linq join出错

Question

我尝试加入两个表，最后收到此错误消息。

  

无法从查询中推断出类型参数。

和

  

错误12 join子句中某个表达式的类型不正确。呼叫＆＃39;加入＆＃39;

时，类型推断失败

以下是代码：

    var query = (from packagingMainCustom in this.Connection.Tbl_PackagingMaster_Main_Customs
                 join packagingMainSync in this.Connection.Tbl_PackagingMaster_Main_Codes
                     on new { packagingMainCustom.ID_Code, packagingMainCustom.ID_Version }
                     equals new {  packagingMainSync.ID_Code, packagingMainSync.ID_CurrentVersion }
                 select packagingMainCustom);

这个linq查询正在运行：

    var query = (from packagingMainCustom in this.Connection.Tbl_PackagingMaster_Main_Customs
                 join packagingMainSync in this.Connection.Tbl_PackagingMaster_Main_Codes
                     on new { packagingMainCustom.ID_Code }
                     equals new {  packagingMainSync.ID_Code }
                 select packagingMainCustom);

因此，错误消息可能与packagingMainSync.ID_CurrentVersion和packagingMainCustom.ID_Version有关。

以下是两个属性的声明：

//packagingMainCustom
private long _iD_Version;
[System.ComponentModel.DataAnnotations.Required()]
[System.ComponentModel.DataAnnotations.Key()]
public virtual long ID_Version
{
    get
    {
        return this._iD_Version;
    }
    set
    {
        if(this._iD_Version != value)
        {
            this.OnPropertyChanging("ID_Version");
            this._iD_Version = value;
            this.OnPropertyChanged("ID_Version");
        }
    }
}

//packagingMainSync
private long _iD_CurrentVersion;
[System.ComponentModel.DataAnnotations.Required()]
public virtual long ID_CurrentVersion
{
    get
    {
        return this._iD_CurrentVersion;
    }
    set
    {
        if(this._iD_CurrentVersion != value)
        {
            this.OnPropertyChanging("ID_CurrentVersion");
            this._iD_CurrentVersion = value;
            this.OnPropertyChanged("ID_CurrentVersion");
        }
    }
}

两者之间的唯一区别是DataAnnotations.Key() Annontations。

为什么上面的查询不起作用，错误消息是什么意思？

Answer 1

尝试使匿名类型的成员更明显相同：

on new { Code = packagingMainCustom.ID_Code,
    Version = packagingMainCustom.ID_Version }
equals new { Code = packagingMainSync.ID_Code,
    Version = packagingMainSync.ID_CurrentVersion }

这里的主要变化是名称现在匹配，顺序也是如此，大概是类型;这意味着这两个地方实际上现在都是相同的匿名类型（匿名类型基本上由成员名称，顺序，类型和声明程序集的组合定义）。