System.ArgumentNullException: The argument column cannot be null

Question

So for a school project I need to follow the instruction of a book called Programming with C# Databases. And in this chapter I need to make a datagridview with Bindingsources etc. So where I get the error in the title of this thread is when I'm defining a DataRelation. The code is as following:

private void CreateDataRelation()
{
    DataColumn dataColumnParent, dataColumnChild;

    //Hier definieer ik de variabelen dataColumnParent en Child en koppel ik ze aan de keys.
    dataColumnParent = datasetRugby.Tables["RugbyTeams"].Columns["Id"];
    dataColumnChild = datasetRugby.Tables["Spelers"].Columns["TeamId"];

    //Dit is de uiteindelijke relatie die wordt aangemaakt die ik later zal toevoegen aan DataSet.
    DataRelation dataRelationTeamSpeler = new DataRelation("relationTeamSpeler",
        dataColumnParent,
        dataColumnChild);

    //Op regel 118 voeg ik de relatie toe aan de DataSet.
    datasetRugby.Relations.Add(dataRelationTeamSpeler);

    //Hier zet ik de foreignkey van de dataset van Spelers en koppel ik hem aan de dataColumnChild.
    ForeignKeyConstraint foreignKeyConstraintTeamSpeler = dataRelationTeamSpeler.ChildKeyConstraint;

    //Hiermee geef ik aan dat je de team niet mag verwijderen als er nog spelers in dat team spelen.
    foreignKeyConstraintTeamSpeler.DeleteRule = Rule.None;

    //Als de Id van het team veranderd dan moet het teamId meeveranderen van alle spelers die in dat team spelen doormiddel van regel 129.
    foreignKeyConstraintTeamSpeler.UpdateRule = Rule.Cascade;

}

The line that the error occurred is this line of code

DataRelation dataRelationTeamSpeler = new DataRelation("relationTeamSpeler",
            dataColumnParent,
            dataColumnChild);`

I don't know how to fix this aswell as my fellow classmates. We compared our codes and they all have the same code because the books tells us what to do but for some reason the error only occurres with me.

What can I do ? HELP :(

Cheers,

Answer 1

Although this is homework, you've posted a reasonably acceptable MCVE, and made an attempt at the problem, so let me help you with some tips.

The answer to your immediate question is, either (or both) dataColumnParent or dataColumnParent is null. A tip on figuring out what's wrong is Googling the error message you get and also reading the documentation.

• Documentation.
• Troubleshooting tips for the exception.

Documentation states that:

The exception that is thrown when a null reference (Nothing in Visual Basic) is passed to a method that does not accept it as a valid argument.

And if you go to the documentation for the DataRelation class and see it's constructor details, it says the condition for ArgumentNullException is that:

One or both of the DataColumn objects contains null.

Essentially, your problem boils down to passing null values to the constructor when it's expecting non-null values.

In other words, one or both of following lines are returning null.

datasetRugby.Tables["RugbyTeams"].Columns["Id"];
dataColumnChild = datasetRugby.Tables["Spelers"].Columns["TeamId"];

So you should figure out why. Likely, as other users mentioned, you've misspelled a table or a column name.

---

Debugging

Best way to figure out your problem in a situation like this is to debug your code. Assuming you're a complete novice, here's how:

In your Visual Studio editor, you can click the gray vertical bar to the left of line numbers, which will put a break-point indicated by the red dot. Following is an extremely simple example.

Make sure your project is in Debug mode.

Now run your program, and the code execution will pause/halt at the point where you've put a breakpoint.

If you hover your mouse over a variable you can see it's current content.

Also there are Locals, Watch etc. windows that will also help see the contents of your variables.

By inspecting the contents of your variables (in your case, dataColumnParent and dataColumnParent) you can often easily narrow down on your problem.

Finally, here's some good resources on debugging.

• Getting Started with the Debugger.
• Getting Started YouTube video.

Good luck!