last and first word in a string c#

If this is homework, don't hand this in unless you really understand it, have done LINQ (or have a supervisor that approves of off-piste learning and you're prepared to acknowledge you got outside assistance/did background learning) and are willing to explain it if asked:

    Console.WriteLine("please enter a string");
    string str = Console.ReadLine();
    string[] bits = str.Split();
    Console.WriteLine(bits.First() + " " + bits.Last());

For a non-LINQ version:

    Console.WriteLine("please enter a string");
    string str = Console.ReadLine();
    string first = str.Remove(str.IndexOf(' '));
    string last = str.Substring(str.LastIndexOf(' ') + 1);
    Console.WriteLine(first + " " + last);

Bear in mind that these will crash if there are no spaces in the string - the Split version won't

Look at String Remove and Substring

If you want to robust things up so it doesn't crash:

    Console.WriteLine("please enter a string");
    string str = Console.ReadLine();
    if(str.Contains(" ")){
      string first = str.Remove(str.IndexOf(' '));
      string last = str.Substring(str.LastIndexOf(' ') + 1);
      Console.WriteLine(first + " " + last);
    }

I'll leave a "what might we put in an else?" in that last code block, as an exercise for you :)

you can split the string and get first and last...

   var s = str.Split(' ', StringSplitOptions.RemoveEmptyEntries );
   if(s.Length >= 2) 
   {
    var first = s.First();
    var last = s.Last();
    Console.WriteLine($"{first} {last}");
   }

In general case when sentence can contain punctuation, not necessary English letters you can try regular expressions. Let's define

Word is non empty sequence of letters and apostrophes

And so we have

Code:

  using System.Linq;
  using System.Text.RegularExpressions;

  ...

  private static (string first, string last) Solve(string value) {
    if (string.IsNullOrWhiteSpace(value))
      return ("", "");

    var words = Regex
      .Matches(value, @"[\p{L}']+")
      .Cast<Match>()
      .Select(m => m.Value)
      .ToArray();

    return words.Length > 0
      ? (words[0], words[words.Length - 1])
      : ("", ""); 
  }

Demo:

  string[] tests = new string[] {
    "Simple string",                 // Simple Smoke Test
    "Single",                        // Single word which is both first an last
    "",                              // No words at all; let's return empty strings
    "words, punctuations: the end.", // Punctuations
    "Русская (Russian) строка!",     // Punctuations, non-English words
  };

  var result = string.Join(Environment.NewLine, tests
    .Select(test => $"{test,-30} :: {Solve(test)}"));

  Console.Write(result);

Outcome:

Simple string                  :: (Simple, string)
Single                         :: (Single, Single)
                               :: (, )
words, punctuations: the end.  :: (words, end)
Русская (Russian) строка!      :: (Русская, строка)

If you want to get the last and first-word try to do the following:

string sentence = "Hello World"; //Sentence
string first = sentence.Split(" ")[0]; //First word
string last = sentence.Split(" ")[sentence.Split(" ").Length -1]; //Last word
Console.WriteLine(first + " "+ last);