ASP.NET 将 IQueryable 转换为 List答案

【问题标题】：ASP.NET convert IQueryable to ListASP.NET 将 IQueryable 转换为 List
【发布时间】：2015-07-12 05:21:19
【问题描述】：

我收到以下代码，但出现以下错误：

错误 1 无法隐式转换类型 'System.Linq.IQueryable<Receptura_v4.Models.ReceptDTO>' 到 'Receptura_v4.Models.ReceptDTO'。存在显式转换（是你错过了一个演员？）

有什么办法解决吗？

public ReceptDTO GetReceptForUser(string userId)
{
    using (ApplicationDbContext db = new ApplicationDbContext())
    { 
        var q = from r in db.Recipes where r.UserId == userId select new ReceptDTO
            {
                 RecipeID = r.RecipeID,
                 Title = r.Title,
                 Portion = r.Portion,
                 Time = r.Time,
                 Category = r.Category,
                 Preparation = r.Preparation,
                 Difficulty = r.Difficulty,
                 Views = 0,
                 Price = r.Price,
                 Date = DateTime.Now,
                 UserId = userId
            };
        return q;
    }
}

【问题讨论】：

标签： c# asp.net asp.net-mvc linq asp.net-mvc-5

【解决方案1】：

错误意味着您的返回类型和您返回的内容不匹配。你的返回类型是ReceptDTO，你返回的类型是IQueryable<ReceptDTO>

您可以通过从IQueryable 发回第一个对象来返回ReceptDTO 对象

public ReceptDTO GetReceptForUser(string userId)
{
    using (ApplicationDbContext db = new ApplicationDbContext())
    { 
        var q = from r in db.Recipes where r.UserId == userId select new ReceptDTO
            {
                 RecipeID = r.RecipeID,
                 Title = r.Title,
                 Portion = r.Portion,
                 Time = r.Time,
                 Category = r.Category,
                 Preparation = r.Preparation,
                 Difficulty = r.Difficulty,
                 Views = 0,
                 Price = r.Price,
                 Date = DateTime.Now,
                 UserId = userId
            };
        return q.FirstOrDefault();
    }
}

如果你需要返回一个列表，你可以改变你的方法返回类型，返回一个ReceptDTO的列表

public List<ReceptDTO> GetReceptForUser(string userId)
{
    using (ApplicationDbContext db = new ApplicationDbContext())
    { 
        var q = from r in db.Recipes where r.UserId == userId select new ReceptDTO
            {
                 RecipeID = r.RecipeID,
                 Title = r.Title,
                 Portion = r.Portion,
                 Time = r.Time,
                 Category = r.Category,
                 Preparation = r.Preparation,
                 Difficulty = r.Difficulty,
                 Views = 0,
                 Price = r.Price,
                 Date = DateTime.Now,
                 UserId = userId
            };
        return q.ToList();
    }
}

【讨论】：

【解决方案2】：

这段代码：

var q = from r in db.Recipes where r.UserId == userId select new ReceptDTO
                {
                     RecipeID = r.RecipeID,
                     Title = r.Title,
                     Portion = r.Portion,
                     Time = r.Time,
                     Category = r.Category,
                     Preparation = r.Preparation,
                     Difficulty = r.Difficulty,
                     Views = 0,
                     Price = r.Price,
                     Date = DateTime.Now,
                     UserId = userId
                };

将q 分配给IQueryable<ReceptDTO>

虽然您的原型需要单个 ReceptDTO 的返回类型：

public ReceptDTO GetReceptForUser(string userId)

如果您想返回 IQueryable<ReceptDTO>，请将您的原型更改为：

public IQueryable<ReceptDTO> GetReceptForUser(string userId)
    {
        using (ApplicationDbContext db = new ApplicationDbContext())
        { 
            var q = from r in db.Recipes where r.UserId == userId select new ReceptDTO
                {
                     RecipeID = r.RecipeID,
                     Title = r.Title,
                     Portion = r.Portion,
                     Time = r.Time,
                     Category = r.Category,
                     Preparation = r.Preparation,
                     Difficulty = r.Difficulty,
                     Views = 0,
                     Price = r.Price,
                     Date = DateTime.Now,
                     UserId = userId
                };
            return q;
        }
    }

如果您想从查询中返回第一个项目，请添加 FirstOrDefault()：

    public ReceptDTO GetReceptForUser(string userId)
    {
        using (ApplicationDbContext db = new ApplicationDbContext())
        { 
            var q = from r in db.Recipes where r.UserId == userId select new ReceptDTO
                {
                     RecipeID = r.RecipeID,
                     Title = r.Title,
                     Portion = r.Portion,
                     Time = r.Time,
                     Category = r.Category,
                     Preparation = r.Preparation,
                     Difficulty = r.Difficulty,
                     Views = 0,
                     Price = r.Price,
                     Date = DateTime.Now,
                     UserId = userId
                };
            return q.FirstOrDefault();
        }
    }

【讨论】：