NULLs

  • Thread starter Thread starter Mike
  • Start date Start date
M

Mike

Hi.

I have a database from which I am reading some integers into my C# app. The
problem is some of these DB values are NULL, so obviously an error is thrown
when I try to read them. Is there a standard way of representing a NULL
value type? Anyone got any tips on this?

Mike
 
Hi Mike,
Check for DBNull(.Value)
Click to expand...

Yeah, I realise I can check for the NULL first, but I want to be able to
represent this NULL value in my app. Like in the database, I can set an
INTEGER field to NULL, but in C#, I can't set an int variable to NULL. How
do people get around this?

Mike
 
Well, for positive numbers, I would use -1. For both negative and positive numbers ... um ... maybe pick a number, any number and treat it as NULL, Int32.MinValue for instance.
 
Mike said:
Yeah, I realise I can check for the NULL first, but I want to be able to
represent this NULL value in my app. Like in the database, I can set an
INTEGER field to NULL, but in C#, I can't set an int variable to NULL. How
do people get around this?

Mike
Click to expand...

I think you are asking for nullable types which will be available in C# 2.0.
Support for nullable types was added to have the language interact with
databases.
Hence you will in the near future be able to write:

int? a = 5;
int? b = null;
int? c = a + b;

C would then contain null.

If you want nullable types today I guess you have to play around with a
DataSet.

Best Regards
- Michael S
 
Yeah, I realise I can check for the NULL first, but I want to be able to
I think you are asking for nullable types which will be available in C# 2.0.
Support for nullable types was added to have the language interact with
databases.
Hence you will in the near future be able to write:

int? a = 5;
int? b = null;
int? c = a + b;

C would then contain null.
Click to expand...

That's exactly what I was asking, thanks.

Another question on nulls. I'm trying to override the == for a class I have
created:


public class Termination
{
public string CountryName, TerminationName;

public Termination(string countryName, string terminationName)
{
this.CountryName = countryName; this.TerminationName = terminationName;
}

public override string ToString()
{
string str = this.CountryName;
if(this.TerminationName != "" && this.TerminationName != null)
str += " - " + this.TerminationName;
return str;
}

// .................

public static bool operator==(Termination t1, Termination t2)
{
return (t1 == null && t2 == null) ||
(t1 != null && t2 != null && (String.Compare(t1.ToString(),
t2.ToString()) == 0));
}

// .................

}


This is all good if I do this:


Termination t1 = new Termination("USA", "Washington");
Termination t2 = new Termination("UK", "London");
Console.WriteLine(t1 == t2); // False


But this throws "An unhandled exception of type
'System.StackOverflowException'":


Termination t1 = null;
Console.WriteLine(t1 == null); // should be true


presumably because I am defining the == operator WITH the == operator,
causing infinite recursion at runtime. I originally tried this for my
override method:


public static bool operator==(Termination t1, Termination t2)
{
return (String.Compare(t1.ToString(), t2.ToString()) == 0));
}


but that gives "An instance of the object not set" at runtime when comparing
t1 to null.

How do I avoid this?

Mike
 
Hi Mike,


I havent run your code but I think that you are using your overloaded
ToString() method in the == method, then at the same time you are using
ToString() inside the == method, therefore you enter in an infinite cicle
where one method calls the other, hence the stack overflow exception :)


Cheers,
 
Hi Mike,

If you google this you will find a lot of post related to this. Basically
you have two options ( the third being wait until C# 2.0 ships :) )
1- Use an especific integer value to indicate null , this is dependand of
your application .
2- Create/Use a wrapper class.

Cheers,
 
Hi Mike,
I havent run your code but I think that you are using your overloaded
ToString() method in the == method, then at the same time you are using
ToString() inside the == method, therefore you enter in an infinite cicle
where one method calls the other, hence the stack overflow exception :)
Click to expand...

Yes, my stack overfloweth.

Mike
 
Michael and Mike,

Don't forget that you can use the SqlInt32 structure as well, which has
null semantics.

Hope this helps.
 
Nicholas Paldino said:
Michael and Mike,

Don't forget that you can use the SqlInt32 structure as well, which has
null semantics.

Hope this helps.
Click to expand...

Yes, a bit more lean method than my suggestion of using a whole bloated
DataSet! =)

Thanks
- Michael S
 
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