Checking for DBNull with generics

[Leon Mayne]

We currently have lots of checks in our businesslayer object's Load()
functions that look like:

If drCourse("txtTT MCnotes").Equal s(DBNull.Value) Then
Me._txtTTMCnote s = ""
Else
Me._txtTTMCnote s = CStr(drCourse(" txtTTMCnotes"))
End If

and

If drCourse("datTT MCarchived").Eq uals(DBNull.Val ue) Then
Me._datTTMCarch ived = New Nullable(Of DateTime)
Else
Me._datTTMCarch ived = CDate(drCourse( "datTTMCarchive d"))
End If

I'd like to tidy these up with a a static generic function that will check
for null, and then return either the value or a default value (e.g. int=0,
string="", nullables=new nullable) but am having problems with the return
type. I started with this:

Public Shared Function CheckDbNull(Of T)(ByVal pReaderVar As Object) As T
If pReaderVar.Equa ls(DBNull.Value ) Then
Return Nothing
Else
Return CType(pReaderVa r, T)
End If
End Function

Which works, but returns nothing, which is causing null reference
exceptions. I tried writing it like:

If pReaderVar.Equa ls(DBNull.Value ) Then
Select Case GetType(T).Full Name
Case "System.Str ing"
Return ""
Case "System.Int 32"
Return 0
' etc
End Select

But this doesn't compile, as the return type is not T. Also trying to use
e.g. Return CType("", T) doesn't work.

Does anyone know how to return a specific type based on the return type, or
do I have to create a separate function for each type I'm checking for?

[Marc Gravell]

Re: Checking for DBNull with generics

Why not obtain the ordinal of the column (GetOrdinal), then use
IsDBNull to check for null on the column, then finally default(T)* to
use the default value for that type. The only exception you might want
here is string (since it looks like you want "") - although personally
I'd find the null more correct, since you can differentiate between
the null string and the empty string.

(*=this is the C# syntax; I'm sure there is a VB equivalent, but I
don't know what it is ;-p)

Re casting the string (as an exception) - I don't know enough about
VB, but in C# you can trick it by doing something like:
return (T)(object)"";

Marc

[Marc Gravell]

Re: Checking for DBNull with generics

Just for the record, string-compare on GetType is a nasty way to do things -
surely comparing just the types [i.e. GetType(T) vs GetType(String) etc]
would be cleaner? Granted: you can't do this in a switch... but If, ElseIf,
etc should do.

I'm also a little confused as to what the Nullable<boolet c is doing; it
*looks* like it is converting an empty Nullable<boolto object (which
should yield null due to the boxing rules), then converting that null to a
T, which we already know if Nullable<bool(h ence becoming another empty
Nullable<bool>)- so we've got a complicated way of saying "null". In C#,
default(T) here would the same thing... I don't know what VB does, though.

Marc

[Leon Mayne]

Re: Checking for DBNull with generics

"Marc Gravell" <marc.gravell@g mail.comwrote in message
news:uz5%23tjxk IHA.3888@TK2MSF TNGP03.phx.gbl. ..

Just for the record, string-compare on GetType is a nasty way to do
things - surely comparing just the types [i.e. GetType(T) vs
GetType(String) etc] would be cleaner? Granted: you can't do this in a
switch... but If, ElseIf, etc should do.

Yes, that's why I was doing a string compare. Perhaps I should rewrite it as
you suggest for performance.

I'm also a little confused as to what the Nullable<boolet c is doing; it
*looks* like it is converting an empty Nullable<boolto object (which
should yield null due to the boxing rules), then converting that null to a
T, which we already know if Nullable<bool(h ence becoming another empty
Nullable<bool>)- so we've got a complicated way of saying "null". In C#,
default(T) here would the same thing... I don't know what VB does, though.

I just checked this and it seems to work fine. It creates a new nullable(of
boolean) with no value, and HasValue = false. Is this different in C#?
*Leon checks
OK, I just tried:

public static T CheckDBNull<T>( object pReaderVar)
{
if (pReaderVar.Equ als(DBNull.Valu e))
{
if (typeof(T) == typeof(string))
{
return (T)(object)"";
}
else if (typeof(T) == typeof(Nullable <bool>))
{
return (T)(object)new Nullable<bool>( );
}
else
{
return default(T);
}
}
else
{
return (T)pReaderVar;
}
}

and yes, it returns null for a Nullable<bool>. I wonder why it's different
for VB.NET? Would be interesting to look at the MSIL.

[Leon Mayne]

Re: Checking for DBNull with generics


"Leon Mayne" <leon@rmvme.mvp s.orgwrote in message
news:C5A348E0-11EE-48E0-B45B-FAB3BDFA0295@mi crosoft.com...

Would be interesting to look at the MSIL.

I just did this, and it looks like the VB version is using a different call
to unbox the object:

VB:
IL_00b9: box valuetype [mscorlib]System.Nullable `1<bool>
IL_00be: call !!0
[Microsoft.Visua lBasic]Microsoft.Visua lBasic.Compiler Services.Conver sions::ToGeneri cParameter<!!0> (object)

C#:
IL_0066: box valuetype [mscorlib]System.Nullable `1<bool>
IL_006b: unbox.any !!T

So the Conversions::To GenericParamete r works but the direct unboxing
doesn't.

[Leon Mayne]

Re: Checking for DBNull with generics

"Leon Mayne" <leon@rmvme.mvp s.orgwrote in message
news:B27662E5-EC82-4A95-B97A-A023D69262E0@mi crosoft.com...

>
"Leon Mayne" <leon@rmvme.mvp s.orgwrote in message
news:C5A348E0-11EE-48E0-B45B-FAB3BDFA0295@mi crosoft.com...

>Would be interesting to look at the MSIL.

>
I just did this, and it looks like the VB version is using a different
call to unbox the object:
>
VB:
IL_00b9: box valuetype [mscorlib]System.Nullable `1<bool>
IL_00be: call !!0
[Microsoft.Visua lBasic]Microsoft.Visua lBasic.Compiler Services.Conver sions::ToGeneri cParameter<!!0> (object)
>
C#:
IL_0066: box valuetype [mscorlib]System.Nullable `1<bool>
IL_006b: unbox.any !!T
>
So the Conversions::To GenericParamete r works but the direct unboxing
doesn't.

Just tried it myself:

Return
Microsoft.Visua lBasic.Compiler Services.Conver sions.ToGeneric Parameter(Of
T)(New Nullable(Of Boolean))

Works fine in VB.NET, but not in C#:

return
Microsoft.Visua lBasic.Compiler Services.Conver sions.ToGeneric Parameter<T>(ne w
Nullable<Boolea n>());

Something odd going on here!