c# - 无法将类型 'System.Web.Http.Results.NotFoundResult' 隐式转换为 'System.Collections.Generic.IEnumerable<....>'

Question

找不到行如何返回404？但是，以下代码在 return NotFound() 行出错。

Error 1 Cannot implicitly convert type 'System.Web.Http.Results.NotFoundResult' to 'System.Collections.Generic.IEnumerable< webapi.Models.Product>'. An explicit conversion exists (are you missing a cast?)

    public IEnumerable<Product> GetProductsByReportId(int rid)
    {
        using (var db = new MyContext())
        {
            var query = from b in db.table where b.rid == rid select b;
            if (query == null)
            {
                return NotFound(); // Error!
            }
            return query.ToList();
        }
    }

Answer 1

您不能将 System.Web.Http.Results.NotFoundResult 设置/转换为 Product。

当 GetProductsByReportId 的结果为 null 时，您必须修改调用方法以返回 404(或消息)。

public IEnumerable<Product> GetProductsByReportId(int rid)
    {
        using (var db = new MyContext())
        {
            var query = from b in db.table where b.rid == rid select b;
            if (query == null)
            {
                return null;
            }
            return query.ToList();
        }
    }

即

int id = 1;
List<Product> products = GetProductsByReportId(id);
if(products == null) {
    var message = string.Format("Product with id = {0} not found", id);
    HttpError err = new HttpError(message);
    return Request.CreateResponse(HttpStatusCode.NotFound, err);
}