C# Linq SQL 选择要列出的多个列

本文关键字：Linq SQL 选择 | 更新日期: 2023-09-27 18:36:39

嗨，我想从DB中选择4列，然后选择列表

public struct keyData
{
    public int pid;
    public int sid;
    public string proid;
    public string title;
}

Entities dbconn = new Entities();
List<keyData> temRes = (
    from viewData in dbconn.vw1
    join hData in dbconn.tableH
    on new { pid= (int)viewData.pid, proid= viewData.proid}
    equals new { pid= (int)hData .pid, proid= hData .proid}
    into joinSet
    from joinUnit in joinSet.DefaultIfEmpty()
    where joinUnit == null
    select new { pid= (int)viewData.pid, sid= (int)viewData.sid, proid= viewData.proid, Title=viewData.Title }
    ).ToList();

然后它说：

错误 3 无法隐式转换 

type 'System.Collections.Generic.List<AnonymousType#1>'

到'System.Collections.Generic.List<hl.Program.keyData>'

谢谢

使用马尔钦·尤拉谢克的想法

它给了我：

Only parameterless constructors and initializers are supported in LINQ to Entities

C# Linq SQL 选择要列出的多个列

这是因为您的 quere 返回了一个匿名类型的对象。将其更改为返回keyData实例：

    List<keyData> temRes = (
        from viewData in dbconn.vw1
        join hData in dbconn.tableH
        on new { pid= (int)viewData.pid, proid= viewData.proid}
        equals new { pid= (int)hData .pid, proid= hData .proid}
        into joinSet
        from joinUnit in joinSet.DefaultIfEmpty()
        where joinUnit == null
        select new keyData() { pid= (int)viewData.pid, sid= (int)viewData.sid, proid= viewData.proid, Title=viewData.Title }
        ).ToList();

区别在于select条款。我在关键字后添加了您的类名new。

Entities dbconn = new Entities();
List<keyData> temRes = (
    from viewData in dbconn.vw1.ToList()
    join hData in dbconn.tableH.ToList()
    on new { pid= (int)viewData.pid, proid= viewData.proid}
    equals new { pid= (int)hData .pid, proid= hData .proid}
    into joinSet
    from joinUnit in joinSet.DefaultIfEmpty()
    where joinUnit == null
    select new { pid= (int)viewData.pid, sid= (int)viewData.sid, proid= viewData.proid,  Title=viewData.Title }
    ).ToList();

如前所述，您必须返回keyData实例而不是匿名类型 https://stackoverflow.com/a/19529805/1034373。但是结构不能有显式的无参数构造函数，因此您必须将 keyData 类型更改为类或添加结构体

public struct keyData
{
    public keyData(int pid, int sid, string proid, string title)
    {
         this.pid = pid;
         this.sid = sid;
         this.proid = proid;
         this.title = title;
    }
    public int pid;
    public int sid;
    public string proid;
    public string title;
}

并编写这样的代码

List<keyData> temRes = (
    from viewData in dbconn.vw1
    join hData in dbconn.tableH
    on new { pid= (int)viewData.pid, proid= viewData.proid}
    equals new { pid= (int)hData .pid, proid= hData .proid}
    into joinSet
    from joinUnit in joinSet.DefaultIfEmpty()
    where joinUnit == null
    select new { pid= (int)viewData.pid, sid= (int)viewData.sid, proid= viewData.proid, Title=viewData.Title }
    ).ToList().Select(x => new keyData(x.pid, x.sid, x.proid, x.title)).ToList();

我在这里回答了类似的问题。你可以简单地使用SELECT和SELECTMANY来做到这一点。https://stackoverflow.com/a/27755340/2764258

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