ASP.NET Core中DeleteCharacter方法CS0029类型转换错误求助

错误原因 #

DeleteCharacter方法声明返回Task<ServiceResponse<List<GetCharacterDTO>>>，但实际返回的是ServiceResponse<List<GetCharacterDTO>>实例，两者类型不匹配。而正常运行的UpdateCharacter方法因为加了async关键字，编译器会自动将返回值包装成Task，所以无错误。

解决方案 #

方案一：添加async关键字（推荐，与现有代码风格一致） #

给DeleteCharacter方法添加async修饰符，编译器会自动处理Task包装：

public async Task<ServiceResponse<List<GetCharacterDTO>>> DeleteCharacter(int id)
{
    var serviceResponse = new ServiceResponse<List<GetCharacterDTO>>();
    try
    {
        var character = characters.First(c => c.Id == id);

        if (character is null)
        {
            throw new Exception($"Character with Id '{id}' not found");
        }
        characters.Remove(character);
        serviceResponse.Data = characters.Select(c => _mapper.Map<GetCharacterDTO>(c)).ToList();
    }
    catch (Exception ex)
    {
        serviceResponse.Success = false;
        serviceResponse.Message = ex.Message;
    }

    return serviceResponse;
}

即使方法内没有await调用，async关键字依然有效，编译器会生成已完成的Task包装返回值。

方案二：手动包装为Task #

若不想加async，用Task.FromResult()手动包装返回值：

public Task<ServiceResponse<List<GetCharacterDTO>>> DeleteCharacter(int id)
{
    var serviceResponse = new ServiceResponse<List<GetCharacterDTO>>();
    try
    {
        var character = characters.First(c => c.Id == id);

        if (character is null)
        {
            throw new Exception($"Character with Id '{id}' not found");
        }
        characters.Remove(character);
        serviceResponse.Data = characters.Select(c => _mapper.Map<GetCharacterDTO>(c)).ToList();
    }
    catch (Exception ex)
    {
        serviceResponse.Success = false;
        serviceResponse.Message = ex.Message;
    }

    return Task.FromResult(serviceResponse);
}

为什么UpdateCharacter方法没问题？ #

UpdateCharacter方法带有async关键字，C#编译器会自动触发"异步状态机"转换，将返回的ServiceResponse<GetCharacterDTO>自动包装成Task<ServiceResponse<GetCharacterDTO>>，因此不会出现类型转换错误。

内容的提问来源于stack exchange，提问作者DJ Eddie J