C#中DbContext查询返回Null但数据库有数据，求问题排查

问题重现 #

用户提供的API端点代码：

[HttpGet("{language}")]
public async Task<IActionResult> GetAllClozeWords(string language)        
{
    var clozes = await GetClozesForLanguage<Cloze>(language);
    
    if (clozes.IsNullOrEmpty())
    {
        return BadRequest();
    }
 
    return Ok();
}

private async Task<List<T>> GetClozesForLanguage<T>(string language) where T : Cloze 
{
    switch (language.ToLower())
    {
        case "nl":
            return await this._trackerDbContext.NLclozes.ToListAsync() as List<T>;

        case "en":
            return await this._trackerDbContext.ENclozes.ToListAsync() as List<T>;

        default:
            return new List<T>();
    }
}

对应的EF实体模型：

public abstract class Cloze
{
    public int Id { get; set; }
    public string cloze_word { get; set; }
}

public class NLcloze : Cloze
{
    public string note1 { get; set; }
    public string note2 { get; set; }
    public string note3 { get; set; }
    public virtual ICollection<NLexpression> NLexpressions { get; set; }
    public virtual ICollection<ENcloze> ENclozes { get; set; }
}

public class ENcloze : Cloze
{
    public string note1 { get; set; }
    public string note2 { get; set; }
    public string note3 { get; set; }
    public virtual ICollection<ENexpression> ENexpressions { get; set; }
    public virtual ICollection<NLcloze> NLclozes { get; set; }
}

调用端点传入nl参数时，GetClozesForLanguage<Cloze>返回结果始终为Null，尽管数据库存在对应数据。

问题原因 #

核心问题出在as List<T>的类型转换逻辑：

• 调用GetClozesForLanguage<Cloze>时，泛型参数T为抽象基类Cloze
• _trackerDbContext.NLclozes.ToListAsync()返回的是List<NLcloze>类型
• List<T>是不变泛型类型，即便NLcloze继承自Cloze，List<NLcloze>也无法直接转换为List<Cloze>，as转换会返回Null

解决方案 #

将as List<T>替换为显式的元素类型转换，利用Cast<T>方法逐个将子类实例转换为基类类型：

修改后的GetClozesForLanguage方法：

private async Task<List<T>> GetClozesForLanguage<T>(string language) where T : Cloze 
{
    switch (language.ToLower())
    {
        case "nl":
            var nlClozes = await this._trackerDbContext.NLclozes.ToListAsync();
            return nlClozes.Cast<T>().ToList();

        case "en":
            var enClozes = await this._trackerDbContext.ENclozes.ToListAsync();
            return enClozes.Cast<T>().ToList();

        default:
            return new List<T>();
    }
}

补充优化：原API端点的Ok()未返回查询到的clozes数据，建议修改为return Ok(clozes);，否则前端无法获取具体内容。

内容的提问来源于stack exchange，提问作者aldo