Assigning a local variable within a lambda expression

Question

I have a lambda expression in which need to assign a local variable to avoid calling my RetrieveAge(Datetime birthDate) two times per resultset.

My lambda expression looks as following:

result = myList.AsEnumerable().Where(f => DateHelper.RetrieveAge(f.Birthdate) >= 20 && DateHelper.RetrieveAge(f.Birthdate) <= 40).Select(x => new Person { Name = x.Name, Id = x.Id, Alias = x.Alias }).ToList();

I am trying to achieve something like the following:

var result = myList.AsEnumerable().Where(f => { var age = DateHelper.RetrieveAge(f.Birthdate); age >= 20 && age <= 40 }).Select(x => new Person { Name = x.Name, Id = x.Id, Alias = x.Alias }).ToList();

But I can't figure out how to do that properly. Any hints or suggestions would be mostly appreciated.

possible duplicate of C#: Is it possible to declare a local variable in an anonymous method? — oefe
– oefe, Commented Nov 17, 2013 at 12:00

Sergey Kalinichenko · Accepted Answer · 2013-11-17 11:58:32Z

5

You are almost there - you need to add a return and a semicolon:

var result = myList
    .AsEnumerable()
    .Where(f => {
         var age = DateHelper.RetrieveAge(f.Birthdate);
         return age >= 20 && age <= 40; // <<== Here
    }).Select(x => new Person {
         Name = x.Name, Id = x.Id, Alias = x.Alias }
    ).ToList();

answered Nov 17, 2013 at 11:58

Sergey Kalinichenko

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1 Comment

codingjoe Over a year ago

I had actually put the semicolon on one of my attempts, but I had missed the return keyword. It is a cool trick to know about assigning local variables to be used through rest of a lambda expression. :)

atomaras · Accepted Answer · 2013-11-17 11:59:31Z

2

var result = (from f in myList.AsEnumerable()
             let age = DateHelper.RetrieveAge(f.Birthdate)
             where age >= 20 && age <= 40
             select new Person { Name = f.Name, Id = f.Id, Alias = f.Alias }).ToList();

answered Nov 17, 2013 at 11:59

atomaras

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Comments

LINQ2Vodka · Accepted Answer · 2013-11-17 11:58:46Z

0

var result = myList.AsEnumerable()
    .Select(f => new { F = f, X = DateHelper.RetrieveAge(f.Birthdate))
    .Where(f => f.F.age >= 20 && f.F.age <= 40 })
    .Select(x => new Person { Name = f.F.Name, Id = f.F.Id, Alias = f.F.Alias })
    .ToList();

answered Nov 17, 2013 at 11:58

LINQ2Vodka

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Comments

Dmytro Rudenko · Accepted Answer · 2013-11-17 12:07:15Z

0

var result = myList.AsEnumerable().Where(f => 
{ 
    var age = DateHelper.RetrieveAge(f.Birthdate); 
    return age >= 20 && age <= 40 
}).Select(x => new Person { Name = x.Name, Id = x.Id, Alias = x.Alias }).ToList();

answered Nov 17, 2013 at 12:07

Dmytro Rudenko

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