c# trouble getting a null-coalescing operator ?? to work

Question

I have read several articles and questions on Stack Overflow and still cannot see what I am doing wrong. Not using C#6 -- sorry for not posting that at first. VS 2013.

This code works:

if (row.Cells["CSR_Notes"].Value != null)
                    {
                    deliveryEvent.CSR_Notes = row.Cells["CSR_Notes"].Value.ToString();
                    }

and this code works:

 deliveryEvent.CSR_Notes = row.Cells["CSR_Notes"].Value != null 
                        ? row.Cells["CSR_Notes"].Value.ToString() : "";

But this code throws a "Object reference not set..." error if the value is null.

 deliveryEvent.CSR_Notes = row.Cells["CSR_Notes"].Value.ToString() ?? "";

What am I missing?

You're still calling ToString() on null. You need to move the ?? inward to replace Value with something non-null. (row.cells[...].Value ?? "").ToString() — luqui
– luqui, Commented Sep 26, 2016 at 18:38

Jonathon Chase · Accepted Answer · 2016-09-26 18:45:49Z

5

The issue is that Value is the potentially null property. By calling the ToString() method on it, you are creating the Null Reference Exception before the null coalescing operator has a chance to be used. You can fix this with the Null Propagating Operation from C# 6, like so:

deliveryEvent.CSR_Notes = row.Cells["CSR_Notes"].Value?.ToString() ?? "";

Notice the ? after Value. This is the Null Propagating operator. It will pass null through the rest of the expression if Value is null, which will then trigger the Null Coalescing Operator (??) properly.

The C# 5 solution would be to push the coalescing up a bit, as in haim770's answer.

deliveryEvent.CSR_Notes = (rows.Cells["CSR_Notes"].Value ?? "").ToString();

edited Sep 26, 2016 at 18:45

answered Sep 26, 2016 at 18:36

Jonathon Chase

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5 Comments

haim770 Over a year ago

That's only valid for C# 6.0

Jonathon Chase Over a year ago

Yep, I was meaning to add in the fact that it would require C# 6 but missed the version # in the original post. Subsequent edits have corrected this. Thanks @haim770

Joe Ruder Over a year ago

I'm sorry Johathon, I had tried that approach as well without realizing that it was C# 6 only. I am still using vs2013. Looking into changing that soon.

Jonathon Chase Over a year ago

I've added the C# 5 solution as well.

Jonathon Chase Over a year ago

Yeah, that constraint was added after I initially answered. Both solutions are fine, and if there are solutions that are different based on tooling versions it's worth noting where those differences are. Null Propagation is a QoL feature and has a lot of potential alternative implementations.

haim770 · Accepted Answer · 2016-09-26 18:36:55Z

1

You need to check that the row.Cells["CSR_Notes"] object is not null, but your last code snippet is checking the returned value of the entire expression row.Cells["CSR_Notes"].Value.ToString() and by the time it's evaluated it rightly throws because row.Cells["CSR_Notes"] returns null.

Try this instead:

deliveryEvent.CSR_Notes = (row.Cells["CSR_Notes"].Value ?? "").ToString();

See MSDN

answered Sep 26, 2016 at 18:36

haim770

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1 Comment

Joe Ruder Over a year ago

That seems so obvious now haim770....it won't let me accept the answer yet, but I will. This did it.

mason · Accepted Answer · 2016-09-26 18:46:45Z

0

You are calling .ToString() on an object that is null. That's why you receive the error. Here's a simpler reproduction:

MyObject obj = null;
string result = obj.ToString();

You can avoid this by using the null conditional operator (sometimes known as "elvis operator")

deliveryEvent.CSR_Notes = row.Cells["CSR_Notes"].Value?.ToString() ?? "";

This will only call .ToString() on Value if Value is not null.

edited Sep 26, 2016 at 18:46

answered Sep 26, 2016 at 18:38

mason

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3 Comments

Jonathon Chase Over a year ago

The elvis operator usually indicates ?:, which in C# is a ternary conditional operator. This operator is the null conditional operator or the null propagating operator. Does the term elvis operator still apply in this case?

mason Over a year ago

@JonathonChase I think it's still called the "elvis operator". Here's an article where they do so.

Jonathon Chase Over a year ago

I seem to reject that internally, but it's a colloquialism so it is what we decide it is. Thanks.

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