drawing on a form from within a seperate function

drawing on a form from within a seperate function

• gowtham173656

  You know what In a last attempt, could you send me a zip of the project to jvdbeek_13(removeme)@hotmail.com 

  Or, if creating a new project with the steps I provided also works for you, let me know.

  
  
  

• pooyaJoonam

  Here's the exact message I get :
  An unhandled exception of type 'System.NullReferenceException' occurred in system.windows.forms.dll
  
  Additional information: Object reference not set to an instance of an object.
  
  This exception definitely occurs at :
  gr2 = cc-> CreateGraphics();
  
  My function prototype looks like :
  
  BOOL X(System::Windows::Forms::Control* cc, System::Drawing::SolidBrush* sb, System::Drawing::Graphics* gr2, .......
  
  Regards,
  ak
  
  

• FSOL

  Now, I don't get any unhandled exceptions but no red rectangles are drawn either.
  Here's everything I have that deals with the control :
  
  inside the Form definition :
  
      public: static System::Windows::Forms::Control * dd;
      public: static System::Drawing::Graphics * gr2;
      public: static System::Drawing::SolidBrush * sb;
  
  inside InitializeComponent :
  
              this->dd = new System::Windows::Forms::Control();
              //this->gr2 = new System::Drawing::Graphics
              this->sb = new System::Drawing::SolidBrush(Color::Red);
  
              this->Controls->Add(this->dd);
  
  the function call :
   X(dd, sb, gr2, textBox1,...
  
  another Graphic that works fine (the red rectangles are drawn onto this bitmap :
  
  private: System::Void button3_Click(System::Object *  sender, System::EventArgs *  e)
           {
               bitmap = new Bitmap(S"test.bmp");
               this->Invalidate();
               this->Update();
             
  
           }
  
  private: System::Void Form2_Paint(System::Object *  sender, System::Windows::Forms::PaintEventArgs *  e)
           {
  
               Graphics * gr = e->Graphics;
               if (bitmap != NULL)
               {
                   gr->DrawImage(bitmap, 20, 10, 500, 500);
               }
  
  
           }
  
  
  inside the function X :
  
              Graphics* gr2;
              gr2= cc->CreateGraphics();
              SolidBrush* sb = new SolidBrush(Color::Red);
              gr2->FillRectangle(sb, 300+x, 300-y , 5, 5);
              gr2->Dispose();
  
  
  
  Remember, that when gr2 is created & used inside the Form, everything is OK. It's when I'm doing this inside a function that no rectangles are drawn. If everything looks fine, an example of CreateGraphics used in a standalone function would be helpful...
  
  

• bbeckles

  Is this the correct way to initialize the control also, I noticed that the Graphics can not be initialized - is this true
  
      public: static System::Windows::Forms::Control * dd;
      public: static System::Drawing::Graphics * gr2;
      public: static System::Drawing::SolidBrush * sb;
  
      private:
          System::ComponentModel::Container* components;
          void InitializeComponent(void)
          {
              this->dd = new System::Windows::Forms::Control();
              this->sb = new System::Drawing::SolidBrush(Color::Red);
          }
  
  inside the function :
  
              Graphics* gr2;
              gr2= cc->CreateGraphics();
              gr2->FillRectangle(sb, ....
  
  
  

• Thomas Thayil

  I'm sorry man, I'm working on C# only...

  Anyway the CreateGraphics method works on my example, I'm calling it from the form on another loaction and paints the rectangle, sorry I can't help you more...

  
  
  

• Heng-yi Liu

  Can you double check where that exception come from the call to CreateGraphics() will never return a null object.

  
  
  

• clark121121

  You are trying to rebuild his app in C#, while I think he made a mistake in C++, an invalid pointer or something. Could you post the entire code for the control you are building That would clear things up.
  
  

• Interact

  What about the initialize of the Control are you sure that the cc has value
  
  

• KyleHenly

  I appreciate the offer. I would rather try to work with a new project.
  Thanks very much for the help.
  
  
  

• Martinezg

  I've tried to do what your are tring to do, it seems to work fine:

   

  I've define a control:

   

  class DemoControl : Control

  {

  public DemoControl()

  {

  }

  }

  Than, I've add it to a form:

  this.demoControl1 = new ControlsChecker.DemoControl();

  this.demoControl1.Location = new System.Drawing.Point(141, 439);

  this.demoControl1.Name = "demoControl1";

  this.demoControl1.Size = new System.Drawing.Size(75, 23);

  this.demoControl1.TabIndex = 9;

  this.demoControl1.Text = "demoControl1";

  this.Controls.Add(demoControl);

   

  And on a button click, I've try to draw on it:

  private void button1_Click(object sender, EventArgs e)

  {

  Graphics g = demoControl1.CreateGraphics();

  g.FillRectangle(Brushes.Red, ClientRectangle);

  g.Dispose();

  }

   

  Well, Its works...

  If this is the lead of what you are tring to do, maybe you should start from a simple control like this, check if this works, and than add the functionallty slowly, to find where it falls.

  
  
  

• Sushil Chordia

  Ok... tried to reproduce your problem with your code. I can paint in a control in something else than the paint event handler. I also paint the thing when a button is clicked. Some remarks about your code:

  -Why do you create a Control I have used Panel instead of Control, which worked. Control also didn't paint anything, don't know why, but I didn't get the error message.

  -Try not to initialize additional code in the InitializeComponent class. This class is created by the designer, and the creation of your brush will be gone each time you've entered the designer.

  -Why did you make the controls static I didn't, because I didn't see why...

  -Why did you make data members of the graphics object and the brush object Remember that the Graphics object is invalid after the current message has been processed. You need to call CreateGraphics each time. You can't cache it! From the docs:

  "The Graphics is only valid for the duration of the current window's message."

  -Don't forget to dispose of the brush too!

  What I did was:

  1. drag a panel onto a form.

  2. Upon some event, like the click of a button, use this code:

  Graphics* gr2; gr2= panel1->CreateGraphics(); SolidBrush* sb = new SolidBrush(Color::Red); gr2->FillRectangle(sb, 0, 0 , panel1->Width, panel1->Height); gr2->Dispose(); sb->Dispose();

  I think this fixes your problems, doesn't it

  
  
  

• Technobabble80

  Thanks for the help. I've implented your changes but I still get unhandled exception as before. It happens at gr2= panel1->CreateGraphics();  . Looking at the call stack, I see that panel1, gr2, & sb have undefined values - I'm not sure if this means anything
  
  I guess I'll try creating a new project with just the Graphics code to see if that works.
  
  
  
• drawing on a form from within a seperate function