Why does string not require a new like other reference types?

[Ray Cassick]

I can't believe why I had not noticed this before and why I never asked it
before...

When defining a string why is it not required to use the new keyword? Like:

Dim a As String = New String

You can do it when using the string type with some arguments in the
constructor but it look like there is no empty constructor on the String
class.

Seem odd to me that all the other reference types seem to require a new to
create the instance portion of the type and String does not.

Thanks for any insight...

[Brian Gideon]

On Jun 9, 5:40*am, "Cor Ligthert [MVP]" <notmyfirstn... @planet.nl>
wrote:

As a string is declared then the type Is String and the object Is Nothing.
As soon as you use a string then it will create that empty string. Even as
that is If string = nothing

:-)

Cor

That is not a true statement. Consider the following trivial example.

Dim a As String
a.ToString()

The example uses "a" before it is assigned and that generates a
NullReferenceEx ception as you would expect from any reference type.
An empty string did not get created and assigned to "a" at any point.

[Cor Ligthert[MVP]]

Brian,

You are right, however I seldom set a string to a string.

This works
Dim a As String
TextBox1.Text = a

Cor

[Brian Gideon]

On Jun 9, 11:10*am, "Cor Ligthert[MVP]" <notmyfirstn... @planet.nl>
wrote:

Brian,

You are right, however I seldom set a string to a string.

I'm not sure what you mean by that.

>
This works
Dim a As String
TextBox1.Text = a

Cor

That works because TextBox.Text checks to see if the value being
assigned is a null reference first and if it is then it creates an
empty string and assigns that instead (I checked using Reflector).
That behavior is specific to TextBox (and perhaps other classes in the
BCL as well) and cannot be generalized to every use of a string.

[Jack Jackson]

On Mon, 9 Jun 2008 09:48:11 -0700 (PDT), Brian Gideon
<br*********@ya hoo.comwrote:

>On Jun 9, 11:10*am, "Cor Ligthert[MVP]" <notmyfirstn... @planet.nl>
wrote:

>Brian,

You are right, however I seldom set a string to a string.

I'm not sure what you mean by that.

>>
This works
Dim a As String
TextBox1.Tex t = a

Cor

That works because TextBox.Text checks to see if the value being
assigned is a null reference first and if it is then it creates an
empty string and assigns that instead (I checked using Reflector).
That behavior is specific to TextBox (and perhaps other classes in the
BCL as well) and cannot be generalized to every use of a string.

Another oddity of strings is that test for equality with an empty
string doesn't trap if the string reference is Nothing, but any other
use of the reference to Nothing does trap.

Dim a As String = Nothing
If a = "" Then ' This doesn't trap, unlike other reference types
If a.Length 0 Then ' This does trap like other reference types

I'm not sure why it works this way. While this behavior can be
convenient (saving a test for Is Nothing), it only helps when
comparing for the empty string.

It's another example of how String is treated as a cross between
reference and value types.

[=?ISO-8859-1?Q?G=F6ran_Andersson?=]

Jack Jackson wrote:

On Sun, 8 Jun 2008 19:39:09 -0400, "Ray Cassick"
<rc******@enter procity.comwrot e:

>So, I guess my question is.. if String is a reference type why do you NOT
NEED to allocate it using the New keyword every time? Almost seems like
strings are an odd case with respect to this.

I would expect that doing this:

Dim a As String

would simply point to nothing (as it does) just like doing this:

Dim a As Integer

results in a 0.

I am just curious as to why the String type seems to be inconsistent in
behavior with respect to other reference types.

Strings are an odd case because they are a reference type that is
created by using a language constant. If strings required New, then
every use of a string constant would require New:

Dim a As String = New String("abc") + New String("def")
MessageBox.Show (New String("Some message"))

That's a lot of typing and a lot of clutter for no purpose. Requiring
New provides no functionality over just using the constant by itself.

Besides, when you use a literal string it's not created when you use it.
It already exists as a constant in the assembly, so if New was requiered
like that, the compiler would actually replace each New call with the
reference to the string constant.

You can verify that using a string literal doesn't create any new object
like this:

Dim x(1) As String
For i As Integer = 0 to 1
x(i) = "asdf"
Next
Console.WriteLi ne(Object.Refer enceEquals(x(0) , x(1)).ToString( ))

It will write out True, as the code in the loop assigns the reference of
the same string literal to both the items in the array.

You can also verify that string literals are reused like this:

Console.WriteLi ne(Object.Refer enceEquals("asd f", "asdf").ToStrin g())

It will write out True, as the same string constant is used for both
string literals.

--
Göran Andersson
_____
http://www.guffa.com

[=?ISO-8859-1?Q?G=F6ran_Andersson?=]

Cor Ligthert [MVP] wrote:

Goran,

By the way, are you using often C#, because this misunderstandin g is
typically from people only using that?

Cor

I am indeed "using often C#", but I have also used a lot of other
languages, inlcuding some flavours of assembly language and machine
code, so I have a pretty good sense for what's really happening to the
code that I write.

Do you use mostly Visual Basic? Misunderstandin gs like that is typical
for people only using that.

;)

--
Göran Andersson
_____
http://www.guffa.com

[=?ISO-8859-1?Q?G=F6ran_Andersson?=]

Jack Jackson wrote:

On Mon, 9 Jun 2008 09:48:11 -0700 (PDT), Brian Gideon
<br*********@ya hoo.comwrote:

>On Jun 9, 11:10 am, "Cor Ligthert[MVP]" <notmyfirstn... @planet.nl>
wrote:

>>Brian,

You are right, however I seldom set a string to a string.

I'm not sure what you mean by that.

>>This works
Dim a As String
TextBox1.Te xt = a

Cor

That works because TextBox.Text checks to see if the value being
assigned is a null reference first and if it is then it creates an
empty string and assigns that instead (I checked using Reflector).
That behavior is specific to TextBox (and perhaps other classes in the
BCL as well) and cannot be generalized to every use of a string.

Another oddity of strings is that test for equality with an empty
string doesn't trap if the string reference is Nothing, but any other
use of the reference to Nothing does trap.

Dim a As String = Nothing
If a = "" Then ' This doesn't trap, unlike other reference types
If a.Length 0 Then ' This does trap like other reference types

I'm not sure why it works this way. While this behavior can be
convenient (saving a test for Is Nothing), it only helps when
comparing for the empty string.

It's another example of how String is treated as a cross between
reference and value types.

That's simply because the string class overrides the = operator, and the
code that compares strings handles null values.

--
Göran Andersson
_____
http://www.guffa.com

[=?ISO-8859-1?Q?G=F6ran_Andersson?=]

Cor Ligthert [MVP] wrote:

"Göran >>

>>Dim a as integer "results" in a 0
Dim b as string "results" in a ""

Not at all. Declaring a string without assigning a value either results in
a null reference (for member variables) or an undefined variable (for
local method variables).

I especially placed the double quotes around the words results.

That doesn't matter. Neither the result or the "result" is a string.

As a string is declared then the type Is String and the object Is Nothing.
As soon as you use a string then it will create that empty string. Even as
that is If string = nothing

No, it doesn't work that way. A string reference does not automatically
change into something else just because you use it. If it would, you
could never get a null reference exception when using a string.

--
Göran Andersson
_____
http://www.guffa.com

[Brian Gideon]

On Jun 9, 3:09*pm, Jack Jackson <jjack...@cinno vations.netwrot e:

Another oddity of strings is that test for equality with an empty
string doesn't trap if the string reference is Nothing, but any other
use of the reference to Nothing does trap.

Dim a As String = Nothing
If a = "" Then *' This doesn't trap, unlike other reference types
If a.Length 0 Then ' This does trap like other reference types

I'm not sure why it works this way. *While this behavior can be
convenient (saving a test for Is Nothing), it only helps when
comparing for the empty string.

It's another example of how String is treated as a cross between
reference and value types

The first example demonstrates how the VB compiler injects a call to
Microsoft.Visua lBasic.Compiler Services.Operat ors.CompareStri ng. That
method treats null strings the same as empty strings. It is
interesting to note that the C# equivalent of that example results in
a completely different outcome.

[Herfried K. Wagner [MVP]]

"Ray Cassick" <rc******@enter procity.comschr ieb:

So, I guess my question is.. if String is a reference type why do you NOT
NEED to allocate it using the New keyword every time? Almost seems like
strings are an odd case with respect to this.

I would expect that doing this:

Dim a As String

would simply point to nothing (as it does) just like doing this:

Dim a As Integer

results in a 0.

Well, 'Integer' is a value type, 'String' is a reference type. Thus the
value of 'a' in the sample above is a 'Nothing' reference.

--
M S Herfried K. Wagner
M V P <URL:http://dotnet.mvps.org/>
V B <URL:http://dotnet.mvps.org/dotnet/faqs/>