problem using dataRelation in dot net

[Luc Pattyn]

objDA is probably null :|

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[Richard Blythe]

System.NullReferenceException is an exception that never lies. You are obviously typing a string in wrong. To eliminate all possible confusion, try this code:

if (!ds.Tables.Contains("category") || !ds.Tables["category"].Columns.Contains("ID"))
throw new ArgumentException("The category table or column does not exists!")
//
if (!ds.Tables.Contains("item") || !ds.Tables["item"].Columns.Contains("catID"))
throw new ArgumentException("The item table or column does not exists!")

DataColumn c1 = ds.Tables["category"].Columns["ID"];Line
DataColumn c2 = ds.Tables["item"].Columns["catID"];
dRel = new DataRelation("categoryItem", c1, c2);

The mind is like a parachute. It doesn’t work unless it’s open.

[Lost User]

Line 1224 consists of multiple statements:

Line 1224: DataColumn c1 = ds.Tables["category"].Columns["ID"];

I suggest you cut it up into multiple lines, with a breakpoint on each line. What does ds.Tables["category"] evaluate to? Does that new object have a column labeled "ID"?

SayamiSuchi wrote:

i dont know why the error in this line ?

One of those statements on that line evaluates to null.

I are Troll :suss:

[Dhyanga]

objDA is filling ds. ds is getting value.. isn't that means objDA has value ?? Or is there anyway to check the data inside objDA?

Thanks, suchita

[Lost User]

You're having trouble debugging? Start here[^] in order to learn how to inspect those values.

I are Troll :suss:

[Luc Pattyn]

I can't tell what is wrong by seeing only some part of the relevant code. if you think everything is correct, how come the exception? now replace

objDA.Fill(ds);
...

by

try {
if (objDA==null) MessageBox.Show("Oh dear, objDA seems to be null");
objDA.Fill(ds);
...
} catch(Exception exc) {
MessageBox.Show("exception: "+exc.ToString());
}

:|

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[Dhyanga]

Thanks for the reply. yes you were right. it threw the argument exception. the query i used for the dataset ds is:

select i.ID,j.Item,j.catID from category i, item j where i.ID = j.catID

and the dataset is having data . Then why it gave that error ?

suchita

[Richard Blythe]

Don't take it personal SayamiSuchi. Luc is not partial to any codeproject member. He gripes at all of us! :-D

The mind is like a parachute. It doesn’t work unless it’s open.

[Dhyanga]

Thank you for the reply. my code

DataColumn c1 = ds.Tables["category"].Columns["ID"];

is throwing that error. but since my dataset has values, i dont know how come that error ?

suchita

[Richard Blythe]

Which one threw the exception? I agree with Eddy that you need to learn how to perform effecient debugging. Visual Studio wouldn't be worth two cents to me if it didn't have a good debugger.

The mind is like a parachute. It doesn’t work unless it’s open.

[Dhyanga]

Thank you. Both of the argument exception is executing.

suchita

[Dhyanga]

thank you

suchita

[Richard Blythe]

When I use a DataAdapter to fill a DataTable, I have to manually set the DataTable's name. If the same logic applies, You may need to manually set the names of your DataSet tables.

The mind is like a parachute. It doesn’t work unless it’s open.

[Luc Pattyn]

assuming ds is a DataSet, that line would throw a NullRefExc only if ds were null or there were no table called "category" suggestion: split the line into many, and check what gives

if (ds==null) MessageBox.Show("ds is null");
DataTable dtCat=ds.Tables["category"];
if (dtCat==null) MessageBox.Show("dtCat is null");
DataColumn c1=dtCat.Columns["ID"];
MessageBox.Show("we got a DataColumn!");

:)

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[Dhyanga]

ok. But since i'm using join of two tables, how i'm gonna give two tables name in there ? Or is it just whatever name I'm giving for the dataset ?

suchita

[Luc Pattyn]

You know pretty well an open mind is the numero uno prerequisite when debugging some code... :)

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[Richard Blythe]

What is your SELECT query for your data adapter?

The mind is like a parachute. It doesn’t work unless it’s open.

[Dhyanga]

select i.ID,j.Item,j.catID from category i, item j where i.ID = j.catID

suchita

[Dhyanga]

ok now i changed them a little bit.

        string query = @"select i.ID,j.catID,j.item from category i, item j where i.ID = j.catID";
        SqlCommand objCmd = new SqlCommand(query, cnx);
        SqlDataAdapter objDA = new SqlDataAdapter(objCmd);
        DataSet ds = new DataSet();
        objDA.Fill(ds,"category");
        objDA.Fill(ds, "item");
        DataColumn c2 = ds.Tables\["category"\].Columns\["ID"\];
        DataColumn c1 = ds.Tables\["item"\].Columns\["catID"\];
        dRel = new DataRelation("category\_Item", c1, c2);
        ds.Relations.Add(dRel);

And here ID is the unique key for category and catID is the foreign key . But it throws error like:

These columns don't currently have unique values.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.ArgumentException: These columns don't currently have unique values.

Source Error:

Line 1231:
Line 1232: dRel = new DataRelation("category_Item", c1, c2);
Line 1233: ds.Relations.Add(dRel);
Line 1234: Repeater1.DataSource = ds;
Line 1235: Repeater1.DataBind();

But in ID is the unique key in category table and i have unique ID in item table too. I tried replacing catID by ID for the item table but in both the cases, error is same.

suchita

[Dhyanga]

Thank you so much to all who has given interest in this topic. i got my program running now.

suchita