Visual Studio 2019 Form1.cs C# Global Variable

Question

I need to share a variable anywhere in Form1.cs.
Tried using example, https://social.msdn.microsoft.com/Forums/windows/en-US/536b644b-bc4b-42c4-8ad5-2458553c3cd3/how-to-make-certain-variables-global-to-all-forms-c?forum=winforms, but receive the following error;

CS0122 : 'Variables.ChartArray' is inaccessible due to its protection level

Can you share a variable anywhere in Form1.cs?
If so, how?

Accepted Answer

Consider a Singleton class

public sealed class ApplicationSettings
{
    private static readonly Lazy Lazy = 
        new Lazy(() => new ApplicationSettings());

    /// 
    /// Access point to methods and properties
    /// 
    public static ApplicationSettings Instance => Lazy.Value;
    public char[] ChartArray { get; set; }
}

Usage

Read the array

ApplicationSettings.Instance.ChartArray

Update

char[] someArray = new[] { 'c', 'd', 'e' };
ApplicationSettings.Instance.ChartArray = someArray;

Or

ApplicationSettings.Instance.ChartArray[0] = 'a';

Answer

There is really no such thing as a global variable in C#. You can however have a static (shared) value contained in a class. If that class is itself static (shared) then this is about as close to a global variable as you can get. But it is generally not recommended that you do this. It generally indicates you have a design problem. Really the same thing applies to any language that allows globals. Global variables are almost always the wrong solution. There are a couple of well defined cases where such a design may make sense (singletons, for example) but even then they should be rarely used and only if there is not a better option. You can google for the whys.

Back to your question, to expose a value from a class make it public.

public class ParentForm : Form
{
   //Anybody with access to the instance can read or write this value
   public string Title { get; set; }
}

To share across other forms you would simply pass an instance of the desired ParentForm around.

public class ChildForm : Form
{
    public ParentForm { get; set; }

    private void SomeFunction ()
    {
        //Assuming ParentForm has already been set...
        string parentTitle = ParentForm.Title;
    }
}

If you need to access the type without passing an instance around then use a singleton ideally.

public class ParentForm : Form
{
    //Note that normally you make the ctor private but that won't work with a Form
    public ParentForm ()
    {
        ...
        Default = this;
    }

    public static ParentForm Default { get; private set; }
}

By marking the member as static it is now accessible without an instance

string title = ParentForm.Default.Title;

If your type only contains static members then you don't really need an instance and can make the entire type static which saves an allocation.

public static class DataManager
{
    public static DbConnection CreateConnection () { ... }
}

//Somewhere else
using (var conn = DataManager.CreateConnection())
{
}

Visual Studio 2019 Form1.cs C# Global Variable

1 additional answer