Writings from the sticks

It had to happen sooner or later. But now questions about how to do this are appearing. And the answer is “it depends”.

ALWAYS use a copy or have a backup copy of your entire project JUST in case something goes wrong.

IF you have a license that permits you to save as text (Xojo Project) save your project as a text project. You then need to open the single file called projectname.xojo_project

At the very beginning of this file you will see entries like the following :

Type=Desktop
RBProjectVersion=2019.02
MinIDEVersion=20190200
OrigIDEVersion=20190200

Change the number after MinIDEVersion to 20070100 and save. You can now open the project in whatever version you want.

In an XML project you would see

<?xml version="1.0" encoding="UTF-8"?>
<RBProject version="2019r2" FormatVersion="2" MinIDEVersion="20190200">
<block type="Project" ID="0">

Again change the number following MinIDEVersion to 20070100. Make sure you keep the quotes around the value. And save.

Binary is a lot tougher because it is a proprietary format. And to edit that you WILL need a licensed version of Arbed. Once you have that you can open a binary project and select the top entry which represents the project and change the OriginalVersion to 20070100 and the ProjectSavedInVers to 2007.01 and save.

And now you can open your project in old versions again.

Often you’ll hear people say you should use prepared statements to avoid sql injection issues. They’re not wrong. But there can be other reasons to use them.

In some db’s when you create a prepared statement and use it over & over you can avoid a fair amount of a speed hit.

When you initially create a prepared statement many db engines will actually do some work to figure out what the optimal mechanism, or query plan, is to access the data in a table.

And by reusing this over & over you can avoid that computation to determine that optimal query plan.

Not ALL db’s do this, and in some it wont matter if you use a prepared statement because they simply recompute the query plan every time anyways.

But, even if the db engine DOES recompute the query plan over & over you still get all the benefits of avoiding sql injections issues by using them.

I’d say they definitely fall into the category of “best practice”.

Sometimes you run into a situation where you want to add functionality to an existing data type. It might be a string, integer, array or perhaps one of the built in classes that you want to add new functionality to.

Exactly how do you do this ?

You can’t just edit the framework and add new methods to the String type.

And that is probably a good thing. Imagine if every user could edit String and add methods to it. There might be a huge plethora of incompatible implementations of String very rapidly.

Since you cannot edit String what can you do ?

Extension methods!

You CAN create a method type that is almost indistinguishable from a method in the base type itself. First I would suggest creating a Module named with a meaningful name. For a Module to add new method to the String data type I would name the module StringExtensions so its very clear what this module contains.

The in this module you can add GLOBAL methods. And each method in the module should have a signature that uses the extends keyword.

I’ll demonstrate. Suppose we wanted to add a new method to String that checks if the first letter of the string is a digit. We’ll name that method IsDigit.

For create a new desktop project.

Add a Module to the project called StringExtensions.

Then add a method to the StringExtensions module and name the method IsDigit.

The parameters for this method should be

extends aString as String

Note the use of EXTENDS. This is the hint to the compiler that this method should be treated as if it were one of the methods in the framework for the String datatype.

The return type for this method should be Boolean.

The code in the method should be

If aString.Trim = "" Then 
  Return False
End If

If InStr( "0123456789", aString.Left(1) ) > 0 Then
  Return True
End If

return false

The finished method should appear like

Then in the Open, or in 2019r2 the Opening event, you can test the new extension method with code like

Dim s As String

s = "abc"
If s.IsDigit Then
  Break
End If

s = ""
If s.IsDigit Then
  Break
End If

s = "1abc"
If s.IsDigit Then
  Break
End If

The first two cases should return false. And the last one should return true.

The only thing to be careful about with extension methods is :

• you cant name the Module the same as the datatype. The IDE will generally prevent you from doing this.
• you have to not create a new extension that may conflict with one that is in the framework.
• you have to be careful not to create one that will cause the compiler to decide there is an ambiguous use case where it cant decide which one to call. This can happen when you use optional parameters.

But other than that you can extend most datatypes, even ones in the framework, relatively easily.

2019r2 changes a LOT of things.

Some of the things it changed are subtle and search and replace may NOT be the best approach to updating these.

For instance string method like Mid changed from being a 1 based offset for the starting position to a 0 based offset.

If all you do is change mid to middle you will inadvertently cause an off by one error because the start is now 0 based not 1 based.

For instance

var foo as string = "1234"
var midString as string = foo.mid(2,2)
var middleString as string = foo.middle(2,2)

will give you different results in midString and middleString.

There are many other places where the offsets have changed from 1 based to 0 based so be careful updating your code. Make sure you examine all the parameters being used as well to avoid causing yourself a lot of extra work trying to hunt down those off by one errors.