C# Handling Null Values

Question

I have CustomerID declared as

int? CustomerID=null;

I am checking null values while reading DataReader

Id = reader["CustomerId"] is DBNull ? null :Convert.ToInt32(reader["CustomerID"]);

It is throwing

 Type of conditional expression cannot be determined because there 
  is no implicit conversion between '<null>' and 'int'

What is the problem with the Conversion?

By "Id" are you referring to Nullable CustomerID declared at the top? — rkg, Oct 21 '10 at 12:51

score 4 · Answer 1 · answered Oct 21 '10 at 12:52

4

Change your conditon to

  reader["CustomerId"] ==  DBNull.Value

answered Oct 21 '10 at 12:52

Aliostad

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marc_s · Accepted Answer · 2010-10-21T13:41:22.843

4

I think you need to do it this way:

if(! reader.IsDBNull(reader.GetOrdinal("CustomerId"))
{
   Id = Convert.ToInt32(reader["CustomerID"]);
}
else
{
   Id = NULL;
}

You need to use the .IsDBNull method on the reader to determine ahead of time if a column is NULL - if it is, don't even read the value from the reader.

edited Oct 21 '10 at 13:41

answered Oct 21 '10 at 12:52

marc_s

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+1, better solution. If you don't like the multi-line approach, you might be able to add an extension method to the data reader class to allow single-line syntax in your code. – John M Gant Oct 21 '10 at 12:59
Is there an overload of IsDBNull that takes the column name? I can only find the int version on MSDN: http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqldatareader.isdbnull.aspx – Justin Niessner Oct 21 '10 at 13:19
@Justin Niessner: no, sorry - that would be my own extension method.... updated my answer – marc_s Oct 21 '10 at 13:40

score 2 · Answer 3 · answered Oct 21 '10 at 12:52

2

A ?: conditional expression cannot evaluate to two different types on the true and false condition. I think a cast (int?)null should work.

answered Oct 21 '10 at 12:52

softveda

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Yes, casting will solve the problem: Id = reader["CustomerId"] is DBNull ? (int?)null : Convert.ToInt32(reader["CustomerID"]); – Pavel Morshenyuk Oct 21 '10 at 12:53
Well i don't like casting nulls. reader.IsDbNull is much cleaner. – Yves M. Oct 21 '10 at 12:55
I'm talking about "True" part of conditional expression. Not about checking reader if it is DBNull. – Pavel Morshenyuk Oct 21 '10 at 12:58
+1, this would address the specific error message, though it's not the best solution in my opinion. – John M Gant Oct 21 '10 at 13:00

score 2 · Answer 4 · answered Oct 21 '10 at 12:53

2

The problem (assuming that Id is declared properly) is that the conditional statement infers the result type from the true result. In your case, that type is null. It then will try to cast the second type to the same as the first...and there is no cast from int to null.

The solution is to cast the true expression to the desired type:

Id = reader["CustomerId"] == DBNull.Value ?
    (int?) null :
    Convert.ToInt32(reader["CustomerID"]);

answered Oct 21 '10 at 12:53

Justin Niessner

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+1 to reverse (uncommented) downvote. This is the correct answer to "What is the problem with this conversion." – John M Gant Oct 21 '10 at 13:01

score 1 · Answer 5 · answered Oct 21 '10 at 12:54

1

Try this

Id = reader["CustomerId"] is DBNull ? (int?)null : Convert.ToInt32(reader["CustomerID"]);

the types of both parts of the ?: need to be explicit

answered Oct 21 '10 at 12:54

Kleinux

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+1 to reverse (uncommented) downvote. This is the correct answer to "What is the problem with this conversion." – John M Gant Oct 21 '10 at 13:02

Nix · Answer 6 · 2010-10-21T13:41:08.897

0

What's happening is your code is incorrectly evaluating and trying to do the Convert function.

Its readding reader["CustomerId"] is DBNull which it isn't its really a DBNull.Value so then it attempts to do Convert.ToInt32("<null>") and crashes. You need to fix your if statement, and add a (int ?) cast.

It should be:

 Id = reader["CustomerId"] is DBNull.Value ? (int?) null :Convert.ToInt32(reader["CustomerID"]);

edited Oct 21 '10 at 13:41

answered Oct 21 '10 at 12:54

Nix

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Tech, it would produce a new error, but his original error was because he was attempting to do Convert.ToInt32(""); – Nix Oct 21 '10 at 13:40

C# Handling Null Values

6 Answers6