>Hi all,
>I have an array defined in a class. The type of the array is the class type.
>For example:
>
>class MyClass
>    {
>        private int a;
>        private int b;
>        private MyClass[] Array;
>        public MyClass()
>        {
>            a = 5;
>            b = 2;
>            Array = new MyClass[3];
>        }
>
>        public void SetArrayFields()
>        {
>            Array[0].a = 1;
>            Array[0].b = 8;
>        }
>    }
>
>What happens is that whenever I enter SetArrayFields() and try to set values
>to the array there is an error "System.NullReferenceException"
>
>Please help me to solve this problem
>Many thanks

"Jack Jackson" wrote:

> On Thu, 20 Sep 2007 08:30:09 -0700, gol
> <g**@discussions.microsoft.com> wrote:
>
> >Hi all,
> >I have an array defined in a class. The type of the array is the class type.
> >For example:
> >
> >class MyClass
> >    {
> >        private int a;
> >        private int b;
> >        private MyClass[] Array;
> >        public MyClass()
> >        {
> >            a = 5;
> >            b = 2;
> >            Array = new MyClass[3];
> >        }
> >
> >        public void SetArrayFields()
> >        {
> >            Array[0].a = 1;
> >            Array[0].b = 8;
> >        }
> >    }
> >
> >What happens is that whenever I enter SetArrayFields() and try to set values
> >to the array there is an error "System.NullReferenceException"
> >
> >Please help me to solve this problem
> >Many thanks
>
> If you look at this with the debugger you will see that all elements
> of the array are null.
>
> The line:
> Array = new MyClass[3];
> allocates an array with all members set to null.  You need to create
> new instances of your class and store them in the array.
>
> Besides that, your whole structure seems weird to me.  Each instance
> of your class contains an array of three instances of your class. Does
> each of those instances also include 3 more instances?  If so, your
> class will occupy an infinite amount of memory.
>

"gol" <g**@discussions.microsoft.com> wrote in message
news:4B46F1CE-D6F4-4734-B377-321FB044C149@microsoft.com...
> Thanks, Jack
> What do you suggest, then?
> Should I make another class with an array of type MyClass and a public
> method of inserting another element to the array.
> And in the class MyClass - in the constructor, also to call that method
> with
> the element that has just been created.
> How does it sound?
>
> Or maybe it's better to use a list?
> Thanks
>
> "Jack Jackson" wrote:
>
>> On Thu, 20 Sep 2007 08:30:09 -0700, gol
>> <g**@discussions.microsoft.com> wrote:
>>
>> >Hi all,
>> >I have an array defined in a class. The type of the array is the class
>> >type.
>> >For example:
>> >
>> >class MyClass
>> >    {
>> >        private int a;
>> >        private int b;
>> >        private MyClass[] Array;
>> >        public MyClass()
>> >        {
>> >            a = 5;
>> >            b = 2;
>> >            Array = new MyClass[3];
>> >        }
>> >
>> >        public void SetArrayFields()
>> >        {
>> >            Array[0].a = 1;
>> >            Array[0].b = 8;
>> >        }
>> >    }
>> >
>> >What happens is that whenever I enter SetArrayFields() and try to set
>> >values
>> >to the array there is an error "System.NullReferenceException"
>> >
>> >Please help me to solve this problem
>> >Many thanks
>>
>> If you look at this with the debugger you will see that all elements
>> of the array are null.
>>
>> The line:
>> Array = new MyClass[3];
>> allocates an array with all members set to null.  You need to create
>> new instances of your class and store them in the array.
>>
>> Besides that, your whole structure seems weird to me.  Each instance
>> of your class contains an array of three instances of your class. Does
>> each of those instances also include 3 more instances?  If so, your
>> class will occupy an infinite amount of memory.
>>

"Kevin Spencer" wrote:

> When you create an Array, you are only allocating space for its members. You
> have to set each Array member to an instance of the type it contains before
> attempting to work with the members of that instance in the Array.
>
> --
> HTH,
>
> Kevin Spencer
> Microsoft MVP
>
> DSI PrintManager, Miradyne Component Libraries:
> http://www.miradyne.net
>
> "gol" <g**@discussions.microsoft.com> wrote in message
> news:4B46F1CE-D6F4-4734-B377-321FB044C149@microsoft.com...
> > Thanks, Jack
> > What do you suggest, then?
> > Should I make another class with an array of type MyClass and a public
> > method of inserting another element to the array.
> > And in the class MyClass - in the constructor, also to call that method
> > with
> > the element that has just been created.
> > How does it sound?
> >
> > Or maybe it's better to use a list?
> > Thanks
> >
> > "Jack Jackson" wrote:
> >
> >> On Thu, 20 Sep 2007 08:30:09 -0700, gol
> >> <g**@discussions.microsoft.com> wrote:
> >>
> >> >Hi all,
> >> >I have an array defined in a class. The type of the array is the class
> >> >type.
> >> >For example:
> >> >
> >> >class MyClass
> >> >    {
> >> >        private int a;
> >> >        private int b;
> >> >        private MyClass[] Array;
> >> >        public MyClass()
> >> >        {
> >> >            a = 5;
> >> >            b = 2;
> >> >            Array = new MyClass[3];
> >> >        }
> >> >
> >> >        public void SetArrayFields()
> >> >        {
> >> >            Array[0].a = 1;
> >> >            Array[0].b = 8;
> >> >        }
> >> >    }
> >> >
> >> >What happens is that whenever I enter SetArrayFields() and try to set
> >> >values
> >> >to the array there is an error "System.NullReferenceException"
> >> >
> >> >Please help me to solve this problem
> >> >Many thanks
> >>
> >> If you look at this with the debugger you will see that all elements
> >> of the array are null.
> >>
> >> The line:
> >> Array = new MyClass[3];
> >> allocates an array with all members set to null.  You need to create
> >> new instances of your class and store them in the array.
> >>
> >> Besides that, your whole structure seems weird to me.  Each instance
> >> of your class contains an array of three instances of your class. Does
> >> each of those instances also include 3 more instances?  If so, your
> >> class will occupy an infinite amount of memory.
> >>
>
>
>

>Thanks, Jack
>What do you suggest, then?
>Should I make another class with an array of type MyClass and a public
>method of inserting another element to the array.
>And in the class MyClass - in the constructor, also to call that method with
>the element that has just been created.
>How does it sound?
>
>Or maybe it's better to use a list?
>Thanks
>
>"Jack Jackson" wrote:
>
>> On Thu, 20 Sep 2007 08:30:09 -0700, gol
>> <g**@discussions.microsoft.com> wrote:
>>
>> >Hi all,
>> >I have an array defined in a class. The type of the array is the class type.
>> >For example:
>> >
>> >class MyClass
>> >    {
>> >        private int a;
>> >        private int b;
>> >        private MyClass[] Array;
>> >        public MyClass()
>> >        {
>> >            a = 5;
>> >            b = 2;
>> >            Array = new MyClass[3];
>> >        }
>> >
>> >        public void SetArrayFields()
>> >        {
>> >            Array[0].a = 1;
>> >            Array[0].b = 8;
>> >        }
>> >    }
>> >
>> >What happens is that whenever I enter SetArrayFields() and try to set values
>> >to the array there is an error "System.NullReferenceException"
>> >
>> >Please help me to solve this problem
>> >Many thanks
>>
>> If you look at this with the debugger you will see that all elements
>> of the array are null.
>>
>> The line:
>> Array = new MyClass[3];
>> allocates an array with all members set to null.  You need to create
>> new instances of your class and store them in the array.
>>
>> Besides that, your whole structure seems weird to me.  Each instance
>> of your class contains an array of three instances of your class. Does
>> each of those instances also include 3 more instances?  If so, your
>> class will occupy an infinite amount of memory.
>>

>Hi,
>OK, after working on this for a while I've come to this:
>I have a simple class, something like this:
>class MyClass
>    {
>        private int a;
>        private int b;
>        public MyClass()
>        {
>            a = 5;
>            b = 2;
>        }
>
>        public MyClass(int first, int second)
>        {
>            a = first;
>            b = second;
>        }
>
>        public int GetOneField(int Fieldindex)
>        {
>            switch (Fieldindex)
>            {
>                case 1:
>                    return a;
>                    break;
>                case 2:
>                    return b;
>                    break;
>                default:
>                    return 0;
>                    break;
>            }
>        }
>    }
>
>And another class which contains an array of type MyClass, like this:
>
>class MyClassRecords
>    {
>        private MyClass[] RecordsArray;
>        private MyClass OneRecord;
>        private static int index = 0;
>        public MyClassRecords()
>        {
>            OneRecord = new MyClass();
>            RecordsArray = new MyClass[3];
>        }
>
>        public MyClass[] GetRecordsArray()
>        {
>            return RecordsArray;
>        }
>
>        public void Insert()
>        {
>            RecordsArray[index] = OneRecord;
>            index++;
>        }
>
>        public void RecordSet(int a, int b)
>        {
>            OneRecord = new MyClass(a, b);
>        }
>    }
>
>What I want is to insert data to the records array at the beginning, let's
>say, in Form1, and after that use that data in some of the other forms of the
>application.
>
>In Form1 I have an instance of MyClassRecords like this:
>private MyClassRecords Records = new MyClassRecords();
>
>Then I insert data with parameters a,b which I received during the
>application like this:
>Records.RecordSet(a,b);
>Records.Insert();
>
>In other forms I want to use these data this way:
>MyClassRecords Records = new MyClassRecords();
>And then : if(Records.GetRecordsArray()[0].GetOneField(1) == 1)
>{
>//do something
>}
>
>I don't want to change the data in the other forms, only read and use the
>data that has been inserted in Form1.
>
>Now, of course this doesn't work, because here I have a new instance, but I
>want to read the data that I've inserted in Form1.
>In other words, the instance of MyClassRecords which I had in Form1 is no
>longer "alive".
>
>The question is, then, how to transfer information between forms?
>
>In my case, I thought of making the class MyClassRecords as a singleton
>class (a technique which I've learnt only few days ago) and this way its
>array will always remain the same.
>I don't know whether it's good to do it this way here.
>
>Someone also told me I can pass the instance of MyClassRecords as a
>parameter to the constructor of another form. Can I do it? And if so, is it
>worth it? (considering I have to carry this instance with me through all the
>forms of the application).
>
>Sorry for the very long explanation,
>Thanks a lot for your answer

"Jack Jackson" wrote:

> On Mon, 24 Sep 2007 01:30:00 -0700, gol
> <g**@discussions.microsoft.com> wrote:
>
> >Hi,
> >OK, after working on this for a while I've come to this:
> >I have a simple class, something like this:
> >class MyClass
> >    {
> >        private int a;
> >        private int b;
> >        public MyClass()
> >        {
> >            a = 5;
> >            b = 2;
> >        }
> >
> >        public MyClass(int first, int second)
> >        {
> >            a = first;
> >            b = second;
> >        }
> >
> >        public int GetOneField(int Fieldindex)
> >        {
> >            switch (Fieldindex)
> >            {
> >                case 1:
> >                    return a;
> >                    break;
> >                case 2:
> >                    return b;
> >                    break;
> >                default:
> >                    return 0;
> >                    break;
> >            }
> >        }
> >    }
> >
> >And another class which contains an array of type MyClass, like this:
> >
> >class MyClassRecords
> >    {
> >        private MyClass[] RecordsArray;
> >        private MyClass OneRecord;
> >        private static int index = 0;
> >        public MyClassRecords()
> >        {
> >            OneRecord = new MyClass();
> >            RecordsArray = new MyClass[3];
> >        }
> >
> >        public MyClass[] GetRecordsArray()
> >        {
> >            return RecordsArray;
> >        }
> >
> >        public void Insert()
> >        {
> >            RecordsArray[index] = OneRecord;
> >            index++;
> >        }
> >
> >        public void RecordSet(int a, int b)
> >        {
> >            OneRecord = new MyClass(a, b);
> >        }
> >    }
> >
> >What I want is to insert data to the records array at the beginning, let's
> >say, in Form1, and after that use that data in some of the other forms of the
> >application.
> >
> >In Form1 I have an instance of MyClassRecords like this:
> >private MyClassRecords Records = new MyClassRecords();
> >
> >Then I insert data with parameters a,b which I received during the
> >application like this:
> >Records.RecordSet(a,b);
> >Records.Insert();
> >
> >In other forms I want to use these data this way:
> >MyClassRecords Records = new MyClassRecords();
> >And then : if(Records.GetRecordsArray()[0].GetOneField(1) == 1)
> >{
> >//do something
> >}
> >
> >I don't want to change the data in the other forms, only read and use the
> >data that has been inserted in Form1.
> >
> >Now, of course this doesn't work, because here I have a new instance, but I
> >want to read the data that I've inserted in Form1.
> >In other words, the instance of MyClassRecords which I had in Form1 is no
> >longer "alive".
> >
> >The question is, then, how to transfer information between forms?
> >
> >In my case, I thought of making the class MyClassRecords as a singleton
> >class (a technique which I've learnt only few days ago) and this way its
> >array will always remain the same.
> >I don't know whether it's good to do it this way here.
> >
> >Someone also told me I can pass the instance of MyClassRecords as a
> >parameter to the constructor of another form. Can I do it? And if so, is it
> >worth it? (considering I have to carry this instance with me through all the
> >forms of the application).
> >
> >Sorry for the very long explanation,
> >Thanks a lot for your answer
>
> There are a couple of problems with your code.
>
> First, you create one instance of MyClass which you put in
> MyClassRecords.OneClass.  Then in Insert() you assign that one
> instance to the next entry in the array.  That means that each entry
> in the array points to the same instance of the class.  You need to
> create a new instance each time.
>
> Why is SetRecord separate from Insert?  Don't you always want to call
> both?  If so, get rid of Insert and change SetRecord to:
>
>         public void RecordSet(int a, int b)
>         {
>             RecordsArray[index] = New MyClass(a, b);
>             index++;
>         }
>
> Do you really want the class hard coded to have at most three
> instances of the class?  You at least need a check to make sure you
> don't call Insert more than 3 times.  I would probably use and
> ArrayList or the generic List instead of the array.
>
> I would pass the reference to the instance of MyClassRecords to the
> other forms.  You can to it either as a constructor parameter or by
> calling a method in the other forms.
>

>Thank you very much for your answer, Jack
>I took your advice of uniting Insert and RecordSet, and also removed the
>instance of MyClass and creating a new instance each time.
>
>About the hard coded size of the array – I know about it, it is only
>temporary, and I know that I have to do something about it.
>
>I understand now that I can pass the class as a parameter to the
>constructor, or to a method.
>Can you, please, tell me why is it not so good to leave it the way I did
>(make the class as a singleton class)?
>Thank you very much
>
>
>"Jack Jackson" wrote:
>
>> On Mon, 24 Sep 2007 01:30:00 -0700, gol
>> <g**@discussions.microsoft.com> wrote:
>>
>> >Hi,
>> >OK, after working on this for a while I've come to this:
>> >I have a simple class, something like this:
>> >class MyClass
>> >    {
>> >        private int a;
>> >        private int b;
>> >        public MyClass()
>> >        {
>> >            a = 5;
>> >            b = 2;
>> >        }
>> >
>> >        public MyClass(int first, int second)
>> >        {
>> >            a = first;
>> >            b = second;
>> >        }
>> >
>> >        public int GetOneField(int Fieldindex)
>> >        {
>> >            switch (Fieldindex)
>> >            {
>> >                case 1:
>> >                    return a;
>> >                    break;
>> >                case 2:
>> >                    return b;
>> >                    break;
>> >                default:
>> >                    return 0;
>> >                    break;
>> >            }
>> >        }
>> >    }
>> >
>> >And another class which contains an array of type MyClass, like this:
>> >
>> >class MyClassRecords
>> >    {
>> >        private MyClass[] RecordsArray;
>> >        private MyClass OneRecord;
>> >        private static int index = 0;
>> >        public MyClassRecords()
>> >        {
>> >            OneRecord = new MyClass();
>> >            RecordsArray = new MyClass[3];
>> >        }
>> >
>> >        public MyClass[] GetRecordsArray()
>> >        {
>> >            return RecordsArray;
>> >        }
>> >
>> >        public void Insert()
>> >        {
>> >            RecordsArray[index] = OneRecord;
>> >            index++;
>> >        }
>> >
>> >        public void RecordSet(int a, int b)
>> >        {
>> >            OneRecord = new MyClass(a, b);
>> >        }
>> >    }
>> >
>> >What I want is to insert data to the records array at the beginning, let's
>> >say, in Form1, and after that use that data in some of the other forms of the
>> >application.
>> >
>> >In Form1 I have an instance of MyClassRecords like this:
>> >private MyClassRecords Records = new MyClassRecords();
>> >
>> >Then I insert data with parameters a,b which I received during the
>> >application like this:
>> >Records.RecordSet(a,b);
>> >Records.Insert();
>> >
>> >In other forms I want to use these data this way:
>> >MyClassRecords Records = new MyClassRecords();
>> >And then : if(Records.GetRecordsArray()[0].GetOneField(1) == 1)
>> >{
>> >//do something
>> >}
>> >
>> >I don't want to change the data in the other forms, only read and use the
>> >data that has been inserted in Form1.
>> >
>> >Now, of course this doesn't work, because here I have a new instance, but I
>> >want to read the data that I've inserted in Form1.
>> >In other words, the instance of MyClassRecords which I had in Form1 is no
>> >longer "alive".
>> >
>> >The question is, then, how to transfer information between forms?
>> >
>> >In my case, I thought of making the class MyClassRecords as a singleton
>> >class (a technique which I've learnt only few days ago) and this way its
>> >array will always remain the same.
>> >I don't know whether it's good to do it this way here.
>> >
>> >Someone also told me I can pass the instance of MyClassRecords as a
>> >parameter to the constructor of another form. Can I do it? And if so, is it
>> >worth it? (considering I have to carry this instance with me through all the
>> >forms of the application).
>> >
>> >Sorry for the very long explanation,
>> >Thanks a lot for your answer
>>
>> There are a couple of problems with your code.
>>
>> First, you create one instance of MyClass which you put in
>> MyClassRecords.OneClass.  Then in Insert() you assign that one
>> instance to the next entry in the array.  That means that each entry
>> in the array points to the same instance of the class.  You need to
>> create a new instance each time.
>>
>> Why is SetRecord separate from Insert?  Don't you always want to call
>> both?  If so, get rid of Insert and change SetRecord to:
>>
>>         public void RecordSet(int a, int b)
>>         {
>>             RecordsArray[index] = New MyClass(a, b);
>>             index++;
>>         }
>>
>> Do you really want the class hard coded to have at most three
>> instances of the class?  You at least need a check to make sure you
>> don't call Insert more than 3 times.  I would probably use and
>> ArrayList or the generic List instead of the array.
>>
>> I would pass the reference to the instance of MyClassRecords to the
>> other forms.  You can to it either as a constructor parameter or by
>> calling a method in the other forms.
>>

"Jack Jackson" wrote:

> The singleton is OK too.  A slight advantage of passing the reference
> is that if you ever need another instance of the class you could
> easily do it.  Perhaps some day not all forms would share the same
> instance.
>
> On Mon, 24 Sep 2007 10:56:01 -0700, gol
> <g**@discussions.microsoft.com> wrote:
>
> >Thank you very much for your answer, Jack
> >I took your advice of uniting Insert and RecordSet, and also removed the
> >instance of MyClass and creating a new instance each time.
> >
> >About the hard coded size of the array – I know about it, it is only
> >temporary, and I know that I have to do something about it.
> >
> >I understand now that I can pass the class as a parameter to the
> >constructor, or to a method.
> >Can you, please, tell me why is it not so good to leave it the way I did
> >(make the class as a singleton class)?
> >Thank you very much
> >
> >
> >"Jack Jackson" wrote:
> >
> >> On Mon, 24 Sep 2007 01:30:00 -0700, gol
> >> <g**@discussions.microsoft.com> wrote:
> >>
> >> >Hi,
> >> >OK, after working on this for a while I've come to this:
> >> >I have a simple class, something like this:
> >> >class MyClass
> >> >    {
> >> >        private int a;
> >> >        private int b;
> >> >        public MyClass()
> >> >        {
> >> >            a = 5;
> >> >            b = 2;
> >> >        }
> >> >
> >> >        public MyClass(int first, int second)
> >> >        {
> >> >            a = first;
> >> >            b = second;
> >> >        }
> >> >
> >> >        public int GetOneField(int Fieldindex)
> >> >        {
> >> >            switch (Fieldindex)
> >> >            {
> >> >                case 1:
> >> >                    return a;
> >> >                    break;
> >> >                case 2:
> >> >                    return b;
> >> >                    break;
> >> >                default:
> >> >                    return 0;
> >> >                    break;
> >> >            }
> >> >        }
> >> >    }
> >> >
> >> >And another class which contains an array of type MyClass, like this:
> >> >
> >> >class MyClassRecords
> >> >    {
> >> >        private MyClass[] RecordsArray;
> >> >        private MyClass OneRecord;
> >> >        private static int index = 0;
> >> >        public MyClassRecords()
> >> >        {
> >> >            OneRecord = new MyClass();
> >> >            RecordsArray = new MyClass[3];
> >> >        }
> >> >
> >> >        public MyClass[] GetRecordsArray()
> >> >        {
> >> >            return RecordsArray;
> >> >        }
> >> >
> >> >        public void Insert()
> >> >        {
> >> >            RecordsArray[index] = OneRecord;
> >> >            index++;
> >> >        }
> >> >
> >> >        public void RecordSet(int a, int b)
> >> >        {
> >> >            OneRecord = new MyClass(a, b);
> >> >        }
> >> >    }
> >> >
> >> >What I want is to insert data to the records array at the beginning, let's
> >> >say, in Form1, and after that use that data in some of the other forms of the
> >> >application.
> >> >
> >> >In Form1 I have an instance of MyClassRecords like this:
> >> >private MyClassRecords Records = new MyClassRecords();
> >> >
> >> >Then I insert data with parameters a,b which I received during the
> >> >application like this:
> >> >Records.RecordSet(a,b);
> >> >Records.Insert();
> >> >
> >> >In other forms I want to use these data this way:
> >> >MyClassRecords Records = new MyClassRecords();
> >> >And then : if(Records.GetRecordsArray()[0].GetOneField(1) == 1)
> >> >{
> >> >//do something
> >> >}
> >> >
> >> >I don't want to change the data in the other forms, only read and use the
> >> >data that has been inserted in Form1.
> >> >
> >> >Now, of course this doesn't work, because here I have a new instance, but I
> >> >want to read the data that I've inserted in Form1.
> >> >In other words, the instance of MyClassRecords which I had in Form1 is no
> >> >longer "alive".
> >> >
> >> >The question is, then, how to transfer information between forms?
> >> >
> >> >In my case, I thought of making the class MyClassRecords as a singleton
> >> >class (a technique which I've learnt only few days ago) and this way its
> >> >array will always remain the same.
> >> >I don't know whether it's good to do it this way here.
> >> >
> >> >Someone also told me I can pass the instance of MyClassRecords as a
> >> >parameter to the constructor of another form. Can I do it? And if so, is it
> >> >worth it? (considering I have to carry this instance with me through all the
> >> >forms of the application).
> >> >
> >> >Sorry for the very long explanation,
> >> >Thanks a lot for your answer
> >>
> >> There are a couple of problems with your code.
> >>
> >> First, you create one instance of MyClass which you put in
> >> MyClassRecords.OneClass.  Then in Insert() you assign that one
> >> instance to the next entry in the array.  That means that each entry
> >> in the array points to the same instance of the class.  You need to
> >> create a new instance each time.
> >>
> >> Why is SetRecord separate from Insert?  Don't you always want to call
> >> both?  If so, get rid of Insert and change SetRecord to:
> >>
> >>         public void RecordSet(int a, int b)
> >>         {
> >>             RecordsArray[index] = New MyClass(a, b);
> >>             index++;
> >>         }
> >>
> >> Do you really want the class hard coded to have at most three
> >> instances of the class?  You at least need a check to make sure you
> >> don't call Insert more than 3 times.  I would probably use and
> >> ArrayList or the generic List instead of the array.
> >>
> >> I would pass the reference to the instance of MyClassRecords to the
> >> other forms.  You can to it either as a constructor parameter or by
> >> calling a method in the other forms.
> >>
>