I was interested in the POSPrinter class.  They do provide a simulator so that you can test things out using the simulated printer, but it doesn’t support anything other than basic plain text, whereas I needed to use formatted text.

POS printers usually use a control language called ESC/POS.  This was developed by Epson originally for their POS printers, but it has become a standard.  Basically, it is mostly a matter of sending plain text, but you can also send certain escape sequences to change settings on the printer.

Fortunately, Microsoft document a lot of this on MSDN, so you don’t need to work it all out from scratch.  There are things like knowing that the main escape sequence is &H1B or 27 (known as ESC) and then a pipe character followed by optional numbers, small letters or symbols followed by a capital letter.  Eg. ESC|#P cuts the paper, ESC|bC is the code to turn on bold etc.

However, you may find it useful to have some sample code…

Imports Microsoft.PointOfService

Public Class POSPrinter

    Public Const ESC = Chr(&H1B) & "|"
    Public Const SetBold = ESC & "bC"
    Public Const SetUnderline = ESC & "uC"
    Public Const SetItalic = ESC & "iC"
    Public Const SetCentre = ESC & "cA"
    Public Const SetRight = ESC & "rA"
    Public Const ResetFormatting = ESC & "N"

    Public Shared Function SetSize(ByVal Size As Integer) As String
        Return ESC & Size & "C"
    End Function

    Public Shared Function GetAndInitPosPrinter() As Microsoft.PointOfService.PosPrinter
        Dim pe As New PosExplorer()
        Dim ppdi = pe.GetDevice(DeviceType.PosPrinter, My.Settings.PrinterName)
        Dim pp As Microsoft.PointOfService.PosPrinter = pe.CreateInstance(ppdi)
        pp.Open()

        pp.Claim(250)

        pp.DeviceEnabled = True

        Return pp
    End Function

    Public Shared Sub ReleaseAndClosePosPrinter(ByVal pp As Microsoft.PointOfService.PosPrinter)

        pp.Release()

        pp.Close()
    End Sub

    Public Shared Sub PrintTest()

    Dim pp As Microsoft.PointOfService.PosPrinter = Nothing

    pp = GetAndInitPosPrinter()

    Dim msg As String = "This is a test" & vbCrLf & SetBold & SetSize(3) & SetCentre & "it works" & SetSize(1) & " pretty well" & vbCrLf & "OK"

    pp.PrintNormal(PrinterStation.Receipt, msg)

    ReleaseAndClosePosPrinter(pp)

    End Sub

One other thing to watch out for is that if you are running any other POS software on the till, such as Microsoft Dynamics POS, then you might need to go into the settings and tell it to share the printer nicely with the other children applications as some POS software tends to be pretty possessive

Also, always make sure that you release and close any POS devices when you are done with them. For some reason when you start using them you need to open them, claim them and then enable them which seems ridiculous overkill to me, but what do I know?

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An example would be something like

Dim p = DBContext.People.First
Dim q = From a in DBContext.Addresses Where a Is p
Dim addr = q.First

The final line will result in this exception as Linq to SQL is attempting to translate the query into SQL and it (correctly) can’t find any way of writing a query that compares a person to an address!

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The truth is that I originally did the .NET 3.5 approach for a specific project where a client of mine had a large number of Windows 2000 PCs. I wanted to be able to use .NET 3.5, and I didn’t want him to be saddled with the upgrade costs. However, since then, he has been gradually replacing those PCs with Windows XP and now Windows 7 PCs, until he only had 2 or 3 Windows 2000 machines left. I recommended to him that it was better for him to upgrade/replace the last ones rather than me trying to get .NET 4.0 working on the old ones, which probably wouldn’t be as cost efficient.

I realise that this might be a disappointment to some people who have still got lots of Windows 2000 machines out there. If anyone does come up with a way of doing it (possibly based on my .NET 3.5 approach), please let me know and I’ll post it up, crediting you.

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Taken from Is there a way to find a table in a DBML file in Visual Studio 2008? – Stack Overflow.

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The first one is the NullReferenceException, which usually comes with the message “Object reference not set to an instance of an object.”. This Exception occurs when you try to access an object reference which is set to Nothing.

First of all, you need to understand the difference between value types and reference types in .NET. With a value type, we only care about what data is stored within it. With a reference type, the actual instance of the type is important in itself. For example, we may have an Integer, which contains 10. Integers are value types, and there is generally nothing particularly different between one number 10 and another number 10 if they are both Integers, so they are effectively identical.

Reference types on the other hand are different. If we have two customers, and both of them happen to be called “John Smith”, we don’t want to treat them as being the same thing just because they have the same name. The other difference with reference types is that we can have an empty reference – Nothing (or null in c#).

In fact, behind the scenes, reference types store a location in memory of the instance. In older programming languages, such as C/C++, these were known as pointers, and you could actually access them just like a number and change the item that they were pointing to with arithmetic operators (+/-/increment/decrement etc.) This created a number of potential problems and so references were created to protect pointers from causing too much damage. References also the garbage collector to keep track of what is referencing an object instance so that the instance can be cleared up when it is no longer in use.

The NullReferenceException is thrown when you attempt to access an object reference which is set to Nothing. This can easily happen because as mentioned above, all variables of a reference type will be initialised to Nothing by the .NET framework if they have not been explicity set to something else.

For example,

        Dim c As Customer
        c.Name = "John Smith"

The second line in the above piece of code will throw a NullReferenceException as the variable c is still set to Nothing, and when the .NET framework attempts to access the Name field, it will find that the object does not exist.

This is easily resolved by setting c to whatever Customer you wish to use, or to a New Customer.

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In order to get around this problem, you need to take the following steps: -

First of all, configure Visual Studio so that you can launch the test server manually as follows: -

Under the Tools menu, select External Tools.
Add a new entry
Call it something like ASP.NET Development Server
Command is C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\WebDev.WebServer.EXE (you may need to alter the path for your local machine)
Arguments are /port:80 /path:$(ProjectDir) (note that you will need to leave a space on the end of this for it to work properly.  Also, you can change the port number if you wish)

Press OK.  You can now launch the development server from the new entry on your Tools menu.  This will show in your system tray.  You should probably remember to close it when you are done.

The next step is to configure your project to use the server.  Right click on your project and click Property Pages.  If an empty dialogue comes up, press cancel and repeat the process – it should work second time.  Under Start Options, select Use custom server and leave the Base URL blank.  You may wish to change the start action as well.

Once you are done, you can press F5 to start debugging.  Don’t forget that next time you open the project you will need to start the server from the tools menu before you start debugging again, otherwise it won’t work.

Also, bear in mind that if this is a copy of a remote site, things like database connection strings may need changing.  Don’t forget to be careful not to overwrite any settings when you copy back if that is the case!

If you do want to develop on a copy of the site, the Website menu has a useful option to Copy website.

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Novell have been working on a project called Mono over the last few years to create an alternative implementation of .NET for other platforms, including Linux.  They have implemented most of the major features and classes, but there are still a few areas which are patchy.  I usually find that the Handles VB.NET keyword doesn’t work and causes a compiler error, but it is quite possible to make a decent site that operates on Mono.  I’ve even compiled a DLL file in Visual Studio and used it in a Mono project’s Bin folder.  Of course if you do that, you have to be careful only to reference classes which are available in Mono.

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In a project that I am working on at the moment, I have a situation where I have a Person table and a Family table.  The Person table has a FamilyID field, which is a foreign key to the Families ID (primary key) field, and the Family table has a DefaultPersonID, which is another foreign key back to Person to store the default contact for a family.

The problem is that if I create a new Person and Family with the Person being the DefaultContact, I get an InvalidOperationException thrown with a message “A cycle was detected in the set of changes”.  The problem is that Linq to SQL doesn’t know what to store first.  The only way that I have come up with to get around this is to do the following…

            If Person.ID = 0 AndAlso Not Person.Family Is Nothing Then

                Dim fam = Person.Family
                Person.Family = Nothing

                DBContext.SubmitChanges()
                Person.Family = fam

            End If

            DBContext.SubmitChanges()

This works by breaking the circular reference, so that there is only a 1 way reference, then submitting to the database, and once submitted, restoring the removed reference and submitting changes again

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DirectCast takes the object which is passed in and assumes it to be of the casted type or an inherited type.  This means that if you try to DirectCast an Integer to a Single, it won’t work.  If the compiler can see that it won’t work because it isn’t an inherited type, you’ll get a compiler error.  If you try it on a variable in eg. an Object at runtime, you’ll get an InvalidCastException.

The alternative is to use CType, which is much cleverer and will try to find a way to convert from one type to another.  This means that you can Ctype(“2.334″, Single) and it will parse the string and give you a Single length floating point number.  It will also check for overloading of the CType operator to see if it can convert using that.  This makes it very powerful, although it does come with the cost of a lot more processor usage.  However, for most modern applications, unless you are doing something heavily time constrained with a large amount of iterations in a  loop, it is unlikely to be an issue.

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