[C# Err]An object reference is required for the non-static field, method, or property

[jackpotsvr]

Hey was programming some Form and I want to be able to clear its controls with this void, but it won't work. What am I doing wrong?


An object reference is required for the non-static field, method, or property

public void SwitchUC()
        {
            Form1.Controls.Clear();
        }

[donn]

Everything. You can't clear all controls from a form with one line.
You need to:

• Remove the event handler from the event
• Use Remove method to delete the desired control
• Call the Dispose method to release all the resources used by the control.

And this for each individual control you want to remove (you can forloop them).

[jackpotsvr]

Originally Posted by donn

Everything. You can't clear all controls from a form with one line.
You need to:

• Remove the event handler from the event
• Use Remove method to delete the desired control
• Call the Dispose method to release all the resources used by the control.

And this for each individual control you want to remove (you can forloop them).

The line of code did work for aslong I had it on a button, but now it's in a void it does not :/

[donn]

Originally Posted by jackpotsvr

The line of code did work for aslong I had it on a button, but now it's in a void it does not :/

It worked because a button is an object reference. Still, without removing any event handler associated with a control and disposing the resources used by it, you will leave a lot of dirt behind.

[pro4never]

Going to ignore just for now all the things donn is mentioning (he's correct. Just clearing everything on a form will not just 'make it go away') and focus on the actual error.


In C# (well... basically any object oriented programming language) you can either have static objects (can only have one of them) or you can have instances of objects.

Forum1 is NOT a static object. You could (if you wanted to) create all sorts of copies of Forum1 and display them. The error you are getting is because you are not saying WHICH Forum1 you want to perform the action on...

It's like saying "Car.Unlock()" without saying what car to try to unlock.

The reason you get this error inside your void and not within the event handler (button press) is because that button press was already inside the Forum1 class... Therefor you can operate on it because everything is already local to it.

Think of this like being inside a car and me telling you to unlock the doors. It simply makes sense that I'm talking about the car you're inside... not a car that is across the parking lot.

Now... your method has no reference to the instance of a Forum1 object. As such it cannot do what you're asking of it.


Here's a simple (although not recommended) workaround to give you an idea of how it works...


Inside the forum1 code do something like...

public static Forum1 MainWindow;

Then inside the initialization of the forum say...

MainWindow = this;


Done! You can now just say Forum1.MainWindow.Blablabla

This is NOT recommended, it's just an example of how you could handle it if for some really strange reason you HAD to have access to it outside of itself without first passing any thing between the two separate classes.

[I don't have a username]

this.Controls.Clear();

or

Form1 f;

Last one needs to be set as a variable somewhere, I assume you have a static field for that, but since anyways it's not doing that by itself, when creating a form, so I assume you call the form within itself and for that reason you can do number one. If it's from another form you have to declare Form1.

[jackpotsvr]

Originally Posted by pro4never

Going to ignore just for now all the things donn is mentioning (he's correct. Just clearing everything on a form will not just 'make it go away') and focus on the actual error.


In C# (well... basically any object oriented programming language) you can either have static objects (can only have one of them) or you can have instances of objects.

Forum1 is NOT a static object. You could (if you wanted to) create all sorts of copies of Forum1 and display them. The error you are getting is because you are not saying WHICH Forum1 you want to perform the action on...

It's like saying "Car.Unlock()" without saying what car to try to unlock.

The reason you get this error inside your void and not within the event handler (button press) is because that button press was already inside the Forum1 class... Therefor you can operate on it because everything is already local to it.

Think of this like being inside a car and me telling you to unlock the doors. It simply makes sense that I'm talking about the car you're inside... not a car that is across the parking lot.

Now... your method has no reference to the instance of a Forum1 object. As such it cannot do what you're asking of it.


Here's a simple (although not recommended) workaround to give you an idea of how it works...


Inside the forum1 code do something like...

public static Forum1 MainWindow;

Then inside the initialization of the forum say...

MainWindow = this;

Done! You can now just say Forum1.MainWindow.Blablabla

This is NOT recommended, it's just an example of how you could handle it if for some really strange reason you HAD to have access to it outside of itself without first passing any thing between the two separate classes.

Ah I see, well I don't want to clear the things from memory orso, since the thing I will clear is just UserComponent1 so I can get to UserComponent2 (and I want to be able to switch between those, so clearing it from a memory will just make the program running slow I guess). But isn't there anyways to make a void doing the 'Form1.Controls.Clear();' ?

[IAmHawtness]

Originally Posted by jackpotsvr

Ah I see, well I don't want to clear the things from memory orso, since the thing I will clear is just UserComponent1 so I can get to UserComponent2 (and I want to be able to switch between those, so clearing it from a memory will just make the program running slow I guess). But isn't there anyways to make a void doing the 'Form1.Controls.Clear();' ?

Save the instance of the form in a global variable called myForm or something. Then, from your "void", you call myForm.Controls.Clear();
You could assign your global variable (myForm) a value in your Main procedure (or somewhere else, it's all up to you).

Something like this:

(Program.cs)

    static class Program
    {

        private static Form1 myForm;

        public static Form1 MyForm
        {
            get { return myForm; }
        }

        [STAThread]
        static void Main()
        {

            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);

            myForm = new Form1();

            Application.Run(myForm);

        }
    }

Then, when you want to refer to your form from inside your own method, you just do

        public void SwitchUC()
        {
            Program.MyForm.Controls.Clear();
        }

Many will probably argue that this is not the ideal approach, but I doubt you care much about design anyways.

[jackpotsvr]

Originally Posted by IAmHawtness

Save the instance of the form in a global variable called myForm or something. Then, from your "void", you call myForm.Controls.Clear();
You could assign your global variable (myForm) a value in your Main procedure (or somewhere else, it's all up to you).

Something like this:

(Program.cs)

    static class Program
    {

        private static Form1 myForm;

        public static Form1 MyForm
        {
            get { return myForm; }
        }

        [STAThread]
        static void Main()
        {

            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);

            myForm = new Form1();

            Application.Run(myForm);

        }
    }

Then, when you want to refer to your form from inside your own method, you just do

        public void SwitchUC()
        {
            Program.MyForm.Controls.Clear();
        }

Many will probably argue that this is not the ideal approach, but I doubt you care much about design anyways.

Good works great. Anyways, what would be wrong if I wanted to do some designing in here?

[IAmHawtness]

Originally Posted by jackpotsvr

Good works great. Anyways, what would be wrong if I wanted to do some designing in here?

I don't mean design as in visual design. I just meant that normally you'd have the method "SwitchUC" in the UI class itself. It will also make everything a lot easier if you ever introduced multi-threading to your program.

[jackpotsvr]

Originally Posted by IAmHawtness

I don't mean design as in visual design. I just meant that normally you'd have the method "SwitchUC" in the UI class itself. It will also make everything a lot easier if you ever introduced multi-threading to your program.

Oh well in this case im not really planning on doing multithreading, although good to know. I didn't need the void later anyways, since the UserComponent I was wishing to call this void from, can either just do that line of code itself.

The reason I want this is because I use one form, that use multiple usercomponents to switch between interfaces.
Now how I fixed it is
ButtonClick --
Program.MyForm.Controls.Clear();
Program.MyForm.Controls.Add(US2) (US2 is an usercomponent I assigned.

[pro4never]

ummm.... maybe I'm misunderstanding you but you're using a single forum and instead of separating things based on control groups and swapping between them you're simply hiding all controls and adding/showing a different control then swapping back?....

No offense but that's really not the right way to be handling things...

If for some reason you MUST have multiple 'states' for your forum then I'd suggest writing a screenstate type system (mostly used on game programming type environments where you need various gamestats such as menu screen, gameplay, paused etc while within the same program). The idea is that you create an object which represents the current state of the screen and then within the main program (form) you track and can easily swap between them using simple methods rather then the way you're handling it right now.

That being said... if I'm understanding you correctly (unlikely, I tend to make big leaps when I read things ahhaa) then you're prob best off just using some sort of winform control like tab controls.

[jackpotsvr]

Originally Posted by pro4never

ummm.... maybe I'm misunderstanding you but you're using a single forum and instead of separating things based on control groups and swapping between them you're simply hiding all controls and adding/showing a different control then swapping back?....

No offense but that's really not the right way to be handling things...

If for some reason you MUST have multiple 'states' for your forum then I'd suggest writing a screenstate type system (mostly used on game programming type environments where you need various gamestats such as menu screen, gameplay, paused etc while within the same program). The idea is that you create an object which represents the current state of the screen and then within the main program (form) you track and can easily swap between them using simple methods rather then the way you're handling it right now.

That being said... if I'm understanding you correctly (unlikely, I tend to make big leaps when I read things ahhaa) then you're prob best off just using some sort of winform control like tab controls.

The way it works is I have a windows form, and multiple usercompononents. (For example UC1 = LoginScreen). On the login screen I have 2 masked textboxes, ID, Pass connected to mysql). After the button is clicked and everything is filled in correctly the form will clear and will add usercomponent2which is a form with anything else inside it. Etcetc.

I don't really know what's another way to accomplish this. I don't want to switch forms, and with UserComponents I still have a designer.

Anyways I must admit im not an too experienced programmer.. Maybe I should take care of some more basics and follow some course I can find here in this section .

[Heroka]

IAmHawtness>>thx so much for ur job =)