In C# how do access my LINQ var array from another method?

The parts, suppliers and shipments variables are declared in the scope of the MainWindow() method; they will not be accessible anywhere outside of this method. If you want to access these variables in another method within the same class, you should declare them as private to the class.

When a variable is declared, you typically specify an access modifier. In your example, you are creating the variables implicitly using the var keyword. Variables created in such a manner have a default access modifier of private. This means that they are scoped to they containing block in which they are created, which in your case is the MainWindow() method. As others have suggested, I would recommend a review of variable scope and access modifiers. You will need to have a solid grasp of these basics to be productive.

Check out this channel 9 video series on some C# fundamentals for beginners; specifically, episode 5 focuses on variables. This blog post also goes over some basics on variable scope.

The var keyword used for variable declaration means that the variable is implicitly defined based on the assignment. What this means is that the compiler will look at what you are assigning to the variable (ie: what is on the right side of the equals sign) and figure out the type. For technical reasons, you cannot use the var declaration operator for variables defined at the class level (also often referred to as 'fields' of a class if they are private to the class).

Since you cannot use the var keyword to define your class level variables, you must explicitly specify a type. In the example below, I have created a class which represents the original parts anonymous type you were working with in your example.

I also concur with the advice of other on starting out learning C# using a console application. GUI applications bring a lot of other nuances to the table which can only serve to confuse you and cause general frustration when you are learning a language. For example: simple things such as updating a textbox from the non GUI thread with a new value can bomb your program and leave you wondering why it doesn't work.

You can choose to continue to work with an array of anonymous types, however you would need to declare it as dynamic or object. Working with an array of anonymous objects can be a little more challenging. I would recommend that you use a defined type (such as the one part class have illustrated below) for your class fields.

for example:

public class Part
{
    public int PN { get; set; }
    public String PName { get; set; }
    public String Color { get; set; }
    public int Weight { get; set; }
    public String City { get; set; }
}

public partial class MainWindow : Window
{

    private List<Part> _parts;
    public MainWindow()
    {
        InitializeComponent();

        _parts.Add(new Part() { PN = 1, PName = "Nut", Color = "Red", Weight = 12, City = "London" });
        _parts.Add(new Part() { PN = 2, PName = "Bolt", Color = "Green", Weight = 17, City = "Paris" });
        _parts.Add(new Part() { PN = 3, PName = "Screw", Color = "Blue", Weight = 17, City = "Rome" });
        _parts.Add(new Part() { PN = 4, PName = "Screw", Color = "Red", Weight = 14, City = "London" });
        _parts.Add(new Part() { PN = 5, PName = "Cam", Color = "Blue", Weight = 12, City = "Paris" });
        _parts.Add(new Part() { PN = 6, PName = "Cog", Color = "Red", Weight = 19, City = "London" });

    }

    private void SomeMethod()
    {
        string colorChoice = "Blue";

        IEnumerable<string> partResults = _parts
            .Where(clr => String.Equals(clr.Color, colorChoice))
            .Select(part => String.Format("{0}", part.PName));
    }
}