I have created a scrabble game with a computer opponent. If a blank tile is found in the computer's rack during the word generation if needs to be swapped out for every letter in the alphabet. I have my current solution to solve this problem below, but was wondering if there is a better more efficient way to accomplish this task.

if (str.Contains("*"))
                {
                    char c = 'A';
                    String made = "";
                    while(c < 'Z')
                    {
                        made = str.ReplaceFirst("*", c.ToString());
                        if (!made.Contains("*"))
                        {
                            wordsMade.Add(made);
                            if (theGame.theTrie.Search(made) == Trie.SearchResults.Found)
                            {
                                validWords.Add(made);
                            }
                        }
                        else
                        {
                            char ch = 'A';
                            String made2 = "";
                            while (ch < 'Z')
                            {
                                made2 = made.ReplaceFirst("*", c.ToString());
                                wordsMade.Add(made2);
                                if (theGame.theTrie.Search(made2) == Trie.SearchResults.Found)
                                {
                                    validWords.Add(made2);
                                }
                                ch++;
                            }
                        }
                        c++;
                    }