.Net 3.5 Implementation of String.IsNullOrWhitespace with Code Contracts

Question

I'm attempting to use Contracts in my .Net 3.5 (C#) project. I found where I had written something like if (string.IsNullOrEmpty(s) || string.IsNullOrEmpty(s.Trim())) throw new ArgumentException("Required", "s"); at the beginning of methods.

I could keep them there and use legacy contracts. I could change it to Contract.Requires(s!= null && string.IsNullOrEmpty(s.Trim())); . But besides not being very sure about the correctness or performance of the Whitespace part of the condition, those are lame (opinion).

Instead, I tried to encapsulate the condition in a method for use in Contracts. Problem is that I still can't figure out how to describe the Whitespace part declaratively. The static checker finds several issues depending on how I write it.

I'd like to hear comments about the appropriateness of data content checks such as this one for whitespace AS WELL AS a correct definition.

public static class str
{
    [Pure]
    public static bool IsNullOrWhitespace(string s)
    {
      Contract.Ensures(s != null || Contract.Result<bool>());
      Contract.Ensures((s != null && !Contract.ForAll<char>(s, c => char.IsWhiteSpace(c))) || Contract.Result<bool>());

      if (s == null) return true;

      for (var i = 0; i < s.Length; i++)
      {
        if (!char.IsWhiteSpace(s, i))
        {
          Contract.Assume(!Contract.ForAll<char>(s, c => char.IsWhiteSpace(c)));
          return false;
        }
      }


      return true;
    }
}

This might be sloppy, but my initial experimentation method is here. The Asserts on a, b, and c are failing or are unproven.

static void Test()
{
  string a = null;
  string b = "";
  string c = "     ";
  string d = "xyz";
  string e = "  xyz  ";

  if (!str.IsNullOrWhitespace(a))
  {
    Contract.Assert(a != null);
    a = a.Trim();
  }

  if (!str.IsNullOrWhitespace(b))
  {
    Contract.Assert(b != null && b != "");
    b = b.Trim();
  }

  if (!str.IsNullOrWhitespace(c))
  {
    Contract.Assert(c != null && c != "     ");
    c = c.Trim();
  }

  if (!str.IsNullOrWhitespace(d))
  {
    d = d.Trim();
  }

  if (!str.IsNullOrWhitespace(e))
  {
    e = e.Trim();
  }
}

Answer 1

Here's the built-in contract from .NET 4:

Contract.Ensures(
    (Contract.Result<bool>() && ((str == null) || (str.Length == 0)))
    ? ((bool) true)
    : ((Contract.Result<bool>() || (str == null))
      ? ((bool) false)
      : ((bool) (str.Length > 0)))

, null, "Contract.Result<bool>() && (str == null || str.Length == 0) ||\r\n!Contract.Result<bool>() && str != null && str.Length > 0");

Despite its complexity, you can see that there is no mention of whitespace. I've tried several times, but I can't get something working that will satisfy the static checker.

There appear to be a couple of issues stopping me from writing it, so I've filed some questions on the Code Contracts forum.

My contract was:

Contract.Ensures(Contract.Result<bool>() ==
                 (s == null || Contract.Exists(s, char.IsWhiteSpace)))