List<T>, Collection<T>, and FindAll

[David Longnecker]

I'm working to create a base framework for our organization for web and client-side
applications. The framework interfaces with several of our systems and provides
the business and data layer connectivity for basic operations and such.

I've ran into a snag that I just can't think myself out of.

Here's an example:

I have an object for a Student called StudentRecord. It has properties such
as name, grade, identification number, etc.

public class StudentRecord : IComparable
{

private int _studentId;
public int StudentId { get { return _studentId; } set { _studentId = value;
} }

public override string ToString() { get { return string.Format("{0} {1}",
_firstName, _lastName); }

... etc.

}

From there, I have a "roster" of students, thus a collection of those StudentRecord
objects. I would like to inherit from List<StudentRecordto provide Find,
FindAll, Sort, and other functionality to the consumer of the API. (1)*

public class StudentRoster : List<StudentRecord>
{
// I'm not providing any additional functionality here at this time,
// simply providing the easier call rather than requring them to know List<object>.
}

Now, roster.Find works just fine because, I'm assuming, the list is based
off StudentRecord.

StudentRecord studentA = roster.Find(delegate(StudentRecord x) { return x.LastName.Equals("Longnecker");
});

This returns me a populated student object based off a prior created collection
of student Objects.

I run into an issue with:

StudentRoster studentsInGrade8 = roster.FindAll(delegate (StudentRecord x)
{ return x.Grade.Equals("08"); } );

FindAll apparently returns a raw list of List<StudentRecordrather than
StudentRoster (since I haven't specified it). I've attempted to modify my
StudentRoster class to add:

public StudentRoster FindAll(Predicate<StudentRecorditem)
{

StudentRoster items = (StudentRoster) Items;
return items.FindAll(match);

}

However, that fails with the same error--it cannot convert List<StudentRecord>
to StudentRoster. I've attempted to cast it explicitly (using 'as StudentRoster'
and (StudentRoster)) to no avail.

Google hasn't turned up anything thus far and neither has MSDN (though I'm
assuming, perhaps, I'm not searching for the magical word)... What would
be the best way to create "collections of objects" that can be searched through
using Find at the public API level?

(1)* == Note that CLS does specify not to expose List<T>; however, Collection<T>
doesn't have Find, Sort, FindAll, etc. Is there a better option?

Any reference material, etc. would be greatly appreciated!

Thanks in advance.

-dl

---
David R. Longnecker
Web Developer
http://blog.tiredstudent.com

[Nicholas Paldino [.NET/C# MVP]]

David,

You are going to have to do an explicit copy here.

Assuming you don't want to hide the FindAll method (I recommend against
it), and that you expose the constructor for your StudentRoster which takes
an IEnumerable<StudentRecord>, you can do this:

StudentRoster studentsInGrade8 = new StudentRoster(roster.FindAll(delegate
(StudentRecord x) { return x.Grade.Equals("08"); } ));

This way, the List<StudentRecordfeeds the construction of the new
StudentRoster instance.

If you really want to return a StudentRoster instance, then I would
recommend creating a static FindAll method which will take the
IList<StudentRecordimplementation and then perform the conversion to a
StudentRoster instance yourself.

Hope this helps.

--
- Nicholas Paldino [.NET/C# MVP]
- mv*@spam.guard.caspershouse.com

"David Longnecker" <dl*********@community.nospamwrote in message
news:46************************@msnews.microsoft.c om...

I'm working to create a base framework for our organization for web and
client-side applications. The framework interfaces with several of our
systems and provides the business and data layer connectivity for basic
operations and such.

I've ran into a snag that I just can't think myself out of.

Here's an example:

I have an object for a Student called StudentRecord. It has properties
such as name, grade, identification number, etc.

public class StudentRecord : IComparable
{

private int _studentId;
public int StudentId { get { return _studentId; } set { _studentId =
value; } }
public override string ToString() { get { return string.Format("{0} {1}",
_firstName, _lastName); }

... etc.

}

From there, I have a "roster" of students, thus a collection of those
StudentRecord objects. I would like to inherit from List<StudentRecord>
to provide Find, FindAll, Sort, and other functionality to the consumer of
the API. (1)*

public class StudentRoster : List<StudentRecord>
{
// I'm not providing any additional functionality here at this time,
// simply providing the easier call rather than requring them to know
List<object>.
}

Now, roster.Find works just fine because, I'm assuming, the list is based
off StudentRecord.

StudentRecord studentA = roster.Find(delegate(StudentRecord x) { return
x.LastName.Equals("Longnecker"); });

This returns me a populated student object based off a prior created
collection of student Objects.

I run into an issue with:

StudentRoster studentsInGrade8 = roster.FindAll(delegate (StudentRecord x)
{ return x.Grade.Equals("08"); } );

FindAll apparently returns a raw list of List<StudentRecordrather than
StudentRoster (since I haven't specified it). I've attempted to modify my
StudentRoster class to add:

public StudentRoster FindAll(Predicate<StudentRecorditem)
{

StudentRoster items = (StudentRoster) Items;
return items.FindAll(match);

}

However, that fails with the same error--it cannot convert
List<StudentRecordto StudentRoster. I've attempted to cast it
explicitly (using 'as StudentRoster' and (StudentRoster)) to no avail.

Google hasn't turned up anything thus far and neither has MSDN (though I'm
assuming, perhaps, I'm not searching for the magical word)... What would
be the best way to create "collections of objects" that can be searched
through using Find at the public API level?

(1)* == Note that CLS does specify not to expose List<T>; however,
Collection<Tdoesn't have Find, Sort, FindAll, etc. Is there a better
option?

Any reference material, etc. would be greatly appreciated!

Thanks in advance.

-dl

---
David R. Longnecker
Web Developer
http://blog.tiredstudent.com

[David Longnecker]

Okay, wow, it hadn't even occured to me that the reference was just sorta
trying to override itself rather than create a new one--I've been staring
at this for too long today. Thanks.

So, I added two lines to my StudentRoster class. One, the IEnumerable for
creating new, and the empty one:

public class StudentRoster : List<StudentRecord>, IEnumerable
{
public StudentRoster(IEnumerable<StudentRecordrecord) : base (new
List<StudentRecord>(record)) { }
public StudentRoster() : base(new List<StudentRecord>()) { }

}

Then, as you said, the new roster works:

StudentRoster studentsInGrade9 = new StudentRoster(roster.FindAll(delegate(StudentRecor d
x) { return x.Grade.Equals("09"); }));
Console.WriteLine("There are {0} students in the ninth grade.", studentsInGrade9.Count);

The follow up I have to that is that I'm not sure how to please both keepers,
so to speak. I'm wanting to propagate down the .Find, .FindAll, and .Sort
(for example) methods, but CLS (ala FxCop) is complaining that:

"Change %OBJECT% to use Collection<T>, ReadOnlyCollection<T>
or KeyedCollection<K,V>"

Of course, Collection doesn’t have the methods of List. Is there a benefit
to this beyond 100% compliance? If so, is it documented out there ‘best
practices’ of how to create “collections that act like lists�?

Thanks!

-dl

---
David R. Longnecker
Web Developer
http://blog.tiredstudent.com

David,

You are going to have to do an explicit copy here.

Assuming you don't want to hide the FindAll method (I recommend
against it), and that you expose the constructor for your
StudentRoster which takes an IEnumerable<StudentRecord>, you can do
this:

StudentRoster studentsInGrade8 = new
StudentRoster(roster.FindAll(delegate (StudentRecord x) { return
x.Grade.Equals("08"); } ));

This way, the List<StudentRecordfeeds the construction of the
new StudentRoster instance.

If you really want to return a StudentRoster instance, then I
would recommend creating a static FindAll method which will take the
IList<StudentRecordimplementation and then perform the conversion to
a StudentRoster instance yourself.

Hope this helps.

"David Longnecker" <dl*********@community.nospamwrote in message
news:46************************@msnews.microsoft.c om...

>I'm working to create a base framework for our organization for web
and client-side applications. The framework interfaces with several
of our systems and provides the business and data layer connectivity
for basic operations and such.

I've ran into a snag that I just can't think myself out of.

Here's an example:

I have an object for a Student called StudentRecord. It has
properties such as name, grade, identification number, etc.

public class StudentRecord : IComparable
{
private int _studentId;
public int StudentId { get { return _studentId; } set { _studentId =
value; } }
public override string ToString() { get { return string.Format("{0}
{1}",
_firstName, _lastName); }
... etc.

}

From there, I have a "roster" of students, thus a collection of those
StudentRecord objects. I would like to inherit from
List<StudentRecordto provide Find, FindAll, Sort, and other
functionality to the consumer of the API. (1)*

public class StudentRoster : List<StudentRecord>
{
// I'm not providing any additional functionality here at this time,
// simply providing the easier call rather than requring them to know
List<object>.
}
Now, roster.Find works just fine because, I'm assuming, the list is
based off StudentRecord.

StudentRecord studentA = roster.Find(delegate(StudentRecord x) {
return x.LastName.Equals("Longnecker"); });

This returns me a populated student object based off a prior created
collection of student Objects.

I run into an issue with:

StudentRoster studentsInGrade8 = roster.FindAll(delegate
(StudentRecord x) { return x.Grade.Equals("08"); } );

FindAll apparently returns a raw list of List<StudentRecordrather
than StudentRoster (since I haven't specified it). I've attempted to
modify my StudentRoster class to add:

public StudentRoster FindAll(Predicate<StudentRecorditem) {

StudentRoster items = (StudentRoster) Items;
return items.FindAll(match);
}

However, that fails with the same error--it cannot convert
List<StudentRecordto StudentRoster. I've attempted to cast it
explicitly (using 'as StudentRoster' and (StudentRoster)) to no
avail.

Google hasn't turned up anything thus far and neither has MSDN
(though I'm assuming, perhaps, I'm not searching for the magical
word)... What would be the best way to create "collections of
objects" that can be searched through using Find at the public API
level?

(1)* == Note that CLS does specify not to expose List<T>; however,
Collection<Tdoesn't have Find, Sort, FindAll, etc. Is there a
better option?

Any reference material, etc. would be greatly appreciated!

Thanks in advance.

-dl

---
David R. Longnecker
Web Developer
http://blog.tiredstudent.com

[Nicholas Paldino [.NET/C# MVP]]

David,

You don't have to add the IEnumerable interface declaration, List<T>
already does that for you.

Your class should look something like this:

public class StudentRoster : List<StudentRecord>
{
public StudentRoster(IEnumerable<StudentRecordcollection) :
base(collection)
{ }

public StudentRoster() : base()
{ }
}

There is no need for you to create a new list to pass to the base
constructor, since it is looking for an IEnumerable<T(where T is
StudentRecord in this case) as well, you can just pass it directly.

And you also don't need to create a new instance and chain the
constructor for the default parameterless constructor, since you can just
call the base constructor of List<T>.

In order to please both keepers, you would have to derive from
Collection<Tlike FxCop says. Of course, like you mention, it doesn't have
all the methods of List<T>, and you would have to implement those yourself.

You could try and create an explicit and implicit casting operator from
IEnumerable<StudentRecordto StudentRoster, and that might get you a little
further, but personally, I find that feeding the constructor of a new
StudentRoster instance works fine for me. I can see what is happening, and
I'm not polluting the type (List<T>, from which you derive) so to speak.

This brings up an interesting point. If you are not adding any
significant functionality to List<StudentRecordin the StudentRoster class,
then you really shouldn't be deriving from it. You aren't gaining anything
but a different name, and are gaining a headache in the process.
List<StudentRosterseems to work just fine here (unless there is more to
the class than you are exposing here).

Another side note, LINQ is going to make all of this moot, since you
would be able to just do:

// Get the count of students in the 9th grade. I'm using your call to
Equals since I don't know what type the Grade property
// returns. If it was a string, I'd just use ==.
int count = (from student in roster where
student.Grade.Equals("09")).Count();

Once that happens, methods on collection types for searching, ordering,
etc, etc, are going to become pretty useless.

--
- Nicholas Paldino [.NET/C# MVP]
- mv*@spam.guard.caspershouse.com

"David Longnecker" <dl*********@community.nospamwrote in message
news:46************************@msnews.microsoft.c om...

Okay, wow, it hadn't even occured to me that the reference was just sorta
trying to override itself rather than create a new one--I've been staring
at this for too long today. Thanks.

So, I added two lines to my StudentRoster class. One, the IEnumerable for
creating new, and the empty one:

public class StudentRoster : List<StudentRecord>, IEnumerable
{
public StudentRoster(IEnumerable<StudentRecordrecord) : base (new
List<StudentRecord>(record)) { }
public StudentRoster() : base(new List<StudentRecord>()) { }

}

Then, as you said, the new roster works:

StudentRoster studentsInGrade9 = new
StudentRoster(roster.FindAll(delegate(StudentRecor d x) { return
x.Grade.Equals("09"); }));
Console.WriteLine("There are {0} students in the ninth grade.",
studentsInGrade9.Count);

The follow up I have to that is that I'm not sure how to please both
keepers, so to speak. I'm wanting to propagate down the .Find, .FindAll,
and .Sort (for example) methods, but CLS (ala FxCop) is complaining that:

"Change %OBJECT% to use Collection<T>, ReadOnlyCollection<Tor
KeyedCollection<K,V>"

Of course, Collection doesn't have the methods of List. Is there a
benefit to this beyond 100% compliance? If so, is it documented out there
'best practices' of how to create "collections that act like lists"?

Thanks!

-dl
---
David R. Longnecker
Web Developer
http://blog.tiredstudent.com

>David,

You are going to have to do an explicit copy here.

Assuming you don't want to hide the FindAll method (I recommend
against it), and that you expose the constructor for your
StudentRoster which takes an IEnumerable<StudentRecord>, you can do
this:

StudentRoster studentsInGrade8 = new
StudentRoster(roster.FindAll(delegate (StudentRecord x) { return
x.Grade.Equals("08"); } ));

This way, the List<StudentRecordfeeds the construction of the
new StudentRoster instance.

If you really want to return a StudentRoster instance, then I
would recommend creating a static FindAll method which will take the
IList<StudentRecordimplementation and then perform the conversion to
a StudentRoster instance yourself.

Hope this helps.

"David Longnecker" <dl*********@community.nospamwrote in message
news:46************************@msnews.microsoft. com...

>>I'm working to create a base framework for our organization for web
and client-side applications. The framework interfaces with several
of our systems and provides the business and data layer connectivity
for basic operations and such.

I've ran into a snag that I just can't think myself out of.

Here's an example:

I have an object for a Student called StudentRecord. It has
properties such as name, grade, identification number, etc.

public class StudentRecord : IComparable
{
private int _studentId;
public int StudentId { get { return _studentId; } set { _studentId =
value; } }
public override string ToString() { get { return string.Format("{0}
{1}",
_firstName, _lastName); }
... etc.

}

From there, I have a "roster" of students, thus a collection of those
StudentRecord objects. I would like to inherit from
List<StudentRecordto provide Find, FindAll, Sort, and other
functionality to the consumer of the API. (1)*

public class StudentRoster : List<StudentRecord>
{
// I'm not providing any additional functionality here at this time,
// simply providing the easier call rather than requring them to know
List<object>.
}
Now, roster.Find works just fine because, I'm assuming, the list is
based off StudentRecord.

StudentRecord studentA = roster.Find(delegate(StudentRecord x) {
return x.LastName.Equals("Longnecker"); });

This returns me a populated student object based off a prior created
collection of student Objects.

I run into an issue with:

StudentRoster studentsInGrade8 = roster.FindAll(delegate
(StudentRecord x) { return x.Grade.Equals("08"); } );

FindAll apparently returns a raw list of List<StudentRecordrather
than StudentRoster (since I haven't specified it). I've attempted to
modify my StudentRoster class to add:

public StudentRoster FindAll(Predicate<StudentRecorditem) {

StudentRoster items = (StudentRoster) Items;
return items.FindAll(match);
}

However, that fails with the same error--it cannot convert
List<StudentRecordto StudentRoster. I've attempted to cast it
explicitly (using 'as StudentRoster' and (StudentRoster)) to no
avail.

Google hasn't turned up anything thus far and neither has MSDN
(though I'm assuming, perhaps, I'm not searching for the magical
word)... What would be the best way to create "collections of
objects" that can be searched through using Find at the public API
level?

(1)* == Note that CLS does specify not to expose List<T>; however,
Collection<Tdoesn't have Find, Sort, FindAll, etc. Is there a
better option?

Any reference material, etc. would be greatly appreciated!

Thanks in advance.

-dl

---
David R. Longnecker
Web Developer
http://blog.tiredstudent.com

[Jon Skeet [C# MVP]]

On May 2, 2:24 pm, "Nicholas Paldino [.NET/C# MVP]" wrote:

<snip>

Another side note, LINQ is going to make all of this moot, since you
would be able to just do:

// Get the count of students in the 9th grade. I'm using your call to
Equals since I don't know what type the Grade property
// returns. If it was a string, I'd just use ==.
int count = (from student in roster where
student.Grade.Equals("09")).Count();

Once that happens, methods on collection types for searching, ordering,
etc, etc, are going to become pretty useless.

I disagree. While LINQ has massive benefits in complex cases, just
using the extension methods (and existing methods) directly is
simpler. For instance, your query could be written equally effectively
as:

int count = roster.Count (student =student.Grade.Equals("09"));

Personally I'll save the compiler conversion stuff for more
complicated cases.

Jon

[David Longnecker]

Thanks for the information on IEnumerable and cutting that down a bit.

***

This brings up an interesting point. If you are not adding any
significant functionality to List<StudentRecordin the StudentRoster
class, then you really shouldn't be deriving from it. You aren't
gaining anything but a different name, and are gaining a headache in
the process. List<StudentRosterseems to work just fine here (unless
there is more to the class than you are exposing here).

***

You are correct; at this time, the derived class simply recreates List<StudentRecord>
with a more 'friendly' name of StudentRoster. I anticipate extending it
in the future, however, and didn't want to force any dependent applications
to have to change... I'd rather work out the issues now than later and still
require the consumers of the class to change their calls.

***

Another side note, LINQ is going to make all of this moot, since you would

be able to just do:
***

Agreed 100%; I wish the day was closer. As Jon S. points out (further down
the convo), I've dinked around with the lambda expressions on a copy of the
project in Orcas and it's more intuitive, or seem so, than the anonymous
methods.

Thanks for all your help and explainations; I don't feel my eyes crossing
(quite as bad) anymore. ;)

-dl

---
David R. Longnecker
Web Developer
http://blog.tiredstudent.com

David,

You don't have to add the IEnumerable interface declaration,
List<Talready does that for you.

Your class should look something like this:

public class StudentRoster : List<StudentRecord>
{
public StudentRoster(IEnumerable<StudentRecordcollection) :
base(collection)
{ }
public StudentRoster() : base()
{ }
}
There is no need for you to create a new list to pass to the base
constructor, since it is looking for an IEnumerable<T(where T is
StudentRecord in this case) as well, you can just pass it directly.

And you also don't need to create a new instance and chain the
constructor for the default parameterless constructor, since you can
just call the base constructor of List<T>.

In order to please both keepers, you would have to derive from
Collection<Tlike FxCop says. Of course, like you mention, it
doesn't have all the methods of List<T>, and you would have to
implement those yourself.

You could try and create an explicit and implicit casting operator
from IEnumerable<StudentRecordto StudentRoster, and that might get
you a little further, but personally, I find that feeding the
constructor of a new StudentRoster instance works fine for me. I can
see what is happening, and I'm not polluting the type (List<T>, from
which you derive) so to speak.

This brings up an interesting point. If you are not adding any
significant functionality to List<StudentRecordin the StudentRoster
class, then you really shouldn't be deriving from it. You aren't
gaining anything but a different name, and are gaining a headache in
the process. List<StudentRosterseems to work just fine here (unless
there is more to the class than you are exposing here).

Another side note, LINQ is going to make all of this moot, since
you would be able to just do:

// Get the count of students in the 9th grade. I'm using your call to
Equals since I don't know what type the Grade property
// returns. If it was a string, I'd just use ==.
int count = (from student in roster where
student.Grade.Equals("09")).Count();
Once that happens, methods on collection types for searching,
ordering, etc, etc, are going to become pretty useless.

"David Longnecker" <dl*********@community.nospamwrote in message
news:46************************@msnews.microsoft.c om...

>Okay, wow, it hadn't even occured to me that the reference was just
sorta trying to override itself rather than create a new one--I've
been staring at this for too long today. Thanks.

So, I added two lines to my StudentRoster class. One, the
IEnumerable for creating new, and the empty one:

public class StudentRoster : List<StudentRecord>, IEnumerable
{
public StudentRoster(IEnumerable<StudentRecordrecord) : base (new
List<StudentRecord>(record)) { }
public StudentRoster() : base(new List<StudentRecord>()) { }
}

Then, as you said, the new roster works:

StudentRoster studentsInGrade9 = new
StudentRoster(roster.FindAll(delegate(StudentReco rd x) { return
x.Grade.Equals("09"); }));
Console.WriteLine("There are {0} students in the ninth grade.",
studentsInGrade9.Count);
The follow up I have to that is that I'm not sure how to please both
keepers, so to speak. I'm wanting to propagate down the .Find,
.FindAll, and .Sort (for example) methods, but CLS (ala FxCop) is
complaining that:

"Change %OBJECT% to use Collection<T>, ReadOnlyCollection<Tor
KeyedCollection<K,V>"

Of course, Collection doesn't have the methods of List. Is there a
benefit to this beyond 100% compliance? If so, is it documented out
there 'best practices' of how to create "collections that act like
lists"?

Thanks!

-dl
---
David R. Longnecker
Web Developer
http://blog.tiredstudent.com

>>David,

You are going to have to do an explicit copy here.

Assuming you don't want to hide the FindAll method (I recommend
against it), and that you expose the constructor for your
StudentRoster which takes an IEnumerable<StudentRecord>, you can do
this:

StudentRoster studentsInGrade8 = new
StudentRoster(roster.FindAll(delegate (StudentRecord x) { return
x.Grade.Equals("08"); } ));

This way, the List<StudentRecordfeeds the construction of the new
StudentRoster instance.

If you really want to return a StudentRoster instance, then I
would recommend creating a static FindAll method which will take the
IList<StudentRecordimplementation and then perform the conversion
to
a StudentRoster instance yourself.
Hope this helps.

"David Longnecker" <dl*********@community.nospamwrote in message
news:46************************@msnews.microsoft .com...

I'm working to create a base framework for our organization for web
and client-side applications. The framework interfaces with
several of our systems and provides the business and data layer
connectivity for basic operations and such.

I've ran into a snag that I just can't think myself out of.

Here's an example:

I have an object for a Student called StudentRecord. It has
properties such as name, grade, identification number, etc.

public class StudentRecord : IComparable
{
private int _studentId;
public int StudentId { get { return _studentId; } set { _studentId
=
value; } }
public override string ToString() { get { return string.Format("{0}
{1}",
_firstName, _lastName); }
... etc.
}

From there, I have a "roster" of students, thus a collection of
those StudentRecord objects. I would like to inherit from
List<StudentRecordto provide Find, FindAll, Sort, and other
functionality to the consumer of the API. (1)*

public class StudentRoster : List<StudentRecord>
{
// I'm not providing any additional functionality here at this
time,
// simply providing the easier call rather than requring them to
know
List<object>.
}
Now, roster.Find works just fine because, I'm assuming, the list is
based off StudentRecord.
StudentRecord studentA = roster.Find(delegate(StudentRecord x) {
return x.LastName.Equals("Longnecker"); });

This returns me a populated student object based off a prior
created collection of student Objects.

I run into an issue with:

StudentRoster studentsInGrade8 = roster.FindAll(delegate
(StudentRecord x) { return x.Grade.Equals("08"); } );

FindAll apparently returns a raw list of List<StudentRecordrather
than StudentRoster (since I haven't specified it). I've attempted
to modify my StudentRoster class to add:

public StudentRoster FindAll(Predicate<StudentRecorditem) {

StudentRoster items = (StudentRoster) Items;
return items.FindAll(match);
}
However, that fails with the same error--it cannot convert
List<StudentRecordto StudentRoster. I've attempted to cast it
explicitly (using 'as StudentRoster' and (StudentRoster)) to no
avail.

Google hasn't turned up anything thus far and neither has MSDN
(though I'm assuming, perhaps, I'm not searching for the magical
word)... What would be the best way to create "collections of
objects" that can be searched through using Find at the public API
level?

(1)* == Note that CLS does specify not to expose List<T>; however,
Collection<Tdoesn't have Find, Sort, FindAll, etc. Is there a
better option?

Any reference material, etc. would be greatly appreciated!

Thanks in advance.

-dl

---
David R. Longnecker
Web Developer
http://blog.tiredstudent.com