Raising and subscribing to events

kyle

I need help with the syntax to update the BackColor of a label on a form while a while loop is running on another form.

Form1 has a label.

Form2 has an if statement inside a while loop. When the if statement evaluates as true I need to change the BackColor of a label on Form1.

Thanks in advance for your help.

Class MyClass
{
      public delegate void ChangeColorDelegate();
      public event ChangeColorDelegate ChangeColor;
 
     While(true)
 
       if (something == true)
           (HAVING TROUBLE HERE)
           if (ChangeColor != null)
               ChangeColor();
 
}
 
Class MyOtherClass
{
      MyClass test = new MyClass();
      test.ChangeColor += (THIS IS WHERE I AM HAVING DIFFICULTY)

11 Replies

Know the answer? Post it — somebody with the same question will find it here.

Danatas Gervi17y ago
Accepted answer

Possible reason is that you gave the delegate of one instanse of from, but show other. Form – this is simple class, it may be invisible, but existing, if you keep the reference to it…

Take this code:

namespace ChangeLabelBackground

{

   public partial class Form1 : Form

   {

         Diagnose m_DiagForm;

         public delegate void ChangeColorDelegate();

         public event ChangeColorDelegate ChangeColor;

         public Form1()

         {

                InitializeComponent();

                m_DiagForm = new Diagnose();

                ChangeColor += new ChangeColorDelegate(m_DiagForm.ChangeColor);

                m_DiagForm.Show();

         }

         private void button1_Click(object sender, EventArgs e)

         {

                if (ChangeColor != null)

                {

                                ChangeColor();

                }

         }

   }

}

And, Diagnose form (I have added it to project with one label on it)

namespace ChangeLabelBackground

{

   public partial class Diagnose : Form

   {

         public Diagnose()

         {

                InitializeComponent();

         }

         public void ChangeColor()

         {

                //id2_7.BackColor = Color.Green;

                label1.BackColor = Color.Green;

         }

   }

}

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kyle17y ago

I have a question about the code you posted.

namespace ChangeLabelBackground

{

public partial class Form1 : Form

{

Diagnose m_DiagForm; XXXXXXX

public delegate void ChangeColorDelegate();

public event ChangeColorDelegate ChangeColor;

public Form1()

{

InitializeComponent();

m_DiagForm = new Diagnose();XXXXXXXXX

ChangeColor += new ChangeColorDelegate(m_DiagForm.ChangeColor);

m_DiagForm.Show();

Why did you declare m_DiagForm in 2 parts? What would the difference be if you wrote:

Diagnose m_DiagForm = new Diagnose(); in the Form1 constructor instead of breaking it into 2 parts as you did above?

kyle17y ago
Hello Danatas. That indeed was the problem. I had the statements and code for the delegate correct but was showing the wrong instance of the form.

Thanks again for your help. You have been quite helpful.
0

kyle17y ago

Hello Danatas. First let me thank you for your help and patience!

My code is executing correctly now. My ChangeColor metod is being called from the working form (class) but the color is not changing.

The working form with the while loop and if statements:

        Diagnostics diagnostics = new Diagnostics();
        public delegate void ChangeColorDelegate();
        public event ChangeColorDelegate ChangeColor;

        ChangeColor += new ChangeColorDelegate(diagnostics.ChangeColor);

        if (variable == 1)
                {
                   ChangeColor();
                }

The other form with the labels:

public void ChangeColor()
        {
            id2_7.BackColor = Color.Green;
        }

I set a breakpoint at this method and the statement inside the method and it is being called but the label id2_7 is not changing color. I added a statement in the variable id2_7.Refresh() but this was not effective.

Again, thank you for your time and help.

Danatas Gervi17y ago

No. This is not correct.

To subscribe to event you must to call and execute the code of subscription (test.ChangeColor += HandlerOfChangeColor;)

This can be in constructor, additional function, but before you use this event.

Without handler (instance of delegate, "pointer to function" ) event == null.

0
kyle17y ago
That statement is in the form that is not "working" so to speak. The processor is working in the other form where the if statements are located. This statement: test.ChangeColor += HandlerOfChangeColor; is not being "seen" because it is not in a working thread. I think that terminology is correct.
0
Danatas Gervi17y ago

Ok, ok….

First – put a break point at

test.ChangeColor += HandlerOfChangeColor;

row…

This row must be executed before you will try to rise event.

0
kyle17y ago
ChangeColor is still null. The method in the event is never called. If I set a breakpoint at if (ChangeColor != null) it will stop there. If I set it at the statement under the if statement "ChangeColor()" it doesn't stop.
0
Danatas Gervi17y ago
Your still need to subscribe to event an "event handler".

Like:

MyClass test = new MyClass();
test.ChangeColor += HandlerOfChangeColor;

And, pay attention – the instance of "event riser" - MyClass test – should be existing all time, should be "class level variable"

Example:
```
Class MyOtherClass

{
```
```
 
```
```
   MyClass test;
```
```
   public MyOtherClass()//constructor or something else must be used - // for creating and initializing an instance of  MyClass
```
```
 
```
```
   {
```
test = new MyClass();
test.ChangeColor += HandlerOfChangeColor;
```
 
```
```
 
```
```
   }
```
```
 
```
```
}
```
```
 
```
0

kyle17y ago

Hello Danatas. Thank you for your help.

My code stops here:

if (ChangeColor != null)

The if statement below it does not evaluate. How do I prevent 
ChangeColor from being null?

Danatas Gervi17y ago
Replace "(THIS IS WHERE I AM HAVING DIFFICULTY)"

By method name, with any name, but with same returning type and number of parameters, for your case it will be

private void HandlerOfChangeColor() // same as your delegate!

{

Label1.Color = ………;

}

This method will be called each time when
```
if (ChangeColor != null)

               ChangeColor();
```
```
 
```
```
code will executed.
```
```
 
```
```
 
```
Tip – use standard delegate EventHandler for your events.
0