Why does this code work?

[Julie]

I'm supporting an application at work. Below are some code segments
that I can't understand how they work. First let me say, I would never
code this standard. I'm just really creeped out that it works.

Here's the setup. I have a function, called GetEmployeeCert ifications,
that is going to make a call to a SQL DB and return a result set. This
module calls another function, called FillParameters, to determine if
SQL parameters need to be appended to the command object. The
FillParameters function...

does not have a return value
the command object is not globally defined
and the command object is passed by value not reference

But yet the command object, in the GetEmployeeCert ifications function,
has appended to it the parameter objects from the FillParameters
function. How can that be?

Public Shared Function GetEmployeeCert ifications(ByVa l criteria As
SearchCriteria) As EmployeeCertifi cationCollectio n
Dim tblEmployees As New DataTable
Dim connection As New SqlConnection(c onnectionString )
Dim cmdGetEmployees As New SqlCommand
Dim myDataAdapter As New SqlDataAdapter
Dim employees As New EmployeeCertifi cationCollectio n

Try
connection.Open ()
cmdGetEmployees .Connection = connection
cmdGetEmployees .CommandText = "GetEmployeeCer tifications"
cmdGetEmployees .CommandType = CommandType.Sto redProcedure

FillParameters( criteria, cmdGetEmployees ) '<---Here is the
call with no assignment
myDataAdapter.S electCommand = cmdGetEmployees
myDataAdapter.F ill(tblEmployee s)

Catch ex As Exception

Finally
'convert the matching data rows to an
EmployeeCertifi cationCollectio n
If Not tblEmployees Is Nothing Then
Dim row As DataRow
For Each row In tblEmployees.Ro ws
employees.Add(G etEmployeeCerti fication(row))
Next
End If
End Try

'If they searched by cost center, then return the results
sorted by cost center.
'The results come back from the database ordered by last name.
If criteria.CostCe nterFrom.Length 0 Or
criteria.CostCe nterTo.Length 0 Then
employees.Apply Sort("CostCente r",
ComponentModel. ListSortDirecti on.Ascending)
End If
Return employees

End Function

Private Shared Function FillParameters( ByVal criteria As
SearchCriteria, ByVal cmd As SqlCommand)
If criteria.LastNa me.Length 0 Then
cmd.Parameters. Add(New SqlParameter("@ LastName",
SqlEscape(crite ria.LastName)))
End If

If Not (criteria.SiteC ode = "ALL" Or criteria.SiteCo de.Length =
0) Then
cmd.Parameters. Add(New SqlParameter("@ SiteCode",
criteria.SiteCo de))
End If

End Function

[Jon Skeet [C# MVP]]

Julie <ju**********@y ahoo.comwrote:

I'm supporting an application at work. Below are some code segments
that I can't understand how they work. First let me say, I would never
code this standard. I'm just really creeped out that it works.

Here's the setup. I have a function, called GetEmployeeCert ifications,
that is going to make a call to a SQL DB and return a result set. This
module calls another function, called FillParameters, to determine if
SQL parameters need to be appended to the command object. The
FillParameters function...

does not have a return value
the command object is not globally defined
and the command object is passed by value not reference

You're confused about parameter passing, basically. See
http://www.pobox.com/~skeet/csharp/parameters.html

It's written in terms of C#, but the principle is the same - even when
a parameter is passed by value, if it's a reference type (such as
SqlCommand) it's the *reference* which is passed. Changes to the object
made via that reference are still visible to the caller after the
method has terminated.

--
Jon Skeet - <sk***@pobox.co m>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too

[Jesse Houwing]

FillParameters( criteria, cmdGetEmployees ) '<---Here is the

call with no assignment

This is a call by reference. A Command object is by reference always.

So the parameters are filled using the reference.

Jesse

[Jon Skeet [C# MVP]]

Jesse Houwing <je***********@ nospam-sogeti.nlwrote:

FillParameters( criteria, cmdGetEmployees ) '<---Here is the
call with no assignment

This is a call by reference. A Command object is by reference always.

No, the object isn't passed at all. There's a different between passing
an object by reference and passing a reference by value. See
http://www.pobox.com/~skeet/csharp/parameters.html for more on this.

--
Jon Skeet - <sk***@pobox.co m>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too

[Julie]

Jon,

Thanks for the refresher on reference types vs value types. The
confusion comes in on the variable definition in the function name with
the explicit ByVal keyword.

A variable can be assigned a reference. A second variable can be
assigned the value of first. "They are still, however, independent
variables themselves. Changing the value of first will not change the
value of second". But change the value of the object using the first
variable will change the second.

StringBuilder first = new StringBuilder() ;
StringBuilder second = first;
first.Append ("hello");
first = null;
Console.WriteLi ne (second);

Hello will still print because second is a variable whose address
points to an area in core that contains hello. While first has
released its address.

HERE IS MY DISCONNECT

The variable cmd is defined as a SQL command type object. But the
ByValue keyword threw me as we are passing the pointer not creating a
duplicate object. It is possible to create and object in memory that
has the same contents as another object in memory. Thus cmd and
cmdGetEmployees are two different variables pointing to the same
object.

So in the FillParameters function what would have happened if I had
ByVal with a new keyword?

Private Shared Function FillParameters( ByVal criteria As
SearchCriteria, ByVal cmd As new SqlCommand)

Thanks for the whack upside the head.
J
Jon wrote:

Jesse Houwing <je***********@ nospam-sogeti.nlwrote:

FillParameters( criteria, cmdGetEmployees ) '<---Here is the
call with no assignment

This is a call by reference. A Command object is by reference always.

No, the object isn't passed at all. There's a different between passing
an object by reference and passing a reference by value. See
http://www.pobox.com/~skeet/csharp/parameters.html for more on this.

--
Jon Skeet - <sk***@pobox.co m>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too

[Jon Skeet [C# MVP]]

Julie <ju**********@y ahoo.comwrote:

<snip>

So in the FillParameters function what would have happened if I had
ByVal with a new keyword?

Private Shared Function FillParameters( ByVal criteria As
SearchCriteria, ByVal cmd As new SqlCommand)

I'm afraid I don't know enough VB.NET to understand that syntax - it
doesn't seem to make sense judging by the documentation.

--
Jon Skeet - <sk***@pobox.co m>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too

[Cor Ligthert [MVP]]

Jon,

No, the object isn't passed at all. There's a different between passing
an object by reference and passing a reference by value. See

That was in my idea not written by Jesse. I think that we are blind as soon
as we read "reference" .

A reference type is always passing in any way an address as the reference
not the complete object.

:-)

Cor

[Cor Ligthert [MVP]]

Julie,

Do you know why this methode declaration bellow does not give an error. You
write this as a Sub, which should work, but better is it to avoid this in
this kind of situations where you are changing values inside a Sub.

Private Shared Function FillParameters( ByVal criteria As
SearchCriteria, ByVal cmd As SqlCommand)

Try to avoid this, try to return what has to be returned, that makes your
program at least more readable.

Private Shared Function FillParameters( ByVal criteria As
SearchCriteria, ByVal cmd As SqlCommand) as SqlCommand
....
return cmd

And than in your code
cmdGetEmloyees = FillParameters( criteria, cmdGetEmployees )

However the strangest in your code is for me that function whithout return
value.

I hope this helps,
"Julie" <ju**********@y ahoo.comschreef in bericht
news:11******** *************@m 73g2000cwd.goog legroups.com...

I'm supporting an application at work. Below are some code segments
that I can't understand how they work. First let me say, I would never
code this standard. I'm just really creeped out that it works.

Here's the setup. I have a function, called GetEmployeeCert ifications,
that is going to make a call to a SQL DB and return a result set. This
module calls another function, called FillParameters, to determine if
SQL parameters need to be appended to the command object. The
FillParameters function...

does not have a return value
the command object is not globally defined
and the command object is passed by value not reference

But yet the command object, in the GetEmployeeCert ifications function,
has appended to it the parameter objects from the FillParameters
function. How can that be?

Public Shared Function GetEmployeeCert ifications(ByVa l criteria As
SearchCriteria) As EmployeeCertifi cationCollectio n
Dim tblEmployees As New DataTable
Dim connection As New SqlConnection(c onnectionString )
Dim cmdGetEmployees As New SqlCommand
Dim myDataAdapter As New SqlDataAdapter
Dim employees As New EmployeeCertifi cationCollectio n

Try
connection.Open ()
cmdGetEmployees .Connection = connection
cmdGetEmployees .CommandText = "GetEmployeeCer tifications"
cmdGetEmployees .CommandType = CommandType.Sto redProcedure

FillParameters( criteria, cmdGetEmployees ) '<---Here is the
call with no assignment
myDataAdapter.S electCommand = cmdGetEmployees
myDataAdapter.F ill(tblEmployee s)

Catch ex As Exception

Finally
'convert the matching data rows to an
EmployeeCertifi cationCollectio n
If Not tblEmployees Is Nothing Then
Dim row As DataRow
For Each row In tblEmployees.Ro ws
employees.Add(G etEmployeeCerti fication(row))
Next
End If
End Try

'If they searched by cost center, then return the results
sorted by cost center.
'The results come back from the database ordered by last name.
If criteria.CostCe nterFrom.Length 0 Or
criteria.CostCe nterTo.Length 0 Then
employees.Apply Sort("CostCente r",
ComponentModel. ListSortDirecti on.Ascending)
End If
Return employees

End Function

Private Shared Function FillParameters( ByVal criteria As
SearchCriteria, ByVal cmd As SqlCommand)
If criteria.LastNa me.Length 0 Then
cmd.Parameters. Add(New SqlParameter("@ LastName",
SqlEscape(crite ria.LastName)))
End If

If Not (criteria.SiteC ode = "ALL" Or criteria.SiteCo de.Length =
0) Then
cmd.Parameters. Add(New SqlParameter("@ SiteCode",
criteria.SiteCo de))
End If

End Function

[Jon Skeet [C# MVP]]

Cor Ligthert [MVP] <no************ @planet.nlwrote :

No, the object isn't passed at all. There's a different between passing
an object by reference and passing a reference by value. See

That was in my idea not written by Jesse. I think that we are blind as soon
as we read "reference" .

A reference type is always passing in any way an address as the reference
not the complete object.

Yes, but Jesse claimed that the object was passed by reference. It
isn't. It isn't passed at all. There are well-defined meaning to "pass
by reference", and ByVal semantics aren't the same, even when what is
passed is a reference.

--
Jon Skeet - <sk***@pobox.co m>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too

[Jon Skeet [C# MVP]]

Cor Ligthert [MVP] <no************ @planet.nlwrote :

Do you know why this methode declaration bellow does not give an error. You
write this as a Sub, which should work, but better is it to avoid this in
this kind of situations where you are changing values inside a Sub.

Private Shared Function FillParameters( ByVal criteria As
SearchCriteria, ByVal cmd As SqlCommand)

Try to avoid this, try to return what has to be returned, that makes your
program at least more readable.

Private Shared Function FillParameters( ByVal criteria As
SearchCriteria, ByVal cmd As SqlCommand) as SqlCommand
...
return cmd

And than in your code
cmdGetEmloyees = FillParameters( criteria, cmdGetEmployees )

However the strangest in your code is for me that function whithout return
value.

Why is that strange? To me, it would be very odd to return the command
when there's no need for it. Why have a return value when it's
unnecessary? *That* would make the code less readable IMO.

Rather than pretending that the return value is important, it's better
to understand reference type semantics. What would you do when the
method modified the objects referred to by two of its parameters? You
can't return them both, so you get back to the same problem of
understanding.

--
Jon Skeet - <sk***@pobox.co m>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too