Hi kuhrty,
You have several issues in your code. First, you have a private field (the
correct term for a "global or class-scoped variable" in your Form) named
"dsPyramid" of type DataSet. But in your method "Load_Pyramid" you declare a
locally-scoped (to your method) variable named "dsPyramid" of the type
DataSet, and you create an instance of it, which populates it from your
database, and then assign a table from it to a DataTable. The problem here
is that you have declared a locally-scoped method variable with the same
name as the class-scoped field. When this happens, the variable in the
method, which is closer in scope to the method code, "shadows" the
class-scoped field. Be aware that because you have explicitly declared it,
it is not a reference to the class-scoped field, but is a variable that is
scoped to the method. So, the class-scoped field is never assigned to, and
when the method exists, the method-scoped variable passes out of scope and
is gone. With each method call, it is created all over again.
It is very important in .Net to understand scope. I recommend reading the
following MSDN2 article:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dndotnet/html/methodscope.asp
Also, I couldn't help but notice the following issue, which will not cause a
problem (other than wasting processor cycles and memory), but indicates to
me a hole in your understanding of the declaration and usage of variables
and fields:
DataTable dt = new DataTable();
int i;
Pyramid dsPyramid = new Pyramid();
dt = dsPyramid.Get_Pyramid;
Note the declaration of a DataTable at the top, which is combined with an
assignment to that variable of a new DataTable. This could also be written
as:
DataTable dt;
dt = new DataTable();
Next, a few lines down, you assign the return value of a property (which
returns a DataTable) to the variable:
dt = dsPyramid.Get_Pyramid;
This has the effect of replacing the new DataTable just assigned to the
variable with the DataTable returned by the property. In other words, you
create a new DataTable, and then throw it away.
It is important to understand what a variable is, and what assignment is. It
is best to think of a variable as a "box" that is specially built to contain
a certain type of data. When you declare a variable, you are essentially
telling your application "I need a box that I can store a DataTable in."
Note that the "box" and the "contents" of the box are completely different
things. When you assign a DataTable to the variable, you are essentially
"putting a DataTable into the box." Now, since the "box" can only contain
*one* item, every time you assign another "item" to the "box" it replaces
whatever is already *in* the box. So, when you put a new DataTable into the
box (DataTable dt = new DataTable()), you are creating the box, creating a
new DataTable, and putting the new DataTable into the box. When you
re-assign it from a property, you remove the original DataTable, and replace
it with the one you assign.
The confusion probably arises from the fact that the language syntax allows
you to refer to the box as if it were what it contains. In fact, this is not
true, but only a convenience for the programmer. It is much like algebra.
Consider the following:
x = y + 1
The variables 'x' and 'y' are "boxes" or containers for numbers. Now, if you
assign a value to either, it determines the value of the other:
x = 2
x = y + 1
Therefore, y = 1
In this algebraic series of statements, we are treating the variables as if
they were the numbers they represent. It is the same in programming. A
variable is not a value, but *represents* a value. It gives the value a
"name" or a "handle" we can refer to it by in statments. When you assign it,
you are also *re*-assigning it to the new value. So, unless you have a
reason for creating a new DataTable, the following would be better:
DataTable dt;
int i;
Pyramid dsPyramid = new Pyramid();
dt = dsPyramid.Get_Pyramid;
Or even:
Pyramid dsPyramid = new Pyramid();
DataTable dt = dsPyramid.Get_Pyramid;
--
HTH,
Kevin Spencer
Microsoft MVP
Professional Chicken Salad Alchemist
I recycle.
I send everything back to the planet it came from.
kuhrty said:
I am using a window form and trying to get a dataset used in the form
globally but I am having an issue setting the property and setting it
locally to the class. When I try to call the dataset later, I end up
re-instaniating the class which I know its not need. My issue is the
code that I am using setting the dt = dsPyramid.Get_Pyramid. When I
change the code I am getting error that the class can't be found. Any
help would be greatly appreciated, I am also new and I am still
learning. There maybe a better way to code this in general.
Mark
//Windows form global variable
DataSet dsPyramid;
//Private method on the form
private void Load_Pyramid()
{
DataTable dt = new DataTable();
int i;
Pyramid dsPyramid = new Pyramid();
dt = dsPyramid.Get_Pyramid; // I am messing up the code to set
the variable
cbPyramid.Items.Clear();
for (i = 0; i < dt.Rows.Count; i++)
{
cbPyramid.Items.Add(dt.Rows["Pyramid"]);
}
}
//Class code
public class Pyramid
{
#region Private Members
private DataSet _dsPyramid;
#region Public Properties
public DataTable Get_Pyramid
{
get { return _dsPyramid.Tables[0]; }
}
#endregion
#region Constructors
public Pyramid()
{
//This is setting up the connection
Database db = DatabaseFactory.CreateDatabase("Test");
//This will execute the record set immediately
_dsPyramid =
db.ExecuteDataSet(CommandType.StoredProcedure,"usp_GetPyramid");
}
#endregion
}
