可重複使用LINQ選擇查詢

在我的應用程序的多個報告，因此需要對一些表，許多領域都在大多數報告常見的，作爲一個樣本：可重複使用LINQ選擇查詢

public class ReportStudent 
{ 
    public int Id {get; set;} 
    public string Name {get; set;} 
    public string Family {get; set;} 
    public DateTime BirthDate {get; set;} 
    public DateTime RegisterDate {get; set;} 
    public Double Average {get; set;} 
    public string FatherName {get; set;} 
    public string MotherName {get; set;} 
} 

var list1 = context.Students.Select(e=> new ReportStudent 
{ 
    Id = e.Id 
    Name = e.Name 
    Family = e.Family 
    BirthDate = e.BirthDate 
    RegisterDate = e.RegisterDate 
    FatherName = e.FatherName 
    MotherName = e.MotherName 
}).ToList(); 

var list2 = context.Students.Select(e=> new ReportStudent 
{ 
    Id = e.Id 
    Name = e.Name 
    Family = e.Family 
    BirthDate = e.BirthDate 
    RegisterDate = e.RegisterDate 
    Average = e.Average 
}).ToList();

我怎麼能寫這個地圖只有一次？這些字段在list1和list2中很常見。

Id = e.Id 
Name = e.Name 
Family = e.Family 
BirthDate = e.BirthDate 
RegisterDate = e.RegisterDate

來源

2016-12-04 Mohammad Akbari

一個字：AutoMapper。 –

首先，定義將包含公共投影的表達需要：

Expression<Func<ReportStudent, ReportStudent>> commonProjection = e => new ReportStudent 
{ 
    Id = e.Id, 
    Name = e.Name, 
    Family = e.Family, 
    BirthDate = e.BirthDate, 
    RegisterDate = e.RegisterDate, 
};

然後具有將修改這個表達式，以反映附加綁定的方法：

public static Expression<Func<ReportStudent, ReportStudent>> MergeBindings(Expression<Func<ReportStudent, ReportStudent>> expr, Expression<Func<ReportStudent, ReportStudent>> newExpr) 
{ 
    var reportStudentType = typeof(ReportStudent); 
    var eParameter = expr.Parameters.First(); 
    var eNew = Expression.New(reportStudentType); 

    var memberInitExpr = expr.Body as MemberInitExpression; 
    var memberInitNewExpr = newExpr.Body as MemberInitExpression; 
    var allBindings = memberInitExpr.Bindings.Concat(memberInitNewExpr.Bindings.Select(x => 
     Expression.Bind(x.Member, Expression.Property(eParameter, x.Member as PropertyInfo) 
    ))); 

    var eInit = Expression.MemberInit(eNew, allBindings); 
    var lambda = Expression.Lambda<Func<ReportStudent, ReportStudent>>(eInit, eParameter); 

    return lambda; 
}

用法：

var withParentsExpr = MergeBindings(commonProjection, e => new ReportStudent 
{ 
    FatherName = e.FatherName, 
    MotherName = e.MotherName 
}); 

var list1 = context.Students.Select(withParentsExpr).ToList(); 

var withAverageExpr = MergeBindings(commonProjection, e => new ReportStudent 
{ 
    Average = e.Average 
}); 

var list2 = context.Students.Select(withAverageExpr).ToList();

（從@Nicholas巴特勒great answer一些幫助）

來源

2016-12-04 07:41:14 haim770

tnx for answer，但存在錯誤「方法的類型參數」Queryable.Select （IQueryable ，Expression >）'不能從該用法推斷。嘗試指定類型參數明確地「 –

@MohammadAkbari，錯誤發生在哪裏？ – haim770

on select，context.Students.Select（withParentsExpr）.ToList（） –

如果你不想每次寫的地圖，你可以用偉大的圖書館http://automapper.org/ 有了這個庫，你可以定義地圖，它會自動映射所有屬性

來源

2016-12-04 07:42:08

您可以創建，讓一個函數說你有

public StudentReport ParseStudentReport(Student e) 
{ 
    return new StutentReport{ 
     Id = e.Id 
     Name = e.Name 
     Family = e.Family 
     BirthDate = e.BirthDate 
     RegisterDate = e.RegisterDate 
    } 
}

那麼你的SELECT語句中使用它

var list2 = context.Students.Select(ParseStudentReport);

然後添加其餘的屬性，或者您可以使用AutoMapper，它可以在github上找到或作爲nuget包。

來源

2016-12-04 08:27:47

如何添加剩餘的屬性，PLZ給出一個例子 –

這不會返回Entity Framework可以轉換成存儲查詢的'Expression' – haim770

可重複使用LINQ選擇查詢

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