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

score 7 · Accepted Answer · edited May 23 '17 at 12:10

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 '17 at 12:10

Community

1
1

answered Dec 04 '16 at 07:41

haim770

48,394
7
105
133

tnx for answer, but an error exist "type arguments for method 'Queryable.Select(IQueryable, Expression>)' cannot be inferred from the usage. Try specifying the type arguments explicitly" – Mohammad Akbari Dec 06 '16 at 10:00
@MohammadAkbari, Where exactly does the error occur? – haim770 Dec 06 '16 at 10:03
on select , context.Students.Select(withParentsExpr).ToList() – Mohammad Akbari Dec 06 '16 at 10:21
@MohammadAkbari, Did you change anything it the signature of `MergeBindings()`? What do you see when you hover on `var withParentsExpr`? – haim770 Dec 06 '16 at 10:51
Expression> – Mohammad Akbari Dec 06 '16 at 11:11
That's strange. What type is `context.students` then? – haim770 Dec 06 '16 at 11:16
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/129898/discussion-between-mohammad-akbari-and-haim770). – Mohammad Akbari Dec 06 '16 at 11:23
context.Students id Student – Mohammad Akbari Dec 06 '16 at 11:35
You just needed () on the Expression. `context.Students.Select(withAverageExpr()).ToList()` – friggle Apr 07 '17 at 16:05

score 0 · Answer 2 · answered Dec 04 '16 at 07:42

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 04 '16 at 07:42

Victor Leontyev

8,488
2
16
36

score 0 · Answer 3 · answered Dec 04 '16 at 08:27

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 04 '16 at 08:27

Henrik Bøgelund Lavstsen

1,451
14
27

how add remaining properties, plz give an example – Mohammad Akbari Dec 04 '16 at 09:35
1

This will not return an `Expression` that Entity Framework can translate into store query – haim770 Dec 04 '16 at 14:20

Reusable linq select query in Entity Framework

3 Answers3