我得到了答案,我写了下面的方法
public async Task<bool> UpdateCommentAsync(Threads thread)
{
var builder = Builders<Comments>.Filter;
var filter = builder.Empty;
var update = Builders<Comments>.Update.Set("Threads.$[i].Content", thread.Content);
var arrayFilters = new List<ArrayFilterDefinition> { new JsonArrayFilterDefinition<Threads>("{'i._id': '" + thread.Id + "'}") };
var updateOptions = new UpdateOptions { ArrayFilters = arrayFilters };
var result = await _context.Comments.UpdateManyAsync(filter, update, updateOptions);
return result.ModifiedCount > 0;
}