by reference and by value

[Andrew Bullock]

Hi,

Why do i get the following error? In other projects I have done (what i
thought was) the same thing, without compiler complaint :-/
1: List<MyClass> myClasses = new List<MyClass>();
2: myClasses.Add(new MyClass(20));
3: myClasses[0].value = 0;
Line 3: Cannot modify the return value of
System.Collections.Generic.List<OPS.MyClass>.this[int]' because it is
not a variable

And why does this print 20, and not 0? Again I thought i had done this
in other applications without this happening
List<MyClass> myClasses= new List<MyClass>();
myClasses.Add(new MyClass(20));
MyClass m = myClasses[0];
m.value = 0;

// prints "20"
Console.WriteLine(myClasses[0].value);
Thanks
Andrew

[Jon Skeet [C# MVP]]

Andrew Bullock <an*********************@ANDntlworldTHIS.com> wrote:

Why do i get the following error? In other projects I have done (what i
thought was) the same thing, without compiler complaint :-/

1: List<MyClass> myClasses = new List<MyClass>();
2: myClasses.Add(new MyClass(20));
3: myClasses[0].value = 0;

Line 3: Cannot modify the return value of
System.Collections.Generic.List<OPS.MyClass>.this[int]' because it is
not a variable
My guess is that despite its name, "MyClass" is a struct. Fields in a
struct which are referenced via a non-variable expression (a method
call, property or indexer) aren't classed as variables.
And why does this print 20, and not 0? Again I thought i had done this
in other applications without this happening

List<MyClass> myClasses= new List<MyClass>();
myClasses.Add(new MyClass(20));
MyClass m = myClasses[0];
m.value = 0;

// prints "20"
Console.WriteLine(myClasses[0].value);

Again, that would be consistent with MyClass being a value type rather
than a reference type.

--
Jon Skeet - <sk***@pobox.com>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too

[Andrew Bullock]

Jon Skeet [C# MVP] wrote:

Andrew Bullock <an*********************@ANDntlworldTHIS.com> wrote:

Why do i get the following error? In other projects I have done (what i
thought was) the same thing, without compiler complaint :-/

1: List<MyClass> myClasses = new List<MyClass>();
2: myClasses.Add(new MyClass(20));
3: myClasses[0].value = 0;

Line 3: Cannot modify the return value of
System.Collections.Generic.List<OPS.MyClass>.this[int]' because it is
not a variable

My guess is that despite its name, "MyClass" is a struct. Fields in a
struct which are referenced via a non-variable expression (a method
call, property or indexer) aren't classed as variables.

And why does this print 20, and not 0? Again I thought i had done this
in other applications without this happening

List<MyClass> myClasses= new List<MyClass>();
myClasses.Add(new MyClass(20));
MyClass m = myClasses[0];
m.value = 0;

// prints "20"
Console.WriteLine(myClasses[0].value);

Again, that would be consistent with MyClass being a value type rather
than a reference type.

Excellent,

Thankyou very much! You are a true legend!

One further question if you don't mind, how can I duplicate my list of
MyClasses?
Thanks,

Andrew

[Jon Skeet [C# MVP]]

Andrew Bullock <an*********************@ANDntlworldTHIS.com> wrote:

Excellent,

Thankyou very much! You are a true legend!

One further question if you don't mind, how can I duplicate my list of
MyClasses?

You want a separate, independent list?

List<MyClass> copy = new List<MyClass>(original);

--
Jon Skeet - <sk***@pobox.com>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too