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Not understand NULL

Posted on 1999-01-14
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Last Modified: 2010-05-03
1. A help file said that var1 in IsNull(var1) should be a Variant type variable, but in an exmaple var1 is a textbox name.  var1 can be what type?
2. If var1 is a txtbox name, or other object name, what means by IsNull(var1) = True?
3. Can we assign Null to a variable other than the ones of Variant type?
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Question by:learn
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traygreen earned 50 total points
ID: 1456303
1) VB will typecast the variable to Variant - so string etc are valid parameters
2)if you pass the name in as parameter var1 and it is null then will return true
3)Typically no.  You have to assign an empty string to a string not null, nor can you assign it to numeric types
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Expert Comment

by:caraf_g
ID: 1456304
1 the reason why the parameter in IsNull is a variant is because you can put in any type of variable - string, integer etcetera.
2 hard to say - if, for example, the data is retrieved from a database using SQL, it means that the contents of the field in the database was NULL.
3 No
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Expert Comment

by:traygreen
ID: 1456305
Sorry the answer #2 is incorrect - It will return false
Basically Null is returned when a variable pointer does not address memory.
In other languages you would use it (or nil) to destroy the address link between a variable and it's memory location.  
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Author Comment

by:learn
ID: 1456306
So you, experts, said
a) we can put any type as the parameter in IsNull(), VB take care of turnning them to Variant;
b) we can not assign Null to any type except Variant.
am I right?

I remember I wrote a code
    Dim s As String
    Print IsNull(s)
and got error message: using Null incurrect....
I will check this code again later.

Thanks a lot
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Author Comment

by:learn
ID: 1456307
Sorry, we should not get any error in the last code.
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