How do I get ExecuteReader() to efficiently add items to a List?

Question

I have a button in the UI that reveals all remaining records in a table.

To handle this, in my controller, I have a simple SQL SELECT * statement with a LIMIT and OFFSET. My ExecuteReader is currently returning my data from the SQL command, which I am adding to a List. The list contains instances of my custom Run class.

In SSMS, the SQL query executes without exception no matter how large of a LIMIT I request. If my limit is > the number of rows in the table, it just returns all rows.

In webAPI, though, when my limit is > 200, it returns an exception. Otherwise, when less than 200, it returns a List of Runs without exception. I'm trying to debug the exception that occurs when I try to return all the data, but when it passes to the catch block, the exception is null. Which is weird.

So, I think there is a step I'm missing. Maybe I shouldn't be transforming the data into the Run class while the Reader is streaming. If I verified that the SQL command is accurate, then this seems to be the step that is causing the bug. Maybe transforming the data is making the Reader sorta time out? I don't understand ExecuteReader well enough to be able to figure out how I can pass all the data to List and then transform the data in that list into Runs after closing the connection. And don't even know if that would solve problem anyway.

All misgivings about potential SQL injections and lack of dbContext, etc. aside, how can I return all my records from the database utilizing ExecuteReader()?

Thanks.

Edit to add:

My exception value in the catch block is {"Data is Null. This method or property cannot be called on Null values."}.

In the debugger output, I my exception Exception thrown: 'System.Data.SqlTypes.SqlNullValueException' in Microsoft.Data.SqlClient.dll.

Edit to comment on the solution.

Ann L. figured this out. I had null values coming from the database. I learned from her and PSGuy that I can check for null values by using DbNull. Thank you!

Note - an easy place to get tripped up is that your class has to allow for nulls or else VS won't allow you to check for nulls in the method in the controller.

Answer 1

Here's one approach to the syntax you'll need to use (although there are lots of other approaches: see here for a bunch of alternatives!)

shoeAge = reader.IsDBNull(13) ? null : reader.GetInt64(13)

This assumes shoeAge is a nullable Int64 . If it isn't, you'll get another error since you won't be able to assign null to it.