使用 FirstOrDefaultAsync 扩展方法:

Use FirstOrDefaultAsync extension method:

[ResponseType(typeof(Vocab))]
public async Task<IHttpActionResult> GetVocabByLesson(int lessonId)
{
        Vocab vocab = await db.Vocabs.FirstOrDefaultAsync(a => a.LessonId == lessonId);
        if (vocab == null)
            return NotFound();

        return Ok(vocab);
}

通过您的代码，我可以推断出您只想返回一个元素，这就是为什么我建议使用FirstOrDefaultAsync的原因.但是，如果您想获得多个满足某些条件的元素，请使用 ToListAsync :

By your code I can deduct you want to return just an element, that's why I have suggested to use FirstOrDefaultAsync. But in case you want to get more than one element that meets some condition then use ToListAsync:

[ResponseType(typeof(Vocab))]
public async Task<IHttpActionResult> GetVocabByLesson(int lessonId)
{
        var result= await db.Vocabs.Where(a => a.LessonId == lessonId).ToListAsync();
        if (!result.Any())
            return NotFound();

        return Ok(result);
}