{"id":406,"date":"2019-11-07T11:02:12","date_gmt":"2019-11-07T18:02:12","guid":{"rendered":"https:\/\/www.great-white-software.com\/blog\/?p=406"},"modified":"2019-11-19T09:49:34","modified_gmt":"2019-11-19T16:49:34","slug":"extension-methods","status":"publish","type":"post","link":"https:\/\/www.great-white-software.com\/blog\/2019\/11\/07\/extension-methods\/","title":{"rendered":"Extension methods"},"content":{"rendered":"\n

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.<\/p>\n\n\n\n

Exactly how do you do this ?<\/p>\n\n\n\n

You can’t just edit the framework and add new methods to the String<\/em> type.<\/p>\n\n\n\n

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

Since you cannot edit String<\/em> what can you do ?<\/p>\n\n\n\n

Extension methods!<\/p>\n\n\n\n

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<\/em> data type I would name the module StringExtensions<\/em> so its very clear what this module contains.<\/p>\n\n\n\n

The in this module you can add GLOBAL methods. And each method in the module should have a signature that uses the extends<\/em> keyword.<\/p>\n\n\n\n

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<\/em>. <\/p>\n\n\n\n

For create a new desktop project.<\/p>\n\n\n\n

Add a Module to the project called StringExtensions<\/em>.<\/p>\n\n\n\n

Then add a method to the StringExtensions<\/em> module and name the method IsDigit<\/em>.<\/p>\n\n\n\n

The parameters for this method should be<\/p>\n\n\n\n

extends aString as String<\/code><\/pre>\n\n\n\n
Note the use of EXTENDS<\/em>. 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<\/em> datatype.<\/p>\n\n\n\n
The return type for this method should be Boolean<\/em>.<\/p>\n\n\n\n
The code in the method should be<\/p>\n\n\n\n
If aString.Trim = \"\" Then \n  Return False\nEnd If\n\nIf InStr( \"0123456789\", aString.Left(1) ) > 0 Then\n  Return True\nEnd If\n\nreturn false\n\n<\/code><\/pre>\n\n\n\n
The finished method should appear like<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
Then in the Open, or in 2019r2 the Opening event, you can test the new extension method with code like <\/p>\n\n\n\n
\nDim s As String\n\ns = \"abc\"\nIf s.IsDigit Then\n  Break\nEnd If\n\ns = \"\"\nIf s.IsDigit Then\n  Break\nEnd If\n\ns = \"1abc\"\nIf s.IsDigit Then\n  Break\nEnd If\n<\/code><\/pre>\n\n\n\n
The first two cases should return false. And the last one should return true.<\/p>\n\n\n\n
The only thing to be careful about with extension methods is :<\/p>\n\n\n\n