Code Contracts: Ensure unproven on string method

Question

I'm playing around with code contracts and got a simple method inserting a given count of space characters between each character.

e.g.

Hello -> H e l l o
World -> W   o   r   l   d

The method InsertSpaceBetweenLetters is implemented within a class that offers some string-property S, the string that should be returned after modification. Here's the code

public string InsertSpaceBetweenLetters(int spaceWidth)
{
    Contract.Requires(spaceWidth > 0);
    Contract.Requires(S.Length > 0);
    Contract.Ensures(Contract.Result<string>() != null);
    Contract.Ensures(Contract.Result<string>().Length == S.Length + (S.Length - 1) * spaceWidth);

    string result = String.Empty;

    Contract.Assume(S.Length >= 0);
    for(int i = 0; i < S.Length; i++)
    {
        result += S[i];
        if (i < S.Length - 1)
            result += new String(' ', spaceWidth);
    }
    return result;
}

The static checker gives me the following warning:

ensures unproven: Contract.Result<string>().Length == S.Length + (S.Length - 1) * spaceWidth

I thought I could get rid of this warning with the assumption I'm doing before the loop:

Contract.Assume(S.Length >= 0);

But the warning is still there. What assumptions have to be made to get rid of the warning?

Thank you in advance.

Answer 1

Fundamentally, I think you're asking too much of the static checker in this case. It would be pretty impressive if it really could work that out. In this case I think you'll just have to live with it being an execution-time check rather than a compile-time check.

I don't know if there's a specific way of saying "please ensure this as a post-condition, but don't try to prove it" - calling Contract.Assume with the post-condition at the end of the method may do it, but it would probably mean evaluating it twice :(

(As an aside, your implementation is currently really inefficient. That's not the topic of this question, but is still worth looking at.)