Data Reader

[MeanMachine]

Hello All!

Having a problem with assigning the data reader to the field which I want to do. I know how this is done in VB.NET but how does this work in C# syntax code?

VB.NET

DR = Command.ExecuteReader

While DR.Read

txtFirstName.Text = (DR! FirstName)

End While

Now when I try this in C# code I have all sorts of problems:

C#

DR = Command.ExecuteReader();

while (DR.Read())
{
txtFirstName.Text = (DR! FirstName); < Problems Here?
}

Any ideas???

[jmcilhinney]

That notation is a VB6 hold-over that I recommend not using. This code:

txtFirstName.Text = DR("FirstName")

is valid in both VB.NET and C#.

[Wen Lie]

Hello,

Try this one :

DR = Command.ExecuteReader();

while (DR.Read())
{
    txtFirstName.Text = Convert.ToString(DR["FirstName"]);
}

hope it works

To specify the field name you should use [] instead of ().
I also have no idea why. But I think it's all about the matter of C# syntax, which is quite different with those in VB.NET.

Regards,
WJ

[jmcilhinney]

To specify the field name you should use [] instead of ().

Oops. Quite right. Parentheses are for argument lists and brackets are for indexes in C syntax, while VB syntax uses parentheses for both.

There's no need for the Convert.ToString though. Presumably a column named "FirstName" contains strings already, so you should be casting, not converting. Converting a string to a string is a bit redundant.

[Wen Lie]

There's no need for the Convert.ToString though. Presumably a column named "FirstName" contains strings already, so you should be casting, not converting. Converting a string to a string is a bit redundant.

Are you sure ?
coz, I've tried before not to use "Convert" syntax, and returned me an error, which sounds like invalid type when putting the value to a textbox. Even if the content of the field is "string".
That's why I mostly use "convert" when I'm trying to insert a value from field to textbox object.

Well, I'm using C# for ASP.NET. Is it different with the one used for Windows App. ?
Coz I think web and windows in .NET use the same concept, if I'm not wrong. =)
Correct me if I'm wrong.

Thanks n Regards,
WJ

[jmcilhinney]

Are you sure ?
coz, I've tried before not to use "Convert" syntax, and returned me an error, which sounds like invalid type when putting the value to a textbox. Even if the content of the field is "string".
That's why I mostly use "convert" when I'm trying to insert a value from field to textbox object.

Well, I'm using C# for ASP.NET. Is it different with the one used for Windows App. ?
Coz I think web and windows in .NET use the same concept, if I'm not wrong. =)
Correct me if I'm wrong.

Thanks n Regards,
WJ

I'm quite sure. You cannot assign anything other than a string to a property of type String so this would be illegal:

txtFirstName.Text = DR["FirstName"];

because the reader returns the field value as an Object reference. The reference may refer to an object of any type, but if it refers to a string then there's no need or point to converting a string to a string. You should cast the reference as type String instead:

txtFirstName.Text = (string)DR["FirstName"];

or:

txtFirstName.Text = DR["FirstName"] as string;

[Wen Lie]

Hmm....
okei, thanks.

anyway,... why is data reader can't do the move next, previous, first, last... like I did in previous VB 6 ?

Regards,
WJ

[jmcilhinney]

Hmm....
okei, thanks.

anyway,... why is data reader can't do the move next, previous, first, last... like I did in previous VB 6 ?

Regards,
WJ

Why can't a car fly? Because it's not designed that way. A DataReader provides read-only, forward-only access to a result set. If you want to be able to navigate the data at will then you need to load it into a DataTable.

[Wen Lie]

=)~
hehehe... Just curious.
thanks man...

Regards,
WJ

[jmcilhinney]

More specifically, ADO.NET is designed around a disconnected model. With ADO you navigated live data in the database via a Recordset. With ADO.NET you retrieve data into a local data store, manipulate it disconnected from the database and then save the changes back again. Retrieval, editing and saving a distinct operations in ADO.NET whereas in ADO they were basically blurred into one.

[Wen Lie]

so ?
do you mean that we better use data-adapter, dataset, and data table, rather than data reader ?

[jmcilhinney]

You use whatever's appropriate for each particular circumstance. You might use a DataReader alone, a DataReader to populate a DataTable or a DataAdapter to populate a DataTable, depending on the circumstances.

[Wen Lie]

hmm...
interesting.
I'll give it a try.
Thanks man...

anyway, I'm also a newbie in C#
=)~

[jmcilhinney]

This code is in VB.NET but the equivalent C# would be almost the same anyway. It shows when you'd use a DataReader alone, when you'd use a DataReader and a DataTable and when you'd use a DataAdapter and a DataTable.