Object Reference not set when using PrinterSettings.StringCollection

Question

I am attempting to list the currently installed printers using PrinterSettings.StringCollection. However, I get this error:

Object Reference not set to an instance of an object

Code is as follows:

namespace DropDownLibrary
{
    public class DropDownExample : DSDropDownBase
    {
        public DropDownExample() : base("item") { }

        public static PrinterSettings.StringCollection InstalledPrinters { get; }

        public override void PopulateItems()
        {
            // The Items collection contains the elements
            // that appear in the list.            

            Items.Clear();

            // Create a number of DynamoDropDownItem objects 
            // to store the items that we want to appear in our list.

            var newItems = new List<DynamoDropDownItem>();
            {
                 foreach (String name in InstalledPrinters)
                 {
                     new DynamoDropDownItem("{0}", name);
                 }
            };

            Items.AddRange(newItems);

            // Set the selected index to something other
            // than -1, the default, so that your list
            // has a pre-selection.

            SelectedIndex = 0;
        }

        public override IEnumerable<AssociativeNode> BuildOutputAst(List<AssociativeNode> inputAstNodes)
        {
            // Build an AST node for the type of object contained in your Items collection.

            var intNode = AstFactory.BuildIntNode((int)Items[SelectedIndex].Item);
            var assign = AstFactory.BuildAssignment(GetAstIdentifierForOutputIndex(0), intNode);

            return new List<AssociativeNode> { assign };
        }
    }
}

Answer 1

This is covered in this post. The 'Object reference not set to instance of an Object" error is caused by you trying to use a variable that is null. For instance, you can get a null reference error by doing:

object nullObject = null;
nullObject.ToString():

In your code, it doesn't look like the value for InstalledPrinters ever gets set.

Before your code reaches this line:

foreach (String name in InstalledPrinters)

It looks like you copy pasted this from this link:

public static PrinterSettings.StringCollection InstalledPrinters { get; }

This is a property on the PrinterSettings class that you can access. You should access it like so:

var installedPrinters = System.Drawing.Printing.PrinterSettings.InstalledPrinters;

foreach (String name in installedPrinters)