Anonymous type etc. in C# 3.0

[Chris Dunaway]

I understand, basically, what this C# 3.0 code does, but I am unclear
on how it determines the data type of num and index in the lambda
expression in the following code. Nor can I see how it gets the array
index into the index parameter?

BTW: Is there a better forum for asking these questions?
public void Linq12() {
int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };

var numsInPlace = numbers.Select((num, index) => new {Num = num,
InPlace = (num == index)});

Console.WriteLine("Number: In-place?");
foreach (var n in numsInPlace) {
Console.WriteLine("{0}: {1}", n.Num, n.InPlace);
}
}

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

Chris,

This is not valid, because the Select method will only take a lambda
expression with one parameter (the item being evaluated for selection).

If you wanted to do this, you would need a type in the enumeration which
exposed the index and the value.

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

"Chris Dunaway" <du******@gmail.com> wrote in message
news:11**********************@g49g2000cwa.googlegr oups.com...

I understand, basically, what this C# 3.0 code does, but I am unclear
on how it determines the data type of num and index in the lambda
expression in the following code. Nor can I see how it gets the array
index into the index parameter?

BTW: Is there a better forum for asking these questions?
public void Linq12() {
int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };

var numsInPlace = numbers.Select((num, index) => new {Num = num,
InPlace = (num == index)});

Console.WriteLine("Number: In-place?");
foreach (var n in numsInPlace) {
Console.WriteLine("{0}: {1}", n.Num, n.InPlace);
}
}

[Chris Dunaway]

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

This is not valid, because the Select method will only take a lambda
expression with one parameter (the item being evaluated for selection).

Not valid? I got that example from this link at MS:

http://msdn.microsoft.com/vcsharp/fu...t.aspx#indexed

[Chris Dunaway]

Would it work like this? (Note that I eliminated the index parameter
off of the lambda expression and used of Array.IndexOf in the anonymous
type)

public void Linq12() {
int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };

var numsInPlace = numbers.Select((num) => new {Num = num,
InPlace = (num == Array.IndexOf( numbers, num ))});

Console.WriteLine("Number: In-place?");
foreach (var n in numsInPlace) {
Console.WriteLine("{0}: {1}", n.Num, n.InPlace);
}

[Chris Dunaway]

Chris Dunaway wrote:

var numsInPlace = numbers.Select((num) => new {Num = num,
InPlace = (num == Array.IndexOf( numbers, num ))});

And as a follow up, where is "Select" defined? Is that now available
because of Linq? Is it usable on all types?

Thanks,

[William Stacey [MVP]]

I think internally it creates an anonymous type like below:

internal class ???
{
public int Num;
public bool InPlace;
}

And returns a collection of that type.
It inferes the class fields/properties easily because it knows num is of
type Int32 and it knows InPlace is a bool because of the expression.

--
William Stacey [MVP]

"Chris Dunaway" <du******@gmail.com> wrote in message
news:11**********************@g49g2000cwa.googlegr oups.com...

I understand, basically, what this C# 3.0 code does, but I am unclear
on how it determines the data type of num and index in the lambda
expression in the following code. Nor can I see how it gets the array
index into the index parameter?

BTW: Is there a better forum for asking these questions?
public void Linq12() {
int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };

var numsInPlace = numbers.Select((num, index) => new {Num = num,
InPlace = (num == index)});

Console.WriteLine("Number: In-place?");
foreach (var n in numsInPlace) {
Console.WriteLine("{0}: {1}", n.Num, n.InPlace);
}
}

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

Chris,

From what I can tell, that should work. Very clever =)

As for the Select being defined, I pulled that from the C# 3.0 language
enhancements specification. You can find that at:

http://download.microsoft.com/downlo...cification.doc

It's defined as an extension method of IEnumerable<T>.
--
- Nicholas Paldino [.NET/C# MVP]
- mv*@spam.guard.caspershouse.com

"Chris Dunaway" <du******@gmail.com> wrote in message
news:11*********************@z14g2000cwz.googlegro ups.com...

Chris Dunaway wrote:

var numsInPlace = numbers.Select((num) => new {Num = num,
InPlace = (num == Array.IndexOf( numbers, num ))});

And as a follow up, where is "Select" defined? Is that now available
because of Linq? Is it usable on all types?

Thanks,

[Chris Dunaway]

William Stacey [MVP] wrote:

It inferes the class fields/properties easily because it knows num is of
type Int32 and it knows InPlace is a bool because of the expression.

I understand that part, but how can it infer the type of "index"?
Where does that come from?

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

Chris,

I believe the example is wrong, see my response with the definition of
Select.

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

"Chris Dunaway" <du******@gmail.com> wrote in message
news:11**********************@z14g2000cwz.googlegr oups.com...

William Stacey [MVP] wrote:

It inferes the class fields/properties easily because it knows num is of
type Int32 and it knows InPlace is a bool because of the expression.

I understand that part, but how can it infer the type of "index"?
Where does that come from?

[Chris Dunaway]

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

Chris,

From what I can tell, that should work. Very clever =)

I assume that the examples are just to illustrate the concepts because
the following code seems easier to read (even with the cast):

public void Linq12() {
int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };

Console.WriteLine("Number: In-place?");
foreach (int n in numbers) {
bool InPlace = (n == (int)Array.IndexOf(numbers,n));
Console.WriteLine("{0}: {1}", n, InPlace);
}
}

[William Stacey [MVP]]

I think because the selector constructor calls for an Int in the second
overload.
(T)
or
(T, int)
So "index" could be called anything. It is a place holder so you can get
the index.

--
William Stacey [MVP]

"Chris Dunaway" <du******@gmail.com> wrote in message
news:11**********************@z14g2000cwz.googlegr oups.com...

William Stacey [MVP] wrote:

It inferes the class fields/properties easily because it knows num is of
type Int32 and it knows InPlace is a bool because of the expression.

I understand that part, but how can it infer the type of "index"?
Where does that come from?

[William Stacey [MVP]]

It compiles and runs fine.

--
William Stacey [MVP]

"Nicholas Paldino [.NET/C# MVP]" <mv*@spam.guard.caspershouse.com> wrote in
message news:eW**************@TK2MSFTNGP14.phx.gbl...

Chris,

I believe the example is wrong, see my response with the definition of
Select.

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

"Chris Dunaway" <du******@gmail.com> wrote in message
news:11**********************@z14g2000cwz.googlegr oups.com...

William Stacey [MVP] wrote:

It inferes the class fields/properties easily because it knows num is of
type Int32 and it knows InPlace is a bool because of the expression.

I understand that part, but how can it infer the type of "index"?
Where does that come from?

[William Stacey [MVP]]

Yes but less efficient I think. IndexOf() requires another search into the
collection. If you use the index parm, then it set while it is traversing
the list.

--
William Stacey [MVP]

"Chris Dunaway" <du******@gmail.com> wrote in message
news:11*********************@g47g2000cwa.googlegro ups.com...

Would it work like this? (Note that I eliminated the index parameter
off of the lambda expression and used of Array.IndexOf in the anonymous
type)

public void Linq12() {
int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };

var numsInPlace = numbers.Select((num) => new {Num = num,
InPlace = (num == Array.IndexOf( numbers, num ))});

Console.WriteLine("Number: In-place?");
foreach (var n in numsInPlace) {
Console.WriteLine("{0}: {1}", n.Num, n.InPlace);
}

[Abubakar]

Hi, how can you compile the C# 3.0 code? Where is the compiler?

Ab.

"William Stacey [MVP]" wrote:

It compiles and runs fine.

--
William Stacey [MVP]

"Nicholas Paldino [.NET/C# MVP]" <mv*@spam.guard.caspershouse.com> wrote in
message news:eW**************@TK2MSFTNGP14.phx.gbl...

Chris,

I believe the example is wrong, see my response with the definition of
Select.

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

"Chris Dunaway" <du******@gmail.com> wrote in message
news:11**********************@z14g2000cwz.googlegr oups.com...

William Stacey [MVP] wrote:
It inferes the class fields/properties easily because it knows num is of
type Int32 and it knows InPlace is a bool because of the expression.
I understand that part, but how can it infer the type of "index"?
Where does that come from?

[William Stacey [MVP]]

http://msdn.microsoft.com/netframework/future/linq/
See the "C# LINQ Tech Preview" link to download the msi.

--
William Stacey [MVP]

"Abubakar" <Ab******@discussions.microsoft.com> wrote in message
news:FE**********************************@microsof t.com...

Hi, how can you compile the C# 3.0 code? Where is the compiler?

Ab.

"William Stacey [MVP]" wrote:

It compiles and runs fine.

--
William Stacey [MVP]

"Nicholas Paldino [.NET/C# MVP]" <mv*@spam.guard.caspershouse.com> wrote
in
message news:eW**************@TK2MSFTNGP14.phx.gbl...

> Chris,
>
> I believe the example is wrong, see my response with the definition
> of
> Select.
>
> --
> - Nicholas Paldino [.NET/C# MVP]
> - mv*@spam.guard.caspershouse.com
>
> "Chris Dunaway" <du******@gmail.com> wrote in message
> news:11**********************@z14g2000cwz.googlegr oups.com...
>> William Stacey [MVP] wrote:
>>> It inferes the class fields/properties easily because it knows num is
>>> of
>>> type Int32 and it knows InPlace is a bool because of the expression.
>>>
>>
>> I understand that part, but how can it infer the type of "index"?
>> Where does that come from?
>>
>
>

[Chris Dunaway]

William Stacey [MVP] wrote:

I think because the selector constructor calls for an Int in the second
overload.
(T)
or
(T, int)
So "index" could be called anything. It is a place holder so you can get
the index.

But where does it get its value? Somehow, the array index gets stuffed
into that variable.

[William Stacey [MVP]]

I think inside of the mojo they create behind the scenes like in an
enumerator or something.

--
William Stacey [MVP]

"Chris Dunaway" <du******@gmail.com> wrote in message
news:11**********************@g49g2000cwa.googlegr oups.com...

William Stacey [MVP] wrote:

I think because the selector constructor calls for an Int in the second
overload.
(T)
or
(T, int)
So "index" could be called anything. It is a place holder so you can get
the index.

But where does it get its value? Somehow, the array index gets stuffed
into that variable.

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

William, Chris,

I've figured out why this works. There is an overload of the Select
method that looks like this:

public static IEnumerable<S> Select<T, S>(this IEnumerable<T> source,
Func<T, int, S> selector)
{
int index = 0;

foreach (T element in source)
{
yield return selector(element, index);
index++;
}
}

This is where the index comes from, and the overload that accepts a
lambda expression with two inputs (the second one being the index).
--
- Nicholas Paldino [.NET/C# MVP]
- mv*@spam.guard.caspershouse.com

"William Stacey [MVP]" <st*****@mvps.org> wrote in message
news:eA*************@tk2msftngp13.phx.gbl...

It compiles and runs fine.

--
William Stacey [MVP]

"Nicholas Paldino [.NET/C# MVP]" <mv*@spam.guard.caspershouse.com> wrote
in message news:eW**************@TK2MSFTNGP14.phx.gbl...

Chris,

I believe the example is wrong, see my response with the definition of
Select.

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

"Chris Dunaway" <du******@gmail.com> wrote in message
news:11**********************@z14g2000cwz.googlegr oups.com...

William Stacey [MVP] wrote:
It inferes the class fields/properties easily because it knows num is
of
type Int32 and it knows InPlace is a bool because of the expression.
I understand that part, but how can it infer the type of "index"?
Where does that come from?