CombineWith()

/// <summary>
/// Combine two (optionally empty) strings the way you expect.
/// </summary>
/// <param name="input">First string to combine</param>
/// <param name="suffix">Second string to append to <paramref name="input"/></param>
/// <param name="separator">The separator to insert between <paramref name="input"/> and <paramref name="suffix"/> (default=a single space)</param>
/// <returns>
/// <c>"{input}{separator}{suffix}"</c> when both are not null/empty,
/// <c>"{input}"</c> when <paramref name="suffix"/> is null/empty,
/// <c>"{suffix}"</c> when <paramref name="input"/> is null/empty, or
/// <c>string.Empty</c> when both are null/empty
/// </returns>
public static string CombineWith(this string input, string suffix, string separator = " ")
{
  if (string.IsNullOrEmpty(input))
  {
    if (string.IsNullOrEmpty(suffix))
    {
      return string.Empty;
    }
    else
    {
      return suffix;
    }
  }
  else
  {
    if (string.IsNullOrEmpty(suffix))
    {
      return input;
    }
    else
    {
      return string.Format("{0}{1}{2}", input, separator, suffix);
    }
  }
}

string firstName = "John";
string lastName = "Doe";
string fullName = firstName.CombineWith(lastName);