structs and 'using' - C# Discussion Boards

29-Oct-04 6:47

A question for C# language experts:

If I have a struct that implements IDisposable:

struct C : IDisposable
{
  public int clan;
  public void Dispose()
  {
  Console.WriteLine("Disposing");
  }
}

And later in the code something like:

using (C s = new C())
{
  s.clan = 1;
}

A compiler error is reported:

error CS0131: The left-hand side of an assignment must be a variable, property or indexer

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leppie29-Oct-04 9:02

leppie

29-Oct-04 9:02

The problem lies with the fact that structs cant be safely casted, in this case IDisposable. The compiler will typical generate the following for the 'using' block:

IDisposable o = new C();
try
{
  C s = o as C; //this is not allowable for structs, 
  // lets assume it would then it would go out of scope before Dispose can be called
  // your code here
}
finally
{
  o.Dispose();
}

Anyways, my best guess :p

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Nemanja Trifunovic29-Oct-04 9:14

29-Oct-04 9:14

Hmmmm, but using as with structs causes

error CS0077: The as operator must be used with a reference type ('sharptest.C' is a value type)

And my code complains about assigning a value to a member of s.

Also, this compiles (and works) fine:

using (C s = new C())
{
  Console.WriteLine(s.clan.ToString());
}

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Ketty Avashia29-Oct-04 9:27

Ketty Avashia

29-Oct-04 9:27

If U have the struct in the same namespace as ur code
the code below works.

C c = new C();
c.clan = 1;
Console.WriteLine(c.clan.ToString());

and if in different namespace -- in that case using can follow the initial using block. I mean immediately after
using System;
using System.Data;
....
using NameSpaceName.C;

C c = new C();
c.clan = 1;
Console.WriteLine(c.clan.ToString());

I hope this help and that i have not missunderstood ur question. Roll eyes | :rolleyes:

Ketty

Nemanja Trifunovic29-Oct-04 9:42

I got an answer on a C# newsgroup

29-Oct-04 9:42

Ketty Avashia wrote:
I hope ... that i have not missunderstood ur question.

Thanks, but you have Smile | :)

I asked about using Statement[^], not using Directive[^].

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Nemanja Trifunovic29-Oct-04 10:00

29-Oct-04 10:00

According to the C# language spec[^]:

Local variables declared in a resource-acquisition are read-only, and must include an initializer. A compile-time error occurs if the embedded statement attempts to modify these local variables (by assignment or the ++ and -- operators) or pass them as ref or out parameters.

Wow!

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Ami Bar29-Oct-04 12:16

Ami Bar

29-Oct-04 12:16

Structures are allocated on the stack and not on the heap as classes.

Write the following code instead, it does the same trick:

C s = new C();
using (s)
{
s.clan = 1;
}

Ami

Nemanja Trifunovic29-Oct-04 12:27

29-Oct-04 12:27

Ami Bar wrote:
Structures are allocated on the stack and not on the heap as classes.

That is correct in this context - I just didn't know that variables declared in using nlock need to be read-only.

BTW, what exactly "read-only" really means in C#? It does not have const methods like C++.

Whatever... Thanks for your answer Smile | :)

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Cant not get the ListView TextAlign Property to center

Ami Bar29-Oct-04 12:51

Ami Bar

29-Oct-04 12:51

C# doesn't have const and its a petty.

"The readonly keyword is a modifier that you can use on fields. When a field declaration includes a readonly modifier, assignments to the fields introduced by the declaration can only occur as part of the declaration or in a constructor in the same class.", MSDN

Here is the link to the full readonly keyword explanation in the MSDN:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/csref/html/vclrfreadonlypg.asp

Tony D. Abel29-Oct-04 5:37

Tony D. Abel

29-Oct-04 5:37

Good day all.

While attempting to use the ListView column Textalign property to 'Center' text with in the first column of my Listbox, it always uses the Left text align variable. I have a three column ListView. The sub items of this List accepts and aligns the text within the other columns 'Centered'.

Thank you in advance for your assistance.

Have a great coding day.

Regards,

Tony D. Abel

Re: Cant not get the ListView TextAlign Property to center

sreejith ss nair29-Oct-04 5:51

sreejith ss nair

29-Oct-04 5:51

Please check all your columns and there TextAlign property in your listview control. And make sure the value of TextAlign property is 'Center'.

Sreejith Nair
[ My Articles ]

User Control taking up too much resources

xiaowenjie29-Oct-04 4:44

29-Oct-04 4:44

hi, i've added user control onto my main form. but when i run my program, i realise it eats up a lot of resources. my com becames very lag. what's the problem?

Chris

sreejith ss nair29-Oct-04 4:47

sreejith ss nair

29-Oct-04 4:47

What sort of user control that you are using ? i mean what it does ?

Sreejith Nair
[ My Articles ]

xiaowenjie29-Oct-04 4:53

29-Oct-04 4:53

i've this main window form where it contains 4 user control. each of this user control connects to the database. after i added these user control onto my main form and run my program, my com became very lag.

Chris

sreejith ss nair29-Oct-04 4:59

sreejith ss nair

29-Oct-04 4:59

Try to create a class which do database communication insted of user controls. And try to get the reference of class which have the implimentation part of data access into your user control.

Sreejith Nair
[ My Articles ]

xiaowenjie29-Oct-04 5:02

29-Oct-04 5:02

ok i try tt. thanks Wink | ;)

chris

xiaowenjie29-Oct-04 7:06

29-Oct-04 7:06

hi, i've managed to put the database connection coding into a separate class.

namespace WindowsApplication2
{
public class db1
{
public OleDbConnection conn;
public string display_SQL;
public OleDbCommand display_cmd;
public OleDbDataAdapter display_adapter;
public DataSet display_ds;

public db1()
{
conn = new System.Data.OleDb.OleDbConnection();
conn.ConnectionString = @"Provider=Microsoft.Jet.OLEDB.4.0;" +
@"Data source =C:\Documents and Settings\Desktop\WindowsApplication2\bin\Debug\mydbms.mdb";

display_SQL = "SELECT * FROM tblYear";
display_cmd = new System.Data.OleDb.OleDbCommand (display_SQL, conn);
display_adapter = new System.Data.OleDb.OleDbDataAdapter(display_cmd);
display_ds = new System.Data.DataSet();
display_adapter.Fill(display_ds,"tblYear");
}

}
i'm not really good in using separate class.so how do we do these same connection into a function in this same class instead of doing it in the constructor? and how do we call that function from a button?

anw is there a wweb site where they teach about windows classes which may assist more??

Chris

xiaowenjie29-Oct-04 7:28