What are the implicit type conversion rules in vba?

Question

I know the following is possible in vba:

If Len(str) Then

Len(str) will evaluate to false if Len(str) is 0, true otherwise.

What are the exact conversion rules for vba? Please provide a link to official documentation if possible.

Answer 1

A boolean variable can have only two states, True and False. That's true in all programming languages (except if a nullable datatype exists, eg bool? in C# where the variable can have also the value null signaling it is not defined).

In VBA, a boolean is stored as 16-bit integer. False is defined as 0 and True as -1. Thats similar in other languages, simply because 0 has all bits set to 0 and -1 has all bits set to 1. But this is an implementation detail and your programming should not care about it. You deal with True and False and Boolean algebra using AND, OR and NOT.

Implicit conversion rules are straight forward: Numeric data types are converted to boolean False if they are 0 and to True in all other cases (that is, if the value has any bit set to 1). Converting a boolean value to a number results in 0 resp. -1.

Dim i As Integer
Dim b As Boolean    
i = 3
b = i   ' Implicit conversion from 3 to TRUE
i = b   ' Implicit conversion from TRUE to -1

Strings are converted to numbers first, and if that fails, you get a runtime error. Dates are implemented as numbers (Double) in VBA, so every date/time is True except the date+time that is represented as 0 (that is 30 Dec 1899 00:00)

But from my point of view (and >30 years of programming), implicit conversions are evil and should be avoided. They are source of numerous errors, they lead to code that is harder to read - and there is simply no reason to rely on implicit conversations. There is no reason to write If len(str) Then. You want to check if the length of a string is greater than 0, so write it: If len(str) > 0 Then. Always.

Have a look to the following example: Obviously, the VBA runtime need to execute an implicit conversion, but can you tell the outcome?

Dim i As Integer
Dim b As Boolean  
i = 3
b = i
If b = i Then Debug.Print "B=I"

---

Here you can find the official documentation from Microsoft: https://learn.microsoft.com/en-US/office/vba/language/reference/user-interface-help/boolean-data-type

Answer 2

I came across this in a recent textbook I was reading on C++ - I'll add a link at the end. Although a different language, both have similarities to some degree. One of these similarities is what you call 'implicit type conversion' or more technically, type casting.

For any Boolean expression in your project when you compile your VBA project (Debug > Compile VBAProject) any nonzero value will be treated as the value true and will treat the value 0 as false. Essentially this comes down to the how the compiler treats these statesments and also to operator precedence rules as well as boolean expression rules.

For further reading please see below links:

1. Operator precedence
2. Boolean data type

In C++ (as far as I am aware) this term is called copy initialisation, you can read more here:

1. Copy initialization

The book that brought this deep thought recently:

Problem Solving with C++: The Object of Programming

I know this doesn't necessarily answer your question, but hopefully some of the references help.