Navigating through a dataset

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VB

' Create two instances of the DataTable object to reference the two tables

Dim tblCategories As DataTable = DS.Tables("Categories")
Dim tblProducts As DataTable = DS.Tables("Products")

' Create two DataColumn objects to reference the columns on which the relation is based

' error message is: Object reference not set to an instance of an object
Dim colCatCategoryID As DataColumn = tblCategories.Columns("CategoryID")
Dim colProdCategoryID As DataColumn = tblProducts.Columns("CategoryID")

What I have tried:

At Dim colCatCategoryID As DataColumn = tblCategories.Columns("CategoryID") I get this msg : Object reference not set to an instance of an object.

Posted 9-Jul-16 6:11am

Member 12575668

Updated 9-Jul-16 12:44pm

Patrice T

v3

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Michael_Davies 9-Jul-16 12:35pm

Use the debugger to walk through and check that

Dim tblCategories As DataTable = DS.Tables("Categories")

actually returns a table called Categories, if it does not check your SQL to see why not.

Patrice T 9-Jul-16 16:32pm

What is the question ? What is the problem?

Member 12575668 9-Jul-16 18:26pm

You can read it at in What I tried: At Dim colCatCategoryID As DataColumn = tblCategories.Columns("CategoryID") I get this msg : Object reference not set to an instance of an object.

Application ""breaks and an error is shown : Object reference not set to an instance of an object.

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Patrice T · Answer 1 · 2016-07-09T12:44:00

Solution 1

The error message mean that tblCategories.Columns("CategoryID") is not the expected object.

Use the debugger to see what append.
Make sure the column exist.

Quote:
CategoryID column exists in Category Table.

Yes but you open a table named Categories.

[Update]
Use the debugger to make sure the database opens correctly.
Use the debugger to know where is the fail.

Posted 9-Jul-16 12:44pm

Patrice T

Updated 9-Jul-16 14:42pm

v3

Comments

Member 12575668 9-Jul-16 19:30pm

CategoryID column exists in Category Table.

Member 12575668 9-Jul-16 20:22pm

Correction in my comment. Is not Category but Categories : CategoryID column exists in Categories Table.

Michael_Davies 10-Jul-16 3:58am

It may exist in the table but does the table exist in the dataset?

Member 12575668 10-Jul-16 10:26am

Using debugger I get : Object reference not set to an instance of an object.
It exits in the Dataset with DS.Tables("Categories") where DS is the Dataset with Dim DS As New DataSet

an0ther1 10-Jul-16 19:41pm

Use the immediate window & check the table name in the dataset - you will probably find that you Categories table does not exist.
If you use your immediate window you can enter the following;

? DS.Tables.Count

This should return a number (0, 1, 2 etc)
Then get the actual names in the dataset by doing as follows;

? DS.Tables(0).TableName
? DS.Tables(1).TableName
Keep going until you have a name for each of your tables.

Things to note: Tables in datasets do not necessarily have a name unless you set it when you created the dataset.
Both Table and column names are case sensitive.
Object reference not set to an instance of an object means that whatever you are looking for is not there

Kind Regards

Member 12575668 11-Jul-16 10:01am

1. With DS.Tables.Count I get a Count of 2. Categories Table and Products Table. Is there. Thas not the problem . I have use Northwind in multiple applications and all the Tables in other Datasets.

2. I am looking at the table Categories right now in the application in Visual Studio.

3. That is not the problem.

I have also made queries with Catagories Table it at MSSMS (Microsoft SQL Server Management Studio). Take a look at this book : Mastering Visual Basic 2010, page 711. "Navigating Through a DataSet" topic. There it is the example exercise.

Thanks anyway

Member 12575668 11-Jul-16 10:04am

Correction in my comment. Typing error. Is Categories not Catagories. But the table exists in the application. Get a count of 2 and also made a query SELECT * FROM Categories and get the result.