Linq“加入”具有IList< T>获得“错误无法创建常量值”。 [英] Linq "join" with a IList<T> getting "Error Unable to create a constant value.."

问题描述

我有一个保存方法，使用Linq查询将手动重新排序列表（在Web表单中）作为参数传递给我的方法，我尝试更新从数据库（EF）中获取的 IEnumerable< VM_CategoryLabel> 的订单属性与列表中的相应值（可能会更清楚我的代码在下面）：

  public static void SaveFromList（IList< VM_CategoryLabelExtra> listTemplate）
 {
 int idCat = listTemplate.Select（x => x.IdCat）.FirstOrDefault（）; 
 var test =（int）listTemplate.Where（z => z.Id == 8）.Select（z => z.Order）.FirstOrDefault（）; 
 
使用（var context = new my_Entities（））
 {
 var requete = from x in context.arc_CatLabel 
其中x.ID_Categorie == idCat 
 orderby x.Sequence_Cat 
选择新的VM_CategoryLabel 
 {
 Id = x.ID_LabelPerso，
 // Order = x.Sequence_Cat，
 Order =（int）listTemplate。其中（z => z.Id == x.ID_LabelPerso）.Select（z => z.Order）.First（），
 Label = x.arc_Label.Label，
 Unit = x .arc_Label.Unit 
}; 
 context.SaveChanges（）; 
} 
} 
  

我使用了testvar 来查看我的子查询是否获得正确的值，但是，当我在Select（已注释的订单行）中使用我的Linq表达式时，我会收到以下错误：

无法创建类型为Namespace.Models.VM_CategoryLabelExtra的常量值。

这是我的课程：

  public class VM_CategoryLabel 
 {
 public int Id {get; set;} 
 public int Order {get; set;} 
 public string label {get; set;} 
 public string Unit {get; set;} 
 public bool检查{get; set;} 
} 
 
 public class VM_CategoryLabelExtra 
 {
 public int Id {get; set;} 
 public int IdCat {get; set;} 
 public int Order {get; set;} 
 public string标签{get; set;} 
 public string Unit {get; set;} 
 public bool检查{get; set;} 
} 
  / pre> 
 
 
所以我想我不应该查询我的查询中的列表，那么如何匹配2个值列表？
 
 
 
我还尝试了以下（在Linq查询中替换为Order = x.Sequence_Cat），这不是beca使用迭代变量是
只读：
  foreach（请求中的var项）
 {
 item.Order = listTemplate.Where（x => x.Id == item.Id）.Select（x => x.Order）.FirstOrDefault（）; 
} 
 
 try 
 {
 context.SaveChanges（）; 
  
 
 
解决方案
我建议使用这个。
 
 
 
这是 let 子句。
  public static void SaveFromList（IList< VM_CategoryLabelExtra> listTemplate）
 {
 int idCat = listTemplate.Select（x => x.IdCat）.FirstOrDefault（）; 
 var test =（int）listTemplate.Where（z => z.Id == 8）.Select（z => z.Order）.FirstOrDefault（）; 
 
使用（var context = new my_Entities（））
 {
 var requete = from x in context.arc_CatLabel 
其中x.ID_Categorie == idCat 
 orderby x.Sequence_Cat 
 let list = listTemplate 
选择新的VM_CategoryLabel 
 {
 Id = x.ID_LabelPerso，
 Order = list.Where（z => z .Id == x.ID_LabelPerso）.Select（z => z.Order）.First（），
 Label = x.arc_Label.Label，
 Unit = x.arc_Label.Unit 
}; 
 context.SaveChanges（）; 
} 
} 
  
 编辑：而不是  你可以做  let list = listTemplate  
 
 
 
 现在应该工作：） 
 
I have a Save Method that saves with a Linq query a manually re-orderd list (in a web form) that is passed as the parameter to my method, and I try to update the Order Property of the IEnumerable<VM_CategoryLabel> I retrieve from the database (EF) with the corresponding value in the list (maybe would that be clearer with my code below):
public static void SaveFromList(IList<VM_CategoryLabelExtra> listTemplate)
    {
        int idCat = listTemplate.Select(x => x.IdCat).FirstOrDefault();
        var test = (int)listTemplate.Where(z => z.Id == 8).Select(z => z.Order).FirstOrDefault();

        using (var context = new my_Entities())
        {
            var requete = from x in context.arc_CatLabel
                          where x.ID_Categorie == idCat
                          orderby x.Sequence_Cat
                          select new VM_CategoryLabel
                          {
                              Id = x.ID_LabelPerso,
                              //Order = x.Sequence_Cat,
                              Order = (int)listTemplate.Where(z => z.Id == x.ID_LabelPerso).Select(z => z.Order).First(),
                              Label = x.arc_Label.Label,
                              Unit = x.arc_Label.Unit
                          };
            context.SaveChanges();
        }
    }
I used the "test" var to see if my "sub-query" gets the correct value, and it does, but when I use my Linq expression inside the Select (the commented Order line), I get the following error:

Unable to create a constant value of type 'Namespace.Models.VM_CategoryLabelExtra. "Only primitive types and enumeration types are supported in this context.

Here are my classes:
       public class VM_CategoryLabel
{
    public int Id { get; set; }
    public int Order { get; set; }
    public string Label { get; set; }
    public string Unit { get; set; }
    public bool Checked { get; set; }
}

public class VM_CategoryLabelExtra
{
    public int Id { get; set; }
    public int IdCat { get; set; }      
    public int Order { get; set; }
    public string Label { get; set; }
    public string Unit { get; set; }
    public bool Checked { get; set; }
}
So I suppose that I should not query the list inside my query ? So how do I "match" the 2 lists of values ?

I also tried the following (after having replace in the Linq query: Order = x.Sequence_Cat)that is not working neither because the iteration variable is
read-only:
foreach (var item in requete)
                {
                    item.Order = listTemplate.Where(x => x.Id == item.Id).Select(x => x.Order).FirstOrDefault();
                }

                try
                {
                    context.SaveChanges();

解决方案
i suggest using this.

It is the let clause.
public static void SaveFromList(IList<VM_CategoryLabelExtra> listTemplate)
    {
        int idCat = listTemplate.Select(x => x.IdCat).FirstOrDefault();
        var test = (int)listTemplate.Where(z => z.Id == 8).Select(z => z.Order).FirstOrDefault();

    using (var context = new my_Entities())
    {
        var requete = from x in context.arc_CatLabel
                      where x.ID_Categorie == idCat
                      orderby x.Sequence_Cat
                      let list = listTemplate                                     
                      select new VM_CategoryLabel
                      {
                          Id = x.ID_LabelPerso,                              
                          Order = list.Where(z => z.Id == x.ID_LabelPerso).Select(z => z.Order).First(),                             
                          Label = x.arc_Label.Label,
                          Unit = x.arc_Label.Unit
                      };
        context.SaveChanges();
    }
}
edit: instead offrom you can just do let list = listTemplate

Should work now :)

                        
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