VB.NET equivalent to C# "x y = z as y"

[Jimi]

I was wondering if there was a VB.NET equivalent to the following C#
statement:
SomeType SomeVar = SomeOtherVar as SomeType;

I know that in VB.NET you could have:
Dim SomeVar As SomeType = CType(SomeOtherVar, SomeType)

but this throws an exception if the cast fails while the C# example
above just sets SomeVar to null if the cast fails. (the C#
equivalent to this is "SomeType SomeVar = (SomeType)SomeOtherVar;".

 

[Herfried K. Wagner [MVP]]

* (e-mail address removed)-spam.invalid (Jimi) scripsit:

I was wondering if there was a VB.NET equivalent to the following C#
statement:
SomeType SomeVar = SomeOtherVar as SomeType;

I know that in VB.NET you could have:
Dim SomeVar As SomeType = CType(SomeOtherVar, SomeType)

but this throws an exception if the cast fails while the C# example
above just sets SomeVar to null if the cast fails. (the C#
equivalent to this is "SomeType SomeVar = (SomeType)SomeOtherVar;".

\\\
Dim SomeOtherVar As SomeType
If TypeOf SomeVar Is SomeType Then
SomeOtherVar = DirectCast(SomeVar, SomeType)
End If
If Not SomeOtherVar Is Nothing Then
...
End If
///

In VB.NET 2004/5:

\\\
Dim SimeOtherVar As SomeType = TryCast(SomeVar, SomeType)
If SomeOtherVar IsNot Nothing Then
...
End If
///

 

[Jimi]

Thanks!
I was hoping for a one line version - but I see from the teaser you
included that that's not there until VS 2004/5 with "TryCast".

 

[Jay B. Harlow [MVP - Outlook]]

Herfried,
Did I miss something?

Dim SimeOtherVar As SomeType = TryCast(SomeVar, SomeType)

I don't remember seeing TryCast in the PDC release of Whidbey...

Jay

 

[Tom Leylan]

Did I miss something?
Dim SimeOtherVar As SomeType = TryCast(SomeVar, SomeType)

I don't remember seeing TryCast in the PDC release of Whidbey...

Jay: Part of the TryCast(), CatchCast(), FinallyCast() system

Tom

 

[Herfried K. Wagner [MVP]]

Jay,

Did I miss something?
Maybe...

Dim SimeOtherVar As SomeType = TryCast(SomeVar, SomeType)

I don't remember seeing TryCast in the PDC release of Whidbey...

I didn't yet have time to take such a close look at VB.NET Whidbey, but
Paul Vick wrote about this operator in his weblog:

<http://www.panopticoncentral.net/PermaLink.aspx/0d6ba439-8126-427e-952e-3f5fbba33904>

 

[Herfried K. Wagner [MVP]]

Tom,

Jay: Part of the TryCast(), CatchCast(), FinallyCast() system

LOL!

 

[Jay B. Harlow [MVP - Outlook]]

Herfried,
I checked the PDC release of Whidbey, its not there!

The first sentence of the second paragraph, on the site you gave, I think
explains it:

<quote>In Whidbey, we're going to introduce YACO (Yet Another Conversion
Operator) called TryCast that's equivalent to the C# 'as' operator. </quote>

I'll have to add Paul's RSS to my RSS reader. I just finished his "The
Visual Basic .NET Programming Language" from Addison Wesley. Picked up a new
trick! You can use a Goto in a Catch block to go to the Try Block. (aka a
simple form of Retry for Try/Catch blocks).

Thanks for the heads up
Jay

 

[Chris Dunaway]

Visual Basic .NET Programming Language" from Addison Wesley. Picked up a new
trick! You can use a Goto in a Catch block to go to the Try Block. (aka a
simple form of Retry for Try/Catch blocks).

Blasphemy!! Won't the programming gods smite you for using the G-keyword
in a program?

Do you put a label before the try or inside the try:

This:

TRYAGAIN:
Try
'blah blah
Catch
If somecondition then
goto TRYAGAIN
endif
Finally
End Try

Or this:

Try
TRYAGAIN:
'blah blah
Catch
If somecondition then
goto TRYAGAIN
endif
Finally
End Try

 

[Herfried K. Wagner [MVP]]

Jay,

I checked the PDC release of Whidbey, its not there!

I hope there will be a beta with more features soon.

The first sentence of the second paragraph, on the site you gave, I think
explains it:

<quote>In Whidbey, we're going to introduce YACO (Yet Another Conversion
Operator) called TryCast that's equivalent to the C# 'as' operator. </quote>

I like 'TryCast' much more than the very undescriptive 'as' (at least
non-native English speakers have problems to understand the purpose of
this operator when seeing it in source code).

I'll have to add Paul's RSS to my RSS reader. I just finished his "The
Visual Basic .NET Programming Language" from Addison Wesley. Picked up a new
trick! You can use a Goto in a Catch block to go to the Try Block. (aka a
simple form of Retry for Try/Catch blocks).

:->

 

[Cor]

Hi,

Again a statement where all the non C programmers (probably not) are
waiting for.

Cor

 

[Jay B. Harlow [MVP - Outlook]]

Chris,

Blasphemy!! Won't the programming gods smite you for using the G-keyword
in a program?

Remember bad uses of Goto does not make Goto itself bad.

Do you put a label before the try or inside the try:

It needs to go Inside, I can see combining it with a MessageBox on specific
exceptions only (ones the user could recover from, such as a file format
errors after Open File dialog).

Try
TRYAGAIN:
'blah blah
Throw New System.IO.FileNotFoundException
Catch ex As System.IO.FileNotFoundException
If MessageBox.Show("What would you like to do?", "File not
found", MessageBoxButtons.RetryCancel, MessageBoxIcon.Question,
MessageBoxDefaultButton.Button2) = DialogResult.Retry Then
GoTo TRYAGAIN
Else
Throw ' let error propagate up
End If
End Try

Whether I rethrow the caught error to let it propagate up or not would
depend on where I was & what the "alternative" (to having the try fail) is.

Hope this helps
Jay

 

[Jay B. Harlow [MVP - Outlook]]

Herfried,
I like TryCast over As (especially considering As is already well defined in
VB.NET).

Not sure when we will see the next beta...

Jay

 

[NM]

Instead of the two If, it could be better to use Try :

Dim SomeOtherVar As SomeType
Try
SomeOtherVar = CType(SomeVar, SomeType)
Catch ex As Exception
SomeOtherVar = Nothing
End Try

 

[Jay B. Harlow [MVP - Outlook]]

NM,
Remember If TypeOf will perform better then a Try/Catch.

Hence Herfried's example is the better way, in this case.

Hope this helps
Jay

Instead of the two If, it could be better to use Try :

Dim SomeOtherVar As SomeType
Try
SomeOtherVar = CType(SomeVar, SomeType)
Catch ex As Exception
SomeOtherVar = Nothing
End Try

 

[NM]

Ok thanks.

NM,
Remember If TypeOf will perform better then a Try/Catch.

Hence Herfried's example is the better way, in this case.

Hope this helps
Jay

 

[Mattias Sjögren]

Jay,

I like TryCast over As

TryCast is certainly better than As for VB. But I don't like the name
TryCast either. Probably because it reminds me of the MC++ operator
__try_cast which *does* throw InvalidCastException on failure, just
like DirectCast does today. Too confusing...

Plus I associate the word "Try" with SEH and exceptions, so it just
doesn't feel right.

I wish they had used some other name, such as SafeCast or
CastButNoExceptionsPlease.



Mattias

 

[Jay B. Harlow [MVP - Outlook]]

Mattias,
Yea when I first read about TryCast, I was trying to think of a different
term, I like SafeCast.

Whatever is used it needs to be "short", CastButNoExceptionsPlease is a
little too long ;-))

Another option might be to stay with DirectCast and add an overloaded
parameter.

x = DirectCast(o, Type) ' causes exception
x = DirectCast(o, Type, True) ' causes exception
x = DirectCast(o, Type, False) ' exception free zone

But I don't know if I care for the above either, considering other
keywords/operators don't work that way.

I'm just glad we are getting the keyword/operator!

Jay