Global variables available across multiple forms?

[Pc_Madness]

Hey guys, just starting out with some C# programming, and atm I have the following,

frmMain - Form

frmAdd - Form

clsVideoLibrary - Class

frmMain declares a clsVideoLibrary, and loads everything into the class.. all fine and dandy.. however, the Add form adds in new items into the class.. atm, I have to pass everything back into frmMain and create a special function purely for the task (I guess it would depend on how my code is written), which is going to suck, as I'm going to basically redeclare this entire class inside of frmMain. Atm, I'm doing this by passing it frmMain.

Back in the old days of VB6, you used to just declare it as a global variable and stick it inside of a module, and all was well and everyone could access the class. But C# is a real poop about such things!

Is there any easy way to make the class available to other forms? As I'm going to have alot of classes and alot of forms, so it mightn't be entirely practical to pass the class files to the forms by reference or whatever.

w00t, 1234 posts :p

[John Veteran]

Global variables don't exist in .NET, and rightfully so. It's a bad idea to use global variables, and if you're finding your code needs one, you should probably look at how your code is designed and change some things so you don't need a global variable.

If your Add form is a data collection window that returns data to your Main form, then you can (should?) add a method to your Main form that the Add form will use. You can also create an object to act as a container for the data between your two forms.

[Express]

Addendum to John's object container idea, you can use the AppDomain to store global data.

Storing:

AppDomain.CurrentDomain.SetData("myglobalvar", "data1");

Retrieving:

object val = AppDomain.CurrentDomain.GetData("myglobalvar");

[$phinX]

So you're trying to open an AddForm from the MainForm that will allow the user to input a new video and add it to your Video library, yeah?

In which case id say you need another object... clsVideo. frmMain has an instance of clsVideoLibrary. frmAdd has a public instance of clsVideo. When the user clicks Save/Add in frmAdd you save the new video to the public instance of clsVideo. Then in your MainForm code where you open the AddForm you can do something like:

//Video library
clsVideoLibrary myVideoLibrary = new clsVideoLibrary();

//Open the AddForm
frmAdd objAddForm = new frmAdd();
objAddForm.ShowModal();

//Add the Video entered in the AddForm to the VideoLibrary
myVideoLibrary.Add( objAddForm.myNewVideo )

You'll probably also wanna do some checks to see if its null first - in case they press Cancel on the AddForm or something.

There are ways to do 'global variables' in .Net - Express showed one. But as John said, if you find yourself needing to use them - then you should be re-designing your code.

Oh... and VB6 sucks! :D

[Pc_Madness]

  $phinX said:

So you're trying to open an AddForm from the MainForm that will allow the user to input a new video and add it to your Video library, yeah?

In which case id say you need another object... clsVideo. frmMain has an instance of clsVideoLibrary. frmAdd has a public instance of clsVideo. When the user clicks Save/Add in frmAdd you save the new video to the public instance of clsVideo. Then in your MainForm code where you open the AddForm you can do something like:

//Video library
clsVideoLibrary myVideoLibrary = new clsVideoLibrary();

//Open the AddForm
frmAdd objAddForm = new frmAdd();
objAddForm.ShowModal();

//Add the Video entered in the AddForm to the VideoLibrary
myVideoLibrary.Add( objAddForm.myNewVideo )

You'll probably also wanna do some checks to see if its null first - in case they press Cancel on the AddForm or something.

There are ways to do 'global variables' in .Net - Express showed one. But as John said, if you find yourself needing to use them - then you should be re-designing your code.

Oh... and VB6 sucks! :D

Hmm, I like, I like alot. :) I have a struct defined for Video already so should be easy to implement. :)

Yeah... but global variables make 'sense'. :( And don't knock VB6, was a great language :( I dunno, I don't like like the idea of having my code thrown all over the place. Inside AddForm it writes the new element to the XML doc, and then adds it to clsVideoLibrary to save my reloading... which I personally see as being related code that should be lumped together....maybe I really should just rip everything out of AddForm and write everything in frmMain? Hmmmmmm

[John Veteran]

Your code isn't "thrown all over the place" in .NET. Rather the opposite; it's organized into objects that make it easier to do your work. Sometimes this may take a little extra work, but in the end, it simplifies things immensely. Global variables make sense to you now, but if/when you learn .NET, you'll discover how truly convoluted VB6 is.

[Pc_Madness]

Ok, so I have it all working, but I'm getting some warnings,

frmAdd.Add -> Add it!

		public VideoCollection.VideoItem VideoToAdd = new VideoCollection.VideoItem();

		private void btnAdd_Click(object sender, EventArgs e)
		{
			VideoToAdd.CatID = 1;
			VideoToAdd.Title = txtTitle.Text;
			VideoToAdd.Location = txtLocation.Text;
			VideoToAdd.ImageID = Int32.Parse(txtImage.Text);
			this.Close();
		}

frmMain.Show Add Form button

 private void btnShowAdd_Click(object sender, EventArgs e)
		{
			frmAdd MyAddForm = new frmAdd();
			MyAddForm.ShowDialog();

//......XML + collection code here
			   XMLNewAttribute.Value = MyAddForm.VideoToAdd.CatID.ToString();

Atm, I get the following warning,

  Quote

Warning 1 Accessing a member on 'VideoLibrary.frmAdd.VideoToAdd' may cause a runtime exception because it is a field of a marshal-by-reference class C:\Documents and Settings\Phillip\My Documents\Visual Studio 2005\Projects\VideoLibrary\VideoLibrary\Form1.cs 193

Any ideas what that is all about? :\

[John Veteran]

What type is frmAdd.VideoItem? And if it is a class/struct of your own, can you post a class diagram of it?

[Pc_Madness]

	public struct VideoItem
	{
		public int ID;
		public int CatID;
		public string Title;
		public string Location;
		public int ImageID;
	}

:)

Ok, copying it to a local variable seems to do the trick. Why though? :\

Edited July 10, 2006 by Pc_Madness

[amrinders87]

  Pc_Madness said:

	public struct VideoItem
	{
		public int ID;
		public int CatID;
		public string Title;
		public string Location;
		public int ImageID;
	}

:)

Ok, copying it to a local variable seems to do the trick. Why though? :\

If it is global then why not make it a class? That way it will be passed around as a reference. Structs will be passed by value.