DocIndex

public class Indexer
{
    private Dictionary<string, HashSet<string>> terms = new Dictionary<string, HashSet<string>>(StringComparer.OrdinalIgnoreCase);
    private Dictionary<string, HashSet<string>> docTerms = new Dictionary<string, HashSet<string>>(StringComparer.OrdinalIgnoreCase);

    public void Index(string docId, string text)
    { 
        var words = new HashSet<string>(text.Split(), StringComparer.OrdinalIgnoreCase);
        HashSet<string> toRemove;
        if (docTerms.TryGetValue(docId, out toRemove))
        {
            toRemove.ExceptWith(words);
        }
	docTerms[docId] = words;
		
        foreach (var term in words)
        {
            HashSet<string> val;
            if (terms.TryGetValue(term, out val) == false)
            {
                val = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
                terms[term] = val;
            }
            val.Add(docId);
        }
		
        if (toRemove != null)
        {
            foreach (var term in toRemove)
            {
                RemoveTerm(term, docId);
            }
        }		
    }

    public List<string> Query(string term)
    {
        HashSet<string> val;
        terms.TryGetValue(term, out val);
        return new List<string>(val ?? Enumerable.Empty<string>());
    }
	
	public void Delete(string docId)
	{
		HashSet<string> toRemove;
		if (docTerms.TryGetValue(docId, out toRemove))
		{
			foreach (var term in toRemove)
			{
				RemoveTerm(term, docId);
			}
			docTerms.Remove(docId);
		}
	}
	
	private void RemoveTerm(string term, string docId)
	{
      HashSet<string> val;
      if (terms.TryGetValue(term, out val))
      {
          val.Remove(docId);
          if (val.Count == 0)
          {
              terms.Remove(term);
          }
      }
	}
}



public class IndexTests
{
    [Fact]
    public void CanIndexAndQuery()
    {
        var index = new Indexer();
        index.Index("users/1", "Oren Eini");
        index.Index("users/2", "Hibernating Rhinos");

        Assert.Contains("users/1", index.Query("eini"));
        Assert.Contains("users/2", index.Query("rhinos"));
    }

    [Fact]
    public void CanUpdate()
    {
        var index = new Indexer();
        index.Index("users/1", "Oren Eini");
        //updating
        index.Index("users/1", "Ayende Rahien");

        Assert.Contains("users/1", index.Query("Rahien"));
        Assert.Empty(index.Query("eini"));
    }
	
	[Fact]
	public void CanDelete()
	{
		var index = new Indexer();
		index.Index("users/1", "Oren Eini");
		index.Index("users/2", "Hibernating Rhinos");
		index.Index("users/3", "Ayende Rahien Hibernating Rhinos");
		//updating
		index.Delete("users/3");

		Assert.Contains("users/1", index.Query("Oren"));
		Assert.Contains("users/2", index.Query("rhinos"));
		Assert.Empty(index.Query("ayende"));
	}
}

If you rename the file to have the .cs extension, the code will be shown with syntax highlighting. This helps readability IMHO.