An ASP.NET application uses a DataSet named EXorderEntry with two DataTable objects, orderNames and OrderDetails, linked by a ForeignKeyConstraint called orderDetailsKey. When you try to delete a row from orderNames that still has related rows in OrderDetails, an exception is thrown. Which delete rule value on the foreign key constraint most likely causes this exception?

Introduction / Context:
In ADO.NET, DataSet objects can enforce relational integrity between DataTable objects by using ForeignKeyConstraint instances. These constraints include rules that control what happens when a parent row is deleted or updated while related child rows exist. Understanding the DeleteRule options is essential for designing correct business logic and for passing Microsoft certification exams focused on data access behavior in .NET applications.

Given Data / Assumptions:

There is a DataSet named EXorderEntry that includes orderNames as the parent table and OrderDetails as the child table.
A ForeignKeyConstraint called orderDetailsKey enforces the relationship between these tables through a key column such as OrderID or a similar field.
You attempt to delete a row from orderNames that still has related rows in OrderDetails.
An exception is thrown when you perform the delete operation, indicating a conflict with the constraint.
The DeleteRule property of the foreign key constraint determines how the DataSet reacts when deleting a parent row.

Concept / Approach:
The ForeignKeyConstraint.DeleteRule property can be set to Rule.Cascade, Rule.SetNull, Rule.SetDefault, or Rule.None. When set to Cascade, deleting a parent row automatically deletes related child rows. When set to SetNull or SetDefault, deleting the parent modifies child rows accordingly. When set to None, the DataSet enforces strict referential integrity and prevents deletion of a parent row if related child rows exist, throwing an exception. Therefore, if a delete operation fails with an exception in this scenario, the most likely delete rule is Rule.None.

Step-by-Step Solution:
1. Confirm that the exception is thrown when deleting a parent record that has related child records in OrderDetails. 2. Consider what Rule.Cascade would do: it would automatically delete all related child rows, so no exception would be thrown for referential integrity. 3. Consider Rule.SetNull and Rule.SetDefault: these rules modify the foreign key values in the child rows instead of blocking the delete, so again the delete would not be blocked solely due to existing children. 4. Recognize that Rule.None instructs the DataSet to disallow the deletion of a parent record if child records exist, which results in an exception when you attempt the operation. 5. Therefore, the delete rule causing the exception is Rule.None.

Verification / Alternative check:
You can verify this by examining the DeleteRule property of the ForeignKeyConstraint in code or by temporarily changing it to Rule.Cascade and repeating the delete test. If the exception disappears and the related child rows are deleted, that confirms that the original configuration with Rule.None was responsible for blocking the delete.

Why Other Options Are Wrong:
Rule.Cascade would remove all child rows automatically and would not usually cause an exception related to existing children.
Rule.SetNull would set the child foreign key fields to null when the parent is deleted, which also allows the delete to succeed without an integrity exception.
Rule.SetDefault would set child foreign key values to a default value defined in the schema, again allowing the parent delete to succeed if the default is valid.

Common Pitfalls:
A common misunderstanding is assuming that referential integrity is enforced only in the database, when in fact DataSet objects can enforce it in memory as well. Developers also sometimes choose Rule.Cascade without fully understanding the implications for business rules. Conversely, selecting Rule.None without providing a safe way to delete parent records can lead to unexpected exceptions during normal operations.

Final Answer:
The exception occurs because the delete rule is set to Rule.None, which prevents deleting parent rows that still have related child rows.