher encryption VB.net - vb.net

Question

I am trying to make an encryption method in VB.net.
I plan to use arrays so you input the value and it then compares two arrays changing the value.
Console.WriteLine("Please input text")
Dim UserInput As String = Console.ReadLine
UserInput = UserInput.ToUpper()
Console.WriteLine("Original Input is " & UserInput)
Dim EncriptedText As String
Dim Numbers() As String = {"1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12", "13", "14", "15", "16", "17", "18", "19", "21", "22", "23", "24", "25", "26", "27"}
Dim Letters() As String = {"A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z", " "}
For counter As Integer = 0 To UserInput.Length - 1
Dim pos As Integer = Array.IndexOf(Numbers, UserInput.Chars(counter))
Dim CharValue As Char = Letters.ElementAt(pos)
UserInput = UserInput + CharValue
Console.WriteLine(UserInput)
Next
Console.Read()
End Sub
It throws an error when I try to run it.
Does anyone have any idea how I could fix it?
Error : Found on The Dim CharValue As Char Line
An unhandled exception of type 'System.ArgumentOutOfRangeException' occurred in mscorlib.dll
Additional information: Index was out of range. Must be non-negative and less than the size of the collection.
I was thinking it could be because I am trying to convert numbers to chars and if so what other methods could I use?

Answer 1

1) You are using the wrong array to get the index of a letter, and so the wrong array to get its enciphered value.
2) When you use Array.IndexOf like that, you need to give it a string to look for rather than a char. In your code, it is returning -1 because it didn't find the char in the array of strings (once the correct array is used).
3) You're modifying the input string in the loop, I guess you meant to use a separate variable for it.
Console.WriteLine("Please input text")
Dim userInput As String = Console.ReadLine()
userInput = userInput.ToUpper()
Console.WriteLine("Original input is " & userInput)
Dim numbers() As String = {"1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12", "13", "14", "15", "16", "17", "18", "19", "21", "22", "23", "24", "25", "26", "27"}
Dim letters() As String = {"A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z", " "}
Dim encryptedText As String = ""
For counter As Integer = 0 To userInput.Length - 1
Dim pos As Integer = Array.IndexOf(letters, (userInput.Chars(counter)))
Dim cipherValue As String = numbers.ElementAt(pos)
encryptedText = encryptedText & cipherValue
Console.WriteLine(encryptedText)
Next
Console.Read()
Please note that it is usual to start variable names with a lowercase letter.
Now that you can get output, you can go on to find the other problems with the code :)