sending DataTable by ref - is there a way to do it, and any sense of doing it ?

[Stefan Turalski \(stic\)]

Hi,

I'm wondering if there is a way to send a method parametrs by ref when
DataTabel is a type of this value ?

I done some sort of select over DataTable columns, just by removing them
froma table is each of them isn't on a stirng[], but right no I have to do
it in a wat of:
1. passing DataTable to method (lets call it SelectOverDataTabel)
2. declare a local (in SelectOver...) DataTable, inicialize it with my value
3. doing all things on this local copy (removing.....)
4. and to the parent method send a local copy by return, sth like this
dtMyTable = clsLocalSQL.SelectOverDataTable( dtMyTable,
StringOfSelectedColumns)

Could I do it without closing DataTable in (object) and unpacking it in body
of my static procedure ??

 

[William Ryan eMVP]

Stefan:

I'm not entirely sure I understand the problem but you can definitely pass a
DataTable by reference. However, remember this is a reference type to begin
with and there's a lot to a datatable to keep track of if it's size is very
big so you may see all sorts of performance degradation depending on use.
It's often easier to pass the individual rows of the datatable which are
small and easier to work with.

I'm not sure what you mean by Closing the datatable though, there is no open
or closed state to a DataTable, only the database if that's where its filled
from. If you are referrring to the underlying connection, there's no reason
to leave it open here that I can see.

I'm not sure what the goal is here but if you could explain it a little
more, I could be of more help. Also, if you look at MS's Data Access
Application Block (DAAB) it returns Datasets (so DataTables are essentially
in the same boat) in the executeDataSet method. You can pass these things
over layers, back and forth to web services, you name it you can pass it.

Here's a typical scenario. I have a method that fills a datatable and
returns it:

Declare a datatable and cal lthe method:

DataTable dt = myDataAccessClass.GetDataTable();

//now a method in myDataAccessClass
private DataTable GetDataTable(){
//Open Connection,
DataTable dt = new DataTable();
//Fill datatable
return dt;
}

This is fine to do, is this essentially what you're asking?

 

[Stefan Turalski \(stic\)]

Hi,

Thanks for you answer but what I need to do is:
1. take one DataTable filled with for example 12 Columns named by letters
A,B,C,...
DataTable dtMyTable = new DataTable();
//sqlconnection code
dataadapert.Fill(dtMyTable) <- when commandtext is "Select A,B,C,D,F,G,...
from sometable
2. take one string[] with allow me to filter this DataTable for examle new
string[] {"A", "C", "H"}
3. pass them to method in static class like this
csLocalSql.SelectOverDataTable(dtMyTable, new string[] {"A", "C", "H"})
4. Then at this method look in loop for every dtMyTable column name, and if
is on this string list I just pass it, if no do
remove("name_with_is_not_at_list_of_string");
5. I return DataTable without columns I don' need.

So as you see I could make local copy of DataTable with I already have afert
fill, and all logic is on local side... And I really don't think that there
is anything unusal - for performence it is even better becouse I done refer
to SQL sever with any select.

Now the question is, how to pass the DataTable to this method as in/out ->
ref, in a way to avoid making local DataTable for with this passed by
reference (by pointer to the structure on stack), and in way to do all this
removes direct on my DataTable..
By closing in (object) I mean boxing and unboxing... I could do it becouse
when I try to do sth like this

csLocalSql.SelecOverDataTabel( ref dtMyTable, new string [] .....);
to method with wait for ref DataTable dtMyTable, string[] str)

compiler shout at me with 'you can't use ref over a this type...

--
best regards
stci

Stefan:

I'm not entirely sure I understand the problem but you can definitely pass a
DataTable by reference. However, remember this is a reference type to begin
with and there's a lot to a datatable to keep track of if it's size is very
big so you may see all sorts of performance degradation depending on use.
It's often easier to pass the individual rows of the datatable which are
small and easier to work with.

I'm not sure what you mean by Closing the datatable though, there is no open
or closed state to a DataTable, only the database if that's where its filled
from. If you are referrring to the underlying connection, there's no reason
to leave it open here that I can see.

I'm not sure what the goal is here but if you could explain it a little
more, I could be of more help. Also, if you look at MS's Data Access
Application Block (DAAB) it returns Datasets (so DataTables are essentially
in the same boat) in the executeDataSet method. You can pass these things
over layers, back and forth to web services, you name it you can pass it.

Here's a typical scenario. I have a method that fills a datatable and
returns it:

Declare a datatable and cal lthe method:

DataTable dt = myDataAccessClass.GetDataTable();

//now a method in myDataAccessClass
private DataTable GetDataTable(){
//Open Connection,
DataTable dt = new DataTable();
//Fill datatable
return dt;
}

This is fine to do, is this essentially what you're asking?

Hi,

I'm wondering if there is a way to send a method parametrs by ref when
DataTabel is a type of this value ?

I done some sort of select over DataTable columns, just by removing them
froma table is each of them isn't on a stirng[], but right no I have to do
it in a wat of:
1. passing DataTable to method (lets call it SelectOverDataTabel)
2. declare a local (in SelectOver...) DataTable, inicialize it with my value
3. doing all things on this local copy (removing.....)
4. and to the parent method send a local copy by return, sth like this
dtMyTable = clsLocalSQL.SelectOverDataTable( dtMyTable,
StringOfSelectedColumns)

Could I do it without closing DataTable in (object) and unpacking it in body
of my static procedure ??

 

[Jon Skeet [C# MVP]]

Thanks for you answer but what I need to do is:
1. take one DataTable filled with for example 12 Columns named by letters
A,B,C,...
DataTable dtMyTable = new DataTable();
//sqlconnection code
dataadapert.Fill(dtMyTable) <- when commandtext is "Select A,B,C,D,F,G,...
from sometable
2. take one string[] with allow me to filter this DataTable for examle new
string[] {"A", "C", "H"}
3. pass them to method in static class like this
csLocalSql.SelectOverDataTable(dtMyTable, new string[] {"A", "C", "H"})
4. Then at this method look in loop for every dtMyTable column name, and if
is on this string list I just pass it, if no do
remove("name_with_is_not_at_list_of_string");
5. I return DataTable without columns I don' need.

So as you see I could make local copy of DataTable with I already have afert
fill, and all logic is on local side... And I really don't think that there
is anything unusal - for performence it is even better becouse I done refer
to SQL sever with any select.

I think you're missing the difference between reference types and value
types. I certainly don't think you need to pass the DataTable by
reference in this case - passing the reference by value (the default)
is fine, I believe.

See http://www.pobox.com/~skeet/csharp/parameters.html

 

[Stefan Turalski \(stic\)]

Thanks for you answer but what I need to do is:
1. take one DataTable filled with for example 12 Columns named by letters
A,B,C,...
DataTable dtMyTable = new DataTable();
//sqlconnection code
dataadapert.Fill(dtMyTable) <- when commandtext is "Select A,B,C,D,F,G,...
from sometable
2. take one string[] with allow me to filter this DataTable for examle new
string[] {"A", "C", "H"}
3. pass them to method in static class like this
csLocalSql.SelectOverDataTable(dtMyTable, new string[] {"A", "C", "H"})
4. Then at this method look in loop for every dtMyTable column name, and if
is on this string list I just pass it, if no do
remove("name_with_is_not_at_list_of_string");
5. I return DataTable without columns I don' need.

So as you see I could make local copy of DataTable with I already have afert
fill, and all logic is on local side... And I really don't think that there
is anything unusal - for performence it is even better becouse I done refer
to SQL sever with any select.

I think you're missing the difference between reference types and value
types. I certainly don't think you need to pass the DataTable by
reference in this case - passing the reference by value (the default)
is fine, I believe.

See http://www.pobox.com/~skeet/csharp/parameters.html

Hi,

Could be ;-)

Ok, so as far I as understood if I use ref before parametr, in method
inicialization / calls these DataTable should be send by value (as I
thought) and all opperation could be done right on this table, or I'm
lossing something and there will be no change when I use it ?

The thing btw. is that you couldn't send DataTable as a ref parametr... -
and that is why I started this topic...

 

[Jon Skeet [C# MVP]]

Could be ;-)

Ok, so as far I as understood if I use ref before parametr, in method
inicialization / calls these DataTable should be send by value (as I
thought) and all opperation could be done right on this table, or I'm
lossing something and there will be no change when I use it ?

The reference is passed by value, but it doesn't create a new copy of
the object itself.

See my article for more information.

The thing btw. is that you couldn't send DataTable as a ref parametr... -
and that is why I started this topic...

You *can* pass a DataTable by reference - but you need to state that on
both the method declaration and the method call itself.