using System;
using System.Linq;

public class Test
{
  public static void Main() {
    string[] tests = { "a", 1.ToString(), "321", "xyz" };
    foreach (var test in tests) {
      Console.WriteLine(test.ToChar());
    }
  }
}

public static class StringExtensions {
  public static char ToChar(this string value) {
    if (string.IsNullOrEmpty(value)) throw new ArgumentException();
    return value.First();
  }
}