Strange null value

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RichB - 30 Aug 2007 10:02 GMT

I have a strange null value, which is displayed in the screen print here:

http://devscreens.blogspot.com/

If you look at the variables in the bottom half of the screen, third line
down is the value of key ["@sch_id], which clearly has a value, whilst in
the row 2 rows below cr.Keys["sch_id"] it is reported as null.

Keys is a hashtable (perhaps with a confusing name in hindsight) which
contains the parameters as the Key, and coresponding values from a stored
procedure.

This works elsewhere in my code with other id fields retrieved from the
database, but before I go providing a whole lot of classes, can somone
explain why there might be a difference betweeen these two rows?

Many Thanks,
Richard

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Jon Skeet [C# MVP] - 30 Aug 2007 10:28 GMT

> I have a strange null value, which is displayed in the screen print here:
>
[quoted text clipped - 3 lines]
> down is the value of key ["@sch_id], which clearly has a value, whilst in
> the row 2 rows below cr.Keys["sch_id"] it is reported as null.

That's entirely reasonable. A hashtable can contain an entry with a
null value, and it looks like that's what's happening here.

For instance:

Hashtable x = new Hashtable();
x["key"] = null;

x.ContainsKey("key") will return true; x["key"] will return null.

Jon

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RichB - 30 Aug 2007 10:36 GMT

Thanks, that is all understood, but I can't see it in this case.

In the secod row the cr.Keys["@sch_id"] contains the value
{35b92307-a090-4c27-9a66-a57b605eff92},
whilst in row 5 cr.Keys["@sch_id"] it is null.

I thought that these were both the same variable, if this is not the case,
then how to I access the {35b92307-a090-4c27-9a66-a57b605eff92}?

Thanks, Richard

>> I have a strange null value, which is displayed in the screen print here:
>>
[quoted text clipped - 15 lines]
>
> Jon

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Jon Skeet [C# MVP] - 30 Aug 2007 10:47 GMT

> Thanks, that is all understood, but I can't see it in this case.
>
[quoted text clipped - 4 lines]
> I thought that these were both the same variable, if this is not the case,
> then how to I access the {35b92307-a090-4c27-9a66-a57b605eff92}?

Oops, yes I see now.

However, you need to look really carefully at the keys. You're asking
for "sch_id" but the only key in there is "@sch_id". Note the "@" at
the front. (Likewise row 5 shows cr.Keys["sch_id"].)

Jon

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RichB - 30 Aug 2007 10:49 GMT

Thanks, good spot....I couldn't see it for looking!!

>> Thanks, that is all understood, but I can't see it in this case.
>>
[quoted text clipped - 13 lines]
>
> Jon

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Jeffrey Tan[MSFT] - 31 Aug 2007 07:12 GMT

Hi Richard,

Is your problem resolved? Yes, while retrieving the Hashtable.Item property
using a key, we should use the same key for it(without missing the "@"
character). As stated in the "Property Value" section in the link below, if
the key you passed is not the previous one(not found), the Hashtable.Item
will return a null reference. This explains the behavior:
http://msdn2.microsoft.com/en-us/library/system.collections.hashtable.item(V
S.71).aspx

Anyway, if you need further help, please feel free to feedback, thanks.

Best regards,
Jeffrey Tan
Microsoft Online Community Support
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Strange null value

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