Hi
VS 2005 beta 2 successfully compiles the following:
using System;
using System.Collecti ons.Generic;
using System.Text;
namespace ConsoleApplicat ion1 {
class Program {
enum A { a, b, c }
static void Main(string[] args) {
A? aaaa = null;
switch (aaaa) {
case null:
Console.WriteLi ne("null");
break;
default:
Console.WriteLi ne("def");
break;
}
}
}
}
Any comments on nullables in switch?
Thank you
Yuriy
Nov 17 '05
27  5639 
How does it cover nullables? SomeEnum type is not base type for Nullable<SomeEn um>
and no impllicit conversion exists from Nullable<A> to A. Yuri,
Quoting the C# Specification, 3rd Edition (p. 228):
"The governing type of a switch statement is established by the switch
expression. If the type of the switch
expression is sbyte, byte, short, ushort, int, uint, long, ulong,
char,
string, or an enum-type,
then that is the governing type of the switch statement. Otherwise,
exactly one user-defined implicit
conversion operator (§13.4) shall exist from the type of the switch
expression or a base type of this type to
one of the following possible governing types: sbyte, byte, short,
ushort, int, uint, long, ulong,
char, string. If no such implicit conversion operator exists, or if
more than one such implicit conversion
operator exists, a compile-time error occurs."
This covers the case of a nullable type as switch expression, right?
The standard is at www.ecma-international.org.
Privet - Octavio
"Yuriy Solodkyy" <y.************ @gmail.com> escribió en el mensaje
news:7c******** *************** ***@msnews.micr osoft.com...
hi,
which types are allowed in switch? string and integral types? or
any type, if there is only one way of implicit convertion to any of
the types mentioned above.
Nullable<> is neither string nor integral type, and I cannot find
anything where it is said that C# 2.0 enhances a list of types
allowed in switch.
yuriy
Yuriy Solodkyy wrote:
Any comments on nullables in switch?
I'm sure it's me, but I don't understand the question. :-)
Oliver Sturm
In this case 42 is converted to int?, but it does not explain how switch
works. Specifications says that it must be either listed type or implicit
conversion to the listed type must exist. Yuriy Solodkyy wrote:
Nullable<Snum> is not enum itself. It is just value type, which has
no implicit conversion to niether the enum or listed type. so,
according to your explanation in cannot be used in switch, but it
works.
I wasn't saying that Nullable<Enum> is an Enum. I'm sure the
conversion that's taking place is actually covered by paragraph 24.2
of the C# 2.0 specs - read it if you like, I guess the relevant point
for your sample is the last one in 24.2.2:
If the nullable conversion is from S? to T, the conversion is
evaluated as an unwrapping from S? to S followed by the underlying
conversion from S to T.
If you decompile a program similar to your sample, you'll see that the
compiler creates calls to the GetValueOrDefau lt method of the
Nullable<T>, before the comparison itself takes place. This allows for
constructs like
int? foo = 42;
if (foo == 42)
...
Or of course for the use of the Nullable<T> in a switch statement,
such as we were discussing.
Oliver Sturm
Hi Octavio,
see comment inline
"Octavio Hernandez" <do****@danysof t.com> schrieb im Newsbeitrag
news:ue******** ******@TK2MSFTN GP14.phx.gbl... Yuri,
Quoting the C# Specification, 3rd Edition (p. 228):
"The governing type of a switch statement is established by the switch
expression. If the type of the switch
expression is sbyte, byte, short, ushort, int, uint, long, ulong, char,
string, or an enum-type,
a nullable type is non if this types, right?
then that is the governing type of the switch statement. Otherwise,
exactly one user-defined implicit
conversion operator (§13.4) shall exist from the type of the switch
expression or a base type of this type to
one of the following possible governing types: sbyte, byte, short, ushort,
int, uint, long, ulong,
char, string. If no such implicit conversion operator exists, or if more
than one such implicit conversion
There is no user-defined conversion operator from a nullable type (if its
not defined by the user.), right?
operator exists, a compile-time error occurs."
This covers the case of a nullable type as switch expression, right?
No
The standard is at www.ecma-international.org.
Privet - Octavio
"Yuriy Solodkyy" <y.************ @gmail.com> escribió en el mensaje
news:7c******** *************** ***@msnews.micr osoft.com... hi,
which types are allowed in switch? string and integral types? or any
type, if there is only one way of implicit convertion to any of the types
mentioned above.
Nullable<> is neither string nor integral type, and I cannot find
anything where it is said that C# 2.0 enhances a list of types allowed in
switch.
yuriy Yuriy Solodkyy wrote:
Any comments on nullables in switch?
I'm sure it's me, but I don't understand the question. :-)
Oliver Sturm
but with VS2005beta2 you can compile it and it works.. Hi Octavio,
see comment inline
"Octavio Hernandez" <do****@danysof t.com> schrieb im Newsbeitrag
news:ue******** ******@TK2MSFTN GP14.phx.gbl...
Yuri,
Quoting the C# Specification, 3rd Edition (p. 228):
"The governing type of a switch statement is established by the
switch
expression. If the type of the switch
expression is sbyte, byte, short, ushort, int, uint, long, ulong,
char,
string, or an enum-type,
a nullable type is non if this types, right?
then that is the governing type of the switch statement. Otherwise,
exactly one user-defined implicit
conversion operator (§13.4) shall exist from the type of the switch
expression or a base type of this type to
one of the following possible governing types: sbyte, byte, short,
ushort, int, uint, long, ulong,
char, string. If no such implicit conversion operator exists, or if
more than one such implicit conversion
There is no user-defined conversion operator from a nullable type (if
its not defined by the user.), right?
operator exists, a compile-time error occurs."
This covers the case of a nullable type as switch expression, right?
No
The standard is at www.ecma-international.org.
Privet - Octavio
"Yuriy Solodkyy" <y.************ @gmail.com> escribió en el mensaje
news:7c******** *************** ***@msnews.micr osoft.com...
hi,
which types are allowed in switch? string and integral types? or
any type, if there is only one way of implicit convertion to any of
the types mentioned above.
Nullable<> is neither string nor integral type, and I cannot find
anything where it is said that C# 2.0 enhances a list of types
allowed in switch.
yuriy
Yuriy Solodkyy wrote:
> Any comments on nullables in switch?
>
I'm sure it's me, but I don't understand the question. :-)
Oliver Sturm
> If the nullable conversion is from S? to T, the conversion is evaluated as an unwrapping from S? to S followed by the underlying conversion from S to T.
The nullable conversion from S? to T is an Explicit conversion.
This allows for constructs like
int? foo = 42;
if (foo == 42)
This is using the lifted == operator, which is covered in 14.2.7 of the ecma
spec.
Sorry, Yura!
Now I understand what you mean. Very interesting!
Regards - Octavio
"Yuriy Solodkyy" <y.************ @gmail.com> escribió en el mensaje
news:7c******** *************** ***@msnews.micr osoft.com... but with VS2005beta2 you can compile it and it works..
Hi Octavio,
see comment inline
"Octavio Hernandez" <do****@danysof t.com> schrieb im Newsbeitrag
news:ue******** ******@TK2MSFTN GP14.phx.gbl...
Yuri,
Quoting the C# Specification, 3rd Edition (p. 228):
"The governing type of a switch statement is established by the
switch
expression. If the type of the switch
expression is sbyte, byte, short, ushort, int, uint, long, ulong,
char,
string, or an enum-type,
a nullable type is non if this types, right?
then that is the governing type of the switch statement. Otherwise,
exactly one user-defined implicit
conversion operator (§13.4) shall exist from the type of the switch
expression or a base type of this type to
one of the following possible governing types: sbyte, byte, short,
ushort, int, uint, long, ulong,
char, string. If no such implicit conversion operator exists, or if
more than one such implicit conversion
There is no user-defined conversion operator from a nullable type (if
its not defined by the user.), right?
operator exists, a compile-time error occurs."
This covers the case of a nullable type as switch expression, right?
No
The standard is at www.ecma-international.org.
Privet - Octavio
"Yuriy Solodkyy" <y.************ @gmail.com> escribió en el mensaje
news:7c******** *************** ***@msnews.micr osoft.com...
hi,
which types are allowed in switch? string and integral types? or
any type, if there is only one way of implicit convertion to any of
the types mentioned above.
Nullable<> is neither string nor integral type, and I cannot find
anything where it is said that C# 2.0 enhances a list of types
allowed in switch.
yuriy
> Yuriy Solodkyy wrote:
>
>> Any comments on nullables in switch?
>>
> I'm sure it's me, but I don't understand the question. :-)
>
> Oliver Sturm
>
> but with VS2005beta2 you can compile it and it works..
I cannot find anyplace in the spec that would explain why it compiles. I
think this is an interesting question and I will take another look when I
have more time.
Note that the C# compiler also lets you switch on bool (which is a compiler
bug).
MarkT [developmentor] wrote: If the nullable conversion is from S? to T, the conversion is evaluated as
an unwrapping from S? to S followed by the underlying conversion from S to T.
The nullable conversion from S? to T is an Explicit conversion.
I would think so, too, but that's nevertheless exactly what the generated
code shows.
Oliver Sturm
--
Expert programming and consulting services available
See http://www.sturmnet.org (try /blog as well)
> Any comments on nullables in switch?
I just spent an hour looking at every place either nullable or switch
appears in the spec, and I could not find anything that explains the
behavior. However, the spec is pretty tough reading so I could easily have
missed something. I tentatively vote "bug" on this one. You could submit it
as a bug in the Microsoft Product Feedback Center to get some official word
on it.
"MarkT [developmentor]" <Ma************ ****@discussion s.microsoft.com > wrote
in message news:C4******** *************** ***********@mic rosoft.com... Any comments on nullables in switch?
I just spent an hour looking at every place either nullable or switch
appears in the spec, and I could not find anything that explains the
behavior. However, the spec is pretty tough reading so I could easily have
missed something. I tentatively vote "bug" on this one. You could submit
it
as a bug in the Microsoft Product Feedback Center to get some official
word
on it.
Note that only the latest v2.0 beta version (2.0.50727) includes the latest
Design Change Request made for VS2005 and the CLR, one of these DCR relate
to nullable types, maybe one could try this using this build.
Note that it's allways safe to file an issue to Product Feedback though.
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