VS2008 complains about T[ ] data = ...

  • Thread starter Thread starter Rich P
  • Start date Start date
R

Rich P

I have been seeing code samples using notation like this:

...
T[] data = source.ToArray();
...

When I try the sample in VS2008 -- I get the following error message:

"The type or namespace name 'T' could not be found (are you missing a
using directive or an assembly reference?"


Is the notation above a VS2010 thing? If yes, is there an equivalent
for VS2008? If no, what do I need to add so that VS2008 does not
complain about T[] something = ... ?

Thanks

Rich
 
Rich P said:
..
T[] data = source.ToArray();
..

When I try the sample in VS2008 -- I get the following error message:

"The type or namespace name 'T' could not be found (are you missing a
using directive or an assembly reference?"
Click to expand...

This will only work if you use it inside a class or method that is
marked as "Generic" and contains the parameter T:

public void MyMethod<T> ()
{
..
T[] data = source.ToArray();
..
}

Note the <T> in the declaration of the method. That is what tells the
compiler to treat the T in T[] as a class name. The real class will be
passed to the method when it is inivoked:

MyMethod<string>();

The preceding causes the "T" to become "string" when caling the method.
You can do a similar thing at the class level by adding <T> after the
class name.
 
Thank you for your reply. This did fix one of the complaints from
VS2008, but he is still complaining about one other guy. Here is the
sample class that I copied from another post with the recommended fix of
adding <T> to ...class EnumerableExtensions :

public static class EnumerableExtensions<T>
{
private static readonly Random random = new Random();

//--VS2008 is complaining about "selecttrandomitem<t>"

public static ienumerable<t> selectrandomitem<t>(this ienumerable<t>
source, int total)
{
if (source == null) yield break;

T[] data = source.ToArray();

if (total < 0 || total > data.Length)
{
throw new ArgumentException();
}

for (int i = 0; i < total; i++)
{
int index = random.Next(data.Length - i);

yield return data[index];

data[index] = data[data.Length - i - 1];
}
}
}

Here is the error message for "selecttrandomitem<t>"

"The body of
'MySampleApp.EnumerableExtensions<T>.selectrandomitem<t>(ienumerable<t>,
int)' cannot be an iterator block because 'ienumerable<t>' is not an
iterator interface type"

This sample class is great for learning Generics in C#. May I ask how I
could fix this error based on the context of the sample above?

Thanks

Rich
 
Thank you for your reply.  This did fix one of the complaints from
VS2008,  but he is still complaining about one other guy.  Here is the
sample class that I copied from another post with the recommended fix of
adding <T> to ...class EnumerableExtensions :

public static class EnumerableExtensions<T>
{
  private static readonly Random random = new Random();

   //--VS2008 is complaining about "selecttrandomitem<t>"

  public static ienumerable<t> selectrandomitem<t>(this ienumerable<t>
Click to expand...

Yes, C# is case sensitive. This should be IEnumerable<T> and so forth.

Matt
 
Try this, just a reminder c# is case sensitive?



using System.Collections.Generic;

public static class EnumerableExtensions
{
private static readonly Random random = new Random();

public static IEnumerable<T> selectrandomitem<T>(this IEnumerable<T>
source, int total)
{
if (source == null) yield break;

T[] data = source.ToArray();

if (total < 0 || total > data.Length)
{
throw new ArgumentException();
}

for (int i = 0; i < total; i++)
{
int index = random.Next(data.Length - i);
yield return data[index];
data[index] = data[data.Length - i - 1];
}
}
}
 
OK. Now I have this class -- which VS is not complaining about. How to
I use a class like this? Could someone share a sample usage?

public static class enumerableextensions<t>
{
private static readonly Random random = new Random();

public static IEnumerable<t> selectrandomitem<t>(this IEnumerable<t>
source, int total)
{
if (source == null) yield break;

t[] data = source.ToArray();

if (total < 0 || total > data.Length)
{
throw new ArgumentException();
}

for (int i = 0; i < total; i++)
{
int index = random.Next(data.Length - i);

yield return data[index];

data[index] = data[data.Length - i - 1];
}
}
}

In the post I copied this from -- VS complains about the following usage
and I don't see a connection to the class above:

static void Main(string[] args)
{
var data = Enumerable.Range(0, 21);

//--VS complains here about "SelectRandomItem"
foreach (var item in data.SelectRandomItem(10))
{
Console.WriteLine(item);
}
}

I tried changing the casing of "SelectRandomItem", VS still complain,
and still I don't see the connection to the class above. The post was
about picking 10 numbers randomly from a list of numbers. I just copied
this post to study usage of Generics.


Rich
 
OK. Now I have this class -- which VS is not complaining about. How to
I use a class like this? Could someone share a sample usage?

public static class enumerableextensions<t>
{
private static readonly Random random = new Random();

public static IEnumerable<t> selectrandomitem<t>(this IEnumerable<t>
source, int total)
{
if (source == null) yield break;

t[] data = source.ToArray();

if (total< 0 || total> data.Length)
{
throw new ArgumentException();
}

for (int i = 0; i< total; i++)
{
int index = random.Next(data.Length - i);

yield return data[index];

data[index] = data[data.Length - i - 1];
}
}
}

In the post I copied this from -- VS complains about the following usage
and I don't see a connection to the class above:

static void Main(string[] args)
{
var data = Enumerable.Range(0, 21);

//--VS complains here about "SelectRandomItem"
foreach (var item in data.SelectRandomItem(10))
{
Console.WriteLine(item);
}
}

I tried changing the casing of "SelectRandomItem", VS still complain,
and still I don't see the connection to the class above. The post was
about picking 10 numbers randomly from a list of numbers. I just copied
this post to study usage of Generics.


Rich
Click to expand...

Hi Rich,

I glad that you benefit something from my code.
Anyway, as for the comments, if you copy my code exactly as it appear,
it should compile fine.
- SelectRandomItem is an extension method and it can only be declared on
a *non-generic* static class. You had mistakenly make the class generic
by adding <t> on the class definition. That's the source of error.
- It is a well known practice to use upper-case letter 'T' as a
placeholder for the generic type. Otherwise, it will upset your collegues.
- You need to work up on proper casing of names.

As for the usage, if you follow my example carefully, it will print 10
random numbers in the range of 0 to 20. Another usage is for example
suppose you would like to calculate the sum of three dices thrown on the
table,

static void Main(string[] args)
{
var dice = Enumerable.Range(1, 6);

var threeDices = new List<int>(dice); // first dice
threeDices.AddRange(dice); // second dice
threeDices.AddRange(dice); // third dice

int sumOfThreeDices = threeDices.SelectRandomItem(3).Sum();

Console.WriteLine("Sum of three dice = {0}", sumOfThreeDices);
}

Regards.
 
SelectRandomItem is an extension method and it can only be declared on a
*non-generic* static class. You had mistakenly make the class generic by
adding <t> on the class definition. That's the source of error.
<

Thank you for your reply. I still get an error if I copy your class
like this:

public static class EnumerableExtensions
{
private static readonly Random random = new Random();

//--VS complains here about selectrandomitem
public static ienumerable<t> selectrandomitem<t>(this ienumerable<t>
source, int total)
{
if (source == null) yield break;

//--VS complains about T[]
T[] data = source.ToArray();

if (total < 0 || total > data.Length)
{
throw new ArgumentException();
}

for (int i = 0; i < total; i++)
{
int index = random.Next(data.Length - i);
//--and VS complains about data[index]
yield return data[index];
data[index] = data[data.Length - i - 1];
}
}
}

Here are the error messages I am getting with this class (VS2008)

--error1
The body of
'ConsoleApplication1.EnumerableExtensions.selectrandomitem<t>(ienumerabl
e<t>, int)' cannot be an iterator block because 'ienumerable<t>' is not
an iterator interface type

--error2
The type or namespace name 'T' could not be found (are you missing a
using directive or an assembly reference?)

--error3
Cannot implicitly convert type 'T' to 'object'

What should I do to eliminate these errors?

Also, not sure if this is of any significance, but I also see that a lot
of code sampels have

* list<something> x = new list<something>() *

on my workstation I don't have a * list *. I have to use uppercase *
List *. Is my VS2008 different? I am pretty sure I have all the latest
patches for it and service packs (version 3.5 SP1).

Thanks,



Rich
 
SelectRandomItem is an extension method and it can only be declared on a
*non-generic* static class. You had mistakenly make the class generic by
adding<t> on the class definition. That's the source of error.
<

Thank you for your reply. I still get an error if I copy your class
like this:

public static class EnumerableExtensions
{
private static readonly Random random = new Random();

//--VS complains here about selectrandomitem
public static ienumerable<t> selectrandomitem<t>(this ienumerable<t>
source, int total)
{
if (source == null) yield break;

//--VS complains about T[]
T[] data = source.ToArray();

if (total< 0 || total> data.Length)
{
throw new ArgumentException();
}

for (int i = 0; i< total; i++)
{
int index = random.Next(data.Length - i);
//--and VS complains about data[index]
yield return data[index];
data[index] = data[data.Length - i - 1];
}
}
}
Click to expand...

As I said previously, If you had copy the code exactly as it appear, it
should compile fine. You had mistakenly changed the casing for
Here are the error messages I am getting with this class (VS2008)

--error1
The body of
'ConsoleApplication1.EnumerableExtensions.selectrandomitem<t>(ienumerabl
e<t>, int)' cannot be an iterator block because 'ienumerable<t>' is not
an iterator interface type
Click to expand...

As others has said, C# is *case-sensitive*. There is no ienumerable<t>
in the framework, only IEnumerable<T> which is defined in
System.Collections.Generic namespace. Since I had use 'yield break' and
'yield return' statement in the method block, the return type of the
SelectRandomItem method must be of type IEnumerable said:
--error2
The type or namespace name 'T' could not be found (are you missing a
using directive or an assembly reference?)
Click to expand...

I had made the SelectRandomItem method generic by adding <T> at the name
of methods. You had mistakenly changed the casing from uppercase <T> to
lowercase <t>. Generic type declaration have to be consistent with the
usage inside the method. If you use lowercase 't' on the method
definition, you must also use the same lowercase 't' inside the method body.
--error3
Cannot implicitly convert type 'T' to 'object'
Click to expand...

If you resolve the error2 above, this error should go away.
What should I do to eliminate these errors?
Click to expand...

Well, read the code carefully and work up on the proper casing.
Also, not sure if this is of any significance, but I also see that a lot
of code sampels have

* list<something> x = new list<something>() *

on my workstation I don't have a * list *. I have to use uppercase *
List *. Is my VS2008 different? I am pretty sure I have all the latest
patches for it and service packs (version 3.5 SP1).
Click to expand...

Care to share where do you get that code sample? Maybe a typing error or
maybe the author of that code did defined a new type named list<T>?
Without the original code, I cannot say for sure but as far as .NET is
concerned, all classes name in the framework starts with the uppercase
letter.

In order to aid your understanding of my code, below are the features
that I had used and the links to MSDN website for the details

1. Generics
http://msdn.microsoft.com/en-us/library/512aeb7t(VS.90).aspx

2. Iterators
http://msdn.microsoft.com/en-us/library/dscyy5s0(VS.90).aspx

3. Extension Methods
http://msdn.microsoft.com/en-us/library/bb383977(VS.90).aspx


Regards.
 
Thank you again for your reply. Here is the post where I originally got
the code sample:

http://www.developersdex.com/csharp/message.asp?p=1111&r=6923729


Re: 10 random numbers from 0 to 20
From: kndg
Date Posted: 6/15/2010 8:55:00 PM
...

Which appears is your code. Thank you for sharing that sample with this
NG. I copied everything exactly is it was posted. But my console
application is complaining, and my goal is to learn/gain more
understanding of C# by trying to get this to work. And this is a great
sample for that learning. I hope I can get your code to work because it
looks real cool and very useful.

Rich
 
Thank you again for your reply. Here is the post where I originally got
the code sample:

http://www.developersdex.com/csharp/message.asp?p=1111&r=6923729


Re: 10 random numbers from 0 to 20
From: kndg
Date Posted: 6/15/2010 8:55:00 PM
...

Which appears is your code. Thank you for sharing that sample with this
NG. I copied everything exactly is it was posted. But my console
application is complaining, and my goal is to learn/gain more
understanding of C# by trying to get this to work. And this is a great
sample for that learning. I hope I can get your code to work because it
looks real cool and very useful.

Rich
Click to expand...

Ahh, now I know what cause the confusion.

Answer: Stop using that website. They don't render code correctly.
Either use a GoogleGroups or get yourself an Nntp Client (ex: Mozilla
Thunderbird)

Here is the link to the original post at GoogleGroups

http://groups.google.com/group/micr...a4/f8812af0d820bd42?lnk=raot#f8812af0d820bd42

If the above link does not work correctly, use this shortcut:
http://tinyurl.com/28fo7za

Anyway, Happy Coding!

Regards.
 
Hahaha...
After re-read back all my replies to you using that website, I couldn't
stop laughing. Hahaha... I said copy exactly - but you did copied
exactly. The other said use the correct casing - but what you saw is all
in lowercase. Hahaha... Its realy really make your life difficult. Maybe
you should file a complaint to that website.
 
OK. Now I have it straight, and your sample code works perfectly! Many
thanks. This was a great lesson on generics and casing. It was
humbling, but a great lesson!

Rich
 
Back
Top