Reusable linq select query in Entity Framework

Question

In my application multiple reports are needed on some table, many of the fields are common in most reports, as a sample:

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();

How can I write this map only once? These fields are common in list1 and list2.

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

One word: AutoMapper.

Gert Arnold
– Gert Arnold

2016-12-04 10:21:53 +00:00
Commented Dec 4, 2016 at 10:21 — Gert Arnold
– Gert Arnold, Commented Dec 4, 2016 at 10:21

Community · Accepted Answer · 2017-05-23 12:10:22Z

7

First, define an expression that will contain your common projection needs:

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

Then have a method that will modify this expression to reflect the additional bindings:

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;
}

Usage:

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();

(With some help from @Nicholas Butler great answer)

edited May 23, 2017 at 12:10

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answered Dec 4, 2016 at 7:41

haim770

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9 Comments

Mohammad Akbari Over a year ago

tnx for answer, but an error exist "type arguments for method 'Queryable.Select<TSource, TResult>(IQueryable<TSource>, Expression<Func<TSource, TResult>>)' cannot be inferred from the usage. Try specifying the type arguments explicitly"

haim770 Over a year ago

@MohammadAkbari, Where exactly does the error occur?

Mohammad Akbari Over a year ago

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

haim770 Over a year ago

@MohammadAkbari, Did you change anything it the signature of MergeBindings()? What do you see when you hover on var withParentsExpr?

Mohammad Akbari Over a year ago

Expression<Func<ReportStudent, ReportStudent>>

|

Victor Leontyev · Accepted Answer · 2016-12-04 07:42:08Z

0

If you don't want to write maps every time, you can use great library http://automapper.org/ With this library, you can define map and it automatically map all properties

answered Dec 4, 2016 at 7:42

Victor Leontyev

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Comments

Henrik Bøgelund Lavstsen · Accepted Answer · 2016-12-04 08:27:47Z

0

You could create a function for that let say you have

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

Then use it within your select statement

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

Then add remaining properties or you could use AutoMapper, which can be found on github or as a nuget package.

answered Dec 4, 2016 at 8:27

Henrik Bøgelund Lavstsen

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2 Comments

Mohammad Akbari Over a year ago

how add remaining properties, plz give an example

haim770 Over a year ago

This will not return an Expression that Entity Framework can translate into store query

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