Error with 'System.NullReferenceException' [modified]

thesad

I had written : for(int i=0; i <10; i++){ em[i]->pct = (gcnew System::Windows::Forms::PictureBox()); em[i]->pct->Location = System::Drawing::Point(250, 250); em[i]->pct->Visible = true; em[i]->pct->Size = System::Drawing::Size(50, 50); } And I got this error :

An unhandled exception of type 'System.NullReferenceException' occurred in Little tanks.exe

Additional information: Object reference not set to an instance of an object.

at this line : em[i]->pct = (gcnew System::Windows::Forms::PictureBox()); what did I do wrong?

Newbie00

How have you defined the em array?

Christian Graus

em[i] is null, you probably created an array and didn't create the objects in it

Christian Graus - Microsoft MVP - C++ Metal Musings - Rex and my new metal blog "I am working on a project that will convert a FORTRAN code to corresponding C++ code.I am not aware of FORTRAN syntax" ( spotted in the C++/CLI forum )

thesad

the em array defined so : static array^ em; and tank is structure : ref struct tank { int step_x, step_y; System::Windows::Forms::PictureBox^ pct; }; What do you mean saying to create objects for array?:)

prasad_som

thesad wrote:

static array^ em;

You have not initialized array here. Array can be initialized as given in MSDN example.

array<MyClass ^> ^ MyArray = gcnew array<MyClass ^>(100);

Prasad Notifier using ATL | Operator new[],delete[][^]

thesad

I wrote taht you had sad, but still at same line is that error

prasad_som

Would interesting to see, how you have done that. There are two possiblitites, 1. You have not allocated memory for it. 2. You are accessing invalid index.

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thesad

Here is all code : #pragma once namespace Littletanks { ref struct tank { int step_x, step_y; System::Windows::Forms::PictureBox^ pct; }; using namespace System; using namespace System::ComponentModel; using namespace System::Collections; using namespace System::Windows::Forms; using namespace System::Data; using namespace System::Drawing; public ref class wnd_main : public System::Windows::Forms::Form { public: wnd_main(void) { InitializeComponent(); } protected: ~wnd_main() { if (components) delete components; } private: System::ComponentModel::IContainer^ components; private: int max_x, max_y, min_x, min_y; private: tank usr, b_usr; private: static array^ em; private: static array^ b_em; #pragma region Windows Form Designer generated code void InitializeComponent(void) { this->components = (gcnew System::ComponentModel::Container()); this->usr.pct = (gcnew System::Windows::Forms::PictureBox()); this->b_usr.pct = (gcnew System::Windows::Forms::PictureBox()); this->SuspendLayout(); // // pct_usr // this->usr.pct->Location = System::Drawing::Point(235, 391); this->usr.pct->Size = System::Drawing::Size(50, 50); this->usr.pct->TabStop = false; // // pct_em // array^ em = gcnew array(10); array^ b_em = gcnew array(10); for(int i=0; i <10; i++){ this->em[i]->pct = gcnew System::Windows::Forms::PictureBox(); this->em[i]->pct->Location = System::Drawing::Point(250, 250); this->em[i]->pct->Visible = true; this->em[i]->pct->Size = System::Drawing::Size(50, 50); this->b_em[i]->pct = gcnew System::Windows::Forms::PictureBox(); this->b_em[i]->pct->Load("b_em[i]->jpg"); this->b_em[i]->pct->Location = System::Drawing::Point(13, 64); this->b_em[i]->pct->Size = System::Drawing::Size(5, 5); this->b_em[i]->pct->Visible = false; this->Controls->Add(this->b_em[i]->pct); this->Controls->Add(this->em[i]->pct); } this->b_usr.pct->Load("b_usr.jpg"); this->b_usr.pct->Location = System::Drawing::Point(97, 142); this->b_usr.pct->Size = System::Drawing::Size(5, 5); this->b_usr.pct->Visible = false; // wnd_main // this->AutoScaleDimensions = System::Drawing::SizeF(6, 13); this->AutoScaleMode = System::Windows::Forms::AutoScaleMode::Font; this->BackColor = System::Drawing::SystemColors::Window; this->ClientSize = System::Drawin

prasad_som

Here, you had made couple of mistakes,

thesad wrote:

array^ em = gcnew array(10); array^ b_em = gcnew array(10);

Here, again you are defining local variables. It should be like this,

this->em = gcnew array<tank^>(10);
this->b_em = gcnew array<tank^>(10);

Again, simply defining array doesn't initializes its members, you need to allocate tank variable for each index. So, your code can be modified like this,

thesad wrote:

for(int i=0; i <10; i++){ this->em[i]->pct = gcnew System::Windows::Forms::PictureBox(); this->em[i]->pct->Location = System::Drawing::Point(250, 250);

Modify this to,

for(int i=0; i <10; i++){
em[i] = gcnew tank;//create tank object for each index
this->em[i]->pct = gcnew System::Windows::Forms::PictureBox();
this->em[i]->pct->Location = System::Drawing::Point(250, 250);
//your code follows

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thesad

thank you for your help :)

Christian Graus

= new gcnew tank(); to create the actual objects in the array. where x is the index into the array ( you'd probably do this in a loop )

Christian Graus - Microsoft MVP - C++ Metal Musings - Rex and my new metal blog "I am working on a project that will convert a FORTRAN code to corresponding C++ code.I am not aware of FORTRAN syntax" ( spotted in the C++/CLI forum )

prasad_som

Christian Graus wrote:

= new gcnew tank();

Typo. :)

Prasad Notifier using ATL | Operator new[],delete[][^]