Even though CSC does its best to detect use of unassigned variables, it
often misses it... for example if you just declare a double in a class
without assigning a default value, it has a default value of 0 and lets you
use it anyway.
My question is, how can I retrieve the default value for a given type? The
CLR obviously has these defaults stored somewhere and I was hoping to get
hold of it.
I hoped the following code would work, but TypeInitializer is null:
double d = 12;
ConstructorInfo con = d.GetType().Typ eInitializer;
Console.WriteLi ne(con.Invoke(n ew object[] { }));
Any ideas?
29  3038 
John, My question is, how can I retrieve the default value for a given type?
The default is all bits zeroed out, meaning 0 or 0.0 for numeric
types, false for bools and null for reference types.
For value types, you get the default value when using the default
constructor.
double d = new double(); // effectively the same as double d = 0.0;
Mattias
--
Mattias Sjögren [MVP] mattias @ mvps.org http://www.msjogren.net/dotnet/ | http://www.dotnetinterop.com
Please reply only to the newsgroup.
Yes, that may be true -- but, given an arbitrary value type, how do I
programmaticall y retrieve the default value for that type? It can't just be
0 bits, because for a string (for example) that won't work.
I'd have hoped that the boxed equivalent of the value type would have a
constructor, but GetConstructor returns null.
"Mattias Sjögren" <ma************ ********@mvps.o rg> wrote in message
news:e4******** ******@TK2MSFTN GP11.phx.gbl... John,
My question is, how can I retrieve the default value for a given type?
The default is all bits zeroed out, meaning 0 or 0.0 for numeric
types, false for bools and null for reference types.
For value types, you get the default value when using the default
constructor.
double d = new double(); // effectively the same as double d = 0.0;
Mattias
--
Mattias Sjögren [MVP] mattias @ mvps.org
http://www.msjogren.net/dotnet/ | http://www.dotnetinterop.com
Please reply only to the newsgroup.
John,
It's actually quite simple. You can use reflection to determine whether
or not the type derives from ValueType in some way. If it does, then you
can just use the default constructor to generate a value with the bits
zeroed out. If it does not derive from ValueType, then it is a reference
type, and the default value is null.
Hope this helps.
--
- Nicholas Paldino [.NET/C# MVP]
- mv*@spam.guard. caspershouse.co m
"John Wood" <sp**@isannoyin g.com> wrote in message
news:uu******** ******@TK2MSFTN GP09.phx.gbl... Yes, that may be true -- but, given an arbitrary value type, how do I
programmaticall y retrieve the default value for that type? It can't just
be 0 bits, because for a string (for example) that won't work.
I'd have hoped that the boxed equivalent of the value type would have a
constructor, but GetConstructor returns null.
"Mattias Sjögren" <ma************ ********@mvps.o rg> wrote in message
news:e4******** ******@TK2MSFTN GP11.phx.gbl... John,
My question is, how can I retrieve the default value for a given type?
The default is all bits zeroed out, meaning 0 or 0.0 for numeric
types, false for bools and null for reference types.
For value types, you get the default value when using the default
constructor.
double d = new double(); // effectively the same as double d = 0.0;
Mattias
--
Mattias Sjögren [MVP] mattias @ mvps.org
http://www.msjogren.net/dotnet/ | http://www.dotnetinterop.com
Please reply only to the newsgroup.
Right, but that won't work for Guid, DateTime or String (to name a few).
Isn't there a method somewhere in the framework that just returns the
default value for a given type?
"Nicholas Paldino [.NET/C# MVP]" <mv*@spam.guard .caspershouse.c om> wrote in
message news:eG******** *****@TK2MSFTNG P12.phx.gbl... John,
It's actually quite simple. You can use reflection to determine
whether or not the type derives from ValueType in some way. If it does, then you
can just use the default constructor to generate a value with the bits
zeroed out. If it does not derive from ValueType, then it is a reference
type, and the default value is null.
Hope this helps.
--
- Nicholas Paldino [.NET/C# MVP]
- mv*@spam.guard. caspershouse.co m
"John Wood" <sp**@isannoyin g.com> wrote in message
news:uu******** ******@TK2MSFTN GP09.phx.gbl... Yes, that may be true -- but, given an arbitrary value type, how do I
programmaticall y retrieve the default value for that type? It can't
just be 0 bits, because for a string (for example) that won't work.
I'd have hoped that the boxed equivalent of the value type would have a
constructor, but GetConstructor returns null.
"Mattias Sjögren" <ma************ ********@mvps.o rg> wrote in message
news:e4******** ******@TK2MSFTN GP11.phx.gbl... John,
>My question is, how can I retrieve the default value for a given
type?
The default is all bits zeroed out, meaning 0 or 0.0 for numeric
types, false for bools and null for reference types.
For value types, you get the default value when using the default
constructor.
double d = new double(); // effectively the same as double d = 0.0;
Mattias
--
Mattias Sjögren [MVP] mattias @ mvps.org
http://www.msjogren.net/dotnet/ | http://www.dotnetinterop.com
Please reply only to the newsgroup.
John,
Yes, actually it would. With Guid and DateTime, since they are
structures, they have default constructors, which you can get through
reflection and call through reflection (once you have the type).
With a string, it is not a value type, so the default value for a string
is null, which the algorithm that I detailed in my previous post will return
to you.
--
- Nicholas Paldino [.NET/C# MVP]
- mv*@spam.guard. caspershouse.co m
"John Wood" <sp**@isannoyin g.com> wrote in message
news:%2******** *******@tk2msft ngp13.phx.gbl.. . Right, but that won't work for Guid, DateTime or String (to name a few).
Isn't there a method somewhere in the framework that just returns the
default value for a given type?
"Nicholas Paldino [.NET/C# MVP]" <mv*@spam.guard .caspershouse.c om> wrote
in message news:eG******** *****@TK2MSFTNG P12.phx.gbl... John,
It's actually quite simple. You can use reflection to determine
whether or not the type derives from ValueType in some way. If it does, then
you can just use the default constructor to generate a value with the bits
zeroed out. If it does not derive from ValueType, then it is a
reference type, and the default value is null.
Hope this helps.
--
- Nicholas Paldino [.NET/C# MVP]
- mv*@spam.guard. caspershouse.co m
"John Wood" <sp**@isannoyin g.com> wrote in message
news:uu******** ******@TK2MSFTN GP09.phx.gbl... Yes, that may be true -- but, given an arbitrary value type, how do I
programmaticall y retrieve the default value for that type? It can't
just be 0 bits, because for a string (for example) that won't work.
I'd have hoped that the boxed equivalent of the value type would have
a constructor, but GetConstructor returns null.
"Mattias Sjögren" <ma************ ********@mvps.o rg> wrote in message
news:e4******** ******@TK2MSFTN GP11.phx.gbl...
> John,
>
> >My question is, how can I retrieve the default value for a given
type? >
> The default is all bits zeroed out, meaning 0 or 0.0 for numeric
> types, false for bools and null for reference types.
>
> For value types, you get the default value when using the default
> constructor.
>
> double d = new double(); // effectively the same as double d = 0.0;
>
>
>
> Mattias
>
> --
> Mattias Sjögren [MVP] mattias @ mvps.org
> http://www.msjogren.net/dotnet/ | http://www.dotnetinterop.com
> Please reply only to the newsgroup.
"Nicholas Paldino [.NET/C# MVP]" <mv*@spam.guard .caspershouse.c om> wrote in
message news:eG******** *****@TK2MSFTNG P12.phx.gbl... John,
It's actually quite simple. You can use reflection to determine
whether
or not the type derives from ValueType in some way. If it does, then you
can just use the default constructor to generate a value with the bits
zeroed out. If it does not derive from ValueType, then it is a reference
type, and the default value is null.
The only problem is most value types don't have default constructors(I think
MC++ allowed this for a while, but don't think any other languages did). The
new int() style syntax supported by C# is actually a syntactic sugar for the
initobj instruction, which tells the CLR to zero out a given value type.
Unfortunatly the langage as it stands doesn't support any syntax to
initialize a variable to its default. Tthe C# 2.0 .default operator should
give the behaviour the OP is looking for, if I understand him correctly, but
I havn't taken the time to verify it again in the spec preview so it could
just be an artifact of the current build, but I don't think so.
Hope this helps.
--
- Nicholas Paldino [.NET/C# MVP]
- mv*@spam.guard. caspershouse.co m
"John Wood" <sp**@isannoyin g.com> wrote in message
news:uu******** ******@TK2MSFTN GP09.phx.gbl... Yes, that may be true -- but, given an arbitrary value type, how do I
programmaticall y retrieve the default value for that type? It can't just
be 0 bits, because for a string (for example) that won't work.
I'd have hoped that the boxed equivalent of the value type would have a
constructor, but GetConstructor returns null.
"Mattias Sjögren" <ma************ ********@mvps.o rg> wrote in message
news:e4******** ******@TK2MSFTN GP11.phx.gbl... > John,
>
> >My question is, how can I retrieve the default value for a given type?
>
> The default is all bits zeroed out, meaning 0 or 0.0 for numeric
> types, false for bools and null for reference types.
>
> For value types, you get the default value when using the default
> constructor.
>
> double d = new double(); // effectively the same as double d = 0.0;
>
>
>
> Mattias
>
> --
> Mattias Sjögren [MVP] mattias @ mvps.org
> http://www.msjogren.net/dotnet/ | http://www.dotnetinterop.com
> Please reply only to the newsgroup.
If you run GetConstructor on the value type it returns null though...
doesn't look like any of the value types can provide a default constructor.
"Nicholas Paldino [.NET/C# MVP]" <mv*@spam.guard .caspershouse.c om> wrote in
message news:OZ******** ******@TK2MSFTN GP12.phx.gbl... John,
Yes, actually it would. With Guid and DateTime, since they are
structures, they have default constructors, which you can get through
reflection and call through reflection (once you have the type).
With a string, it is not a value type, so the default value for a
string is null, which the algorithm that I detailed in my previous post will
return to you.
--
- Nicholas Paldino [.NET/C# MVP]
- mv*@spam.guard. caspershouse.co m
"John Wood" <sp**@isannoyin g.com> wrote in message
news:%2******** *******@tk2msft ngp13.phx.gbl.. . Right, but that won't work for Guid, DateTime or String (to name a few).
Isn't there a method somewhere in the framework that just returns the
default value for a given type?
"Nicholas Paldino [.NET/C# MVP]" <mv*@spam.guard .caspershouse.c om> wrote in message news:eG******** *****@TK2MSFTNG P12.phx.gbl... John,
It's actually quite simple. You can use reflection to determine
whether or not the type derives from ValueType in some way. If it does, then you can just use the default constructor to generate a value with the bits
zeroed out. If it does not derive from ValueType, then it is a reference type, and the default value is null.
Hope this helps.
--
- Nicholas Paldino [.NET/C# MVP]
- mv*@spam.guard. caspershouse.co m
"John Wood" <sp**@isannoyin g.com> wrote in message
news:uu******** ******@TK2MSFTN GP09.phx.gbl...
> Yes, that may be true -- but, given an arbitrary value type, how do
I > programmaticall y retrieve the default value for that type? It can't
just be
> 0 bits, because for a string (for example) that won't work.
>
> I'd have hoped that the boxed equivalent of the value type would
have a > constructor, but GetConstructor returns null.
>
> "Mattias Sjögren" <ma************ ********@mvps.o rg> wrote in message
> news:e4******** ******@TK2MSFTN GP11.phx.gbl...
> > John,
> >
> > >My question is, how can I retrieve the default value for a given
type? > >
> > The default is all bits zeroed out, meaning 0 or 0.0 for numeric
> > types, false for bools and null for reference types.
> >
> > For value types, you get the default value when using the default
> > constructor.
> >
> > double d = new double(); // effectively the same as double d =
0.0; > >
> >
> >
> > Mattias
> >
> > --
> > Mattias Sjögren [MVP] mattias @ mvps.org
> > http://www.msjogren.net/dotnet/ | http://www.dotnetinterop.com
> > Please reply only to the newsgroup.
>
>
I suppose this answers my question... although it's disappointing the
initobj instruction isn't exposed through the language.
Thanks.
"Daniel O'Connell [C# MVP]" <onyxkirx@--NOSPAM--comcast.net> wrote in
message news:OK******** ******@TK2MSFTN GP10.phx.gbl...
"Nicholas Paldino [.NET/C# MVP]" <mv*@spam.guard .caspershouse.c om> wrote
in message news:eG******** *****@TK2MSFTNG P12.phx.gbl... John,
It's actually quite simple. You can use reflection to determine
whether
or not the type derives from ValueType in some way. If it does, then
you can just use the default constructor to generate a value with the bits
zeroed out. If it does not derive from ValueType, then it is a
reference type, and the default value is null.
The only problem is most value types don't have default constructors(I
think MC++ allowed this for a while, but don't think any other languages did).
The new int() style syntax supported by C# is actually a syntactic sugar for
the initobj instruction, which tells the CLR to zero out a given value type.
Unfortunatly the langage as it stands doesn't support any syntax to
initialize a variable to its default. Tthe C# 2.0 .default operator should
give the behaviour the OP is looking for, if I understand him correctly,
but I havn't taken the time to verify it again in the spec preview so it could
just be an artifact of the current build, but I don't think so.
Hope this helps.
--
- Nicholas Paldino [.NET/C# MVP]
- mv*@spam.guard. caspershouse.co m
"John Wood" <sp**@isannoyin g.com> wrote in message
news:uu******** ******@TK2MSFTN GP09.phx.gbl... Yes, that may be true -- but, given an arbitrary value type, how do I
programmaticall y retrieve the default value for that type? It can't
just be 0 bits, because for a string (for example) that won't work.
I'd have hoped that the boxed equivalent of the value type would have a
constructor, but GetConstructor returns null.
"Mattias Sjögren" <ma************ ********@mvps.o rg> wrote in message
news:e4******** ******@TK2MSFTN GP11.phx.gbl...
> John,
>
> >My question is, how can I retrieve the default value for a given
type? >
> The default is all bits zeroed out, meaning 0 or 0.0 for numeric
> types, false for bools and null for reference types.
>
> For value types, you get the default value when using the default
> constructor.
>
> double d = new double(); // effectively the same as double d = 0.0;
>
>
>
> Mattias
>
> --
> Mattias Sjögren [MVP] mattias @ mvps.org
> http://www.msjogren.net/dotnet/ | http://www.dotnetinterop.com
> Please reply only to the newsgroup.
John,
You actually don't need to get a constructor, you can just call the
version of CreateInstance that takes just the type, it will create an
instance with the bits zeroed out, like this:
// Get the type.
Type pobjIntType = typeof(int);
// Create an instance.
object pobjInt = Activator.Creat eInstance(pobjI ntType);
Of course, you only run this code if you have a value type, otherwise,
you just return null (for reference types). The full code I would use is:
public static object GetDefaultValue (Type type)
{
// If type is null, throw an exception.
if (type == null)
// Throw an exception.
throw new ArgumentNullExc eption("type");
// Check to see if the type is a value type.
// If it is not, return null.
if (!type.IsValueT ype)
// Return null.
return null;
// Create an instance of the value type.
return Activator.Creat eInstance(type) ;
}
--
- Nicholas Paldino [.NET/C# MVP]
- mv*@spam.guard. caspershouse.co m
"John Wood" <sp**@isannoyin g.com> wrote in message
news:%2******** ********@TK2MSF TNGP11.phx.gbl. .. If you run GetConstructor on the value type it returns null though...
doesn't look like any of the value types can provide a default
constructor.
"Nicholas Paldino [.NET/C# MVP]" <mv*@spam.guard .caspershouse.c om> wrote
in message news:OZ******** ******@TK2MSFTN GP12.phx.gbl... John,
Yes, actually it would. With Guid and DateTime, since they are
structures, they have default constructors, which you can get through
reflection and call through reflection (once you have the type).
With a string, it is not a value type, so the default value for a string is null, which the algorithm that I detailed in my previous post will
return to you.
--
- Nicholas Paldino [.NET/C# MVP]
- mv*@spam.guard. caspershouse.co m
"John Wood" <sp**@isannoyin g.com> wrote in message
news:%2******** *******@tk2msft ngp13.phx.gbl.. . Right, but that won't work for Guid, DateTime or String (to name a
few). Isn't there a method somewhere in the framework that just returns the
default value for a given type?
"Nicholas Paldino [.NET/C# MVP]" <mv*@spam.guard .caspershouse.c om>
wrote in message news:eG******** *****@TK2MSFTNG P12.phx.gbl...
> John,
>
> It's actually quite simple. You can use reflection to determine
whether
> or not the type derives from ValueType in some way. If it does,
then you > can just use the default constructor to generate a value with the
bits > zeroed out. If it does not derive from ValueType, then it is a
reference > type, and the default value is null.
>
> Hope this helps.
>
>
> --
> - Nicholas Paldino [.NET/C# MVP]
> - mv*@spam.guard. caspershouse.co m
>
> "John Wood" <sp**@isannoyin g.com> wrote in message
> news:uu******** ******@TK2MSFTN GP09.phx.gbl...
> > Yes, that may be true -- but, given an arbitrary value type, how
do I > > programmaticall y retrieve the default value for that type? It
can't just
> be
> > 0 bits, because for a string (for example) that won't work.
> >
> > I'd have hoped that the boxed equivalent of the value type would
have a > > constructor, but GetConstructor returns null.
> >
> > "Mattias Sjögren" <ma************ ********@mvps.o rg> wrote in
message > > news:e4******** ******@TK2MSFTN GP11.phx.gbl...
> > > John,
> > >
> > > >My question is, how can I retrieve the default value for a
given type?
> > >
> > > The default is all bits zeroed out, meaning 0 or 0.0 for numeric
> > > types, false for bools and null for reference types.
> > >
> > > For value types, you get the default value when using the
default > > > constructor.
> > >
> > > double d = new double(); // effectively the same as double d =
0.0; > > >
> > >
> > >
> > > Mattias
> > >
> > > --
> > > Mattias Sjögren [MVP] mattias @ mvps.org
> > > http://www.msjogren.net/dotnet/ | http://www.dotnetinterop.com
> > > Please reply only to the newsgroup.
> >
> >
>
>
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