Mine

    private struct MultiRating
    {
      public float totalRating;
      public int totalVotes;

      public MultiRating(float rating){
        this.totalRating = rating;
        this.totalVotes = 1;
      }

      public float Rating {
        get { return this.totalRating / (float) this.totalVotes; }
        set { this.totalRating += value; this.totalVotes += 1; } }

      public float TotalRating { get { return this.totalRating; } }
      public float TotalVotes  { get { return this.totalVotes; } }
    }

    public List<Song> GenerateNextSongs(int count)
    {
      Dictionary<Song,MultiRating> songs = new Dictionary<Song,MultiRating>();
      foreach(SongRating sr in this.UserRatings)
      {
        if (songs.ContainsKey(sr.Song))
          // PROBLEM HERE:
          songs[sr.Song].Rating = sr.Rating;
        else
          songs.Add(sr.Song, new MultiRating(sr.Rating));
      }
     // ...
    }

// Error:
error CS1612: Cannot modify the return value of `System.Collections.Generic.Dictionary<ClubMix.Song,ClubMix.Room.MultiRating>.this[...]' because it is not a variable