Without seeing the rest of your code, we can't really comment on it! But...
The way I would do it is to create a method which returns a "valid key":
private char GetValidKey(string validChars)
{
while (true)
{
string key = Console.ReadLine().ToUpper();
if (key.Length > 0)
{
char c = key[0];
if (validChars.Contains(c))
{
return c;
}
}
}
}
Then all you have to do is pass the valid date in:
char c = GetValidKey("ABCD1234");