How to Open CashDrawer when Printing starts

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Hi! I am new in creating POS systems.

I want to open my cash drawer when printing starts. I have found some codes in the internet but they do not seem to work. It always gives me this error: Object reference not set to an instance of an object.

This is my codes:

C#

using Microsoft.PointOfService;

CashDrawer myCashDrawer = null;
       PosExplorer explorer;

        public void CashDrawerClass()
        {
            try
            {
                explorer = new PosExplorer();
                DeviceInfo ObjDevicesInfo = explorer.GetDevice("C4141");
                bool check = ObjDevicesInfo.IsDefault;
                myCashDrawer = (CashDrawer)explorer.CreateInstance(ObjDevicesInfo);
            }
            catch (Exception ex)
            {
                MessageBox.Show("CashDrawerClass: " + ex.Message);
            }
        }

        public void OpenCashDrawer()
        {
            try
            {
                myCashDrawer.Open();
                myCashDrawer.Claim(1000);
                myCashDrawer.DeviceEnabled = true;
                myCashDrawer.OpenDrawer();
                myCashDrawer.DeviceEnabled = false;
                myCashDrawer.Release();
                myCashDrawer.Close();
            }
            catch (NullReferenceException ex)
            {
                MessageBox.Show("OpenCashDrawer: " + ex.Message);
            }
        }

The cash drawer is connected to my printer.

Posted 21-Oct-15 16:58pm

Michelle Anne E. Rigor

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George Jonsson · Answer 1 · 2015-10-21T18:12:00

If you add some validation code in the constructor you might get more info.
Or use the debugger to step through the constructor line by line.

You should add some checks:

1. Is ObjDevicesInfo getting a value or is it null?

C#

if (ObjDevicesInfo == null)
   Report an error.

2. Check if myCashDrawer is given a value.

Garth J Lancaster · Answer 2 · 2015-10-21T17:58:00

Solution 1

Thats always the danger of copying bits of code off the net .. there's not a lot in the code you've shown, and you dont say where that error is emitted from, so ....

...ok, assuming the error comes from

C#

myCashDrawer.Open();

when you call OpenCashDrawer(), the most obvious thing, apart from it looking a bit messy design/implementation-wise, is I dont see anything that looks like

C#

myCashDrawer = new CashDrawer();

possibly with arguments being passed into the instantiation of the CashDrawer Object

Posted 21-Oct-15 17:58pm

Garth J Lancaster

Comments

George Jonsson 22-Oct-15 0:07am

There is actually a line "myCashDrawer = (CashDrawer)explorer.CreateInstance(ObjDevicesInfo);" in the constructor.

Garth J Lancaster 22-Oct-15 0:18am

good pickup - so the user actually creates an instance of CashDrawerClass() - so much for it being well structured/commented .. definately +points for your second point :-)

George Jonsson 22-Oct-15 0:39am

Maybe, maybe not as we don't know where the error occurs.
However, using the info in both our solutions should solve the problem.