In VB6 what is the difference between Property Set and Property Let?

Question

I have just created several Property Set methods, and they didn't compile. When I changed them to Property Let, everything was fine.

I have since studied the documentation to find the difference between Property Set and Property Let, but must admit to being none the wiser. Is there any difference, and if so could someone offer a pointer to a proper explanation of it?

Solution 1

Property Set is for objects (e.g., class instances)

Property Let is for "normal" datatypes (e.g., string, boolean, long, etc.)

Solution 2

Property Let is more versatile than Property Set. The latter is restricted to object references only. If you have this property in a class

Private m_oPicture          As StdPicture

Property Get Picture() As StdPicture
    Set Picture = m_oPicture
End Property

Property Set Picture(oValue As StdPicture)
    Set m_oPicture = oValue
End Property

Property Let Picture(oValue As StdPicture)
    Set m_oPicture = oValue
End Property

You can call Property Set Picture with

Set oObj.Picture = Me.Picture

You can call Property Let Picture with both

Let oObj.Picture = Me.Picture
oObj.Picture = Me.Picture

Implementing Property Set is what other developers expect for properties that are object references but sometimes even Microsoft provide only Property Let for reference properties, leading to the unusual syntax oObj.Object = MyObject without Set statement. In this case using Set statement leads to compile-time or run-time error because there is no Property Set Object implemented on oObj class.

I tend to implement both Property Set and Property Let for properties of standard types -- fonts, pictures, etc -- but with different semantics. Usually on Property Let I tend to perform "deep copy", i.e. cloning the StdFont instead of just holding a reference to the original object.

Solution 3

Property Set is for object-like variables (ByRef) whereas Property Let is for value-like variables (ByVal)

Comments

[Bob77]

It is unrelated to WithEvents, it is for returning object references as properties. There are many times when you want to do this, most often when you have any kind of object hierarchy.

[Bob77]

It isn't a ByRef vs. ByVal issue, but is strictly used for returning object references to the caller.

[mwolfe02]

Wrong. Property Get is used for returning object references as properties, not Property Set.

[mwolfe02]

msdn.microsoft.com/en-us/library/aa266202(VS.60).aspx: Like a Function and Property Get procedure, a Property Set procedure is a separate procedure that can take arguments, perform a series of statements, and change the value of its arguments. However, unlike a Function and Property Get procedure, both of which return a value, you can only use a Property Set procedure on the left side of an object reference assignment (Set statement).

[mwolfe02]

It's true that it's not a ByRef/ByVal issue. But it is not used for returning object references to the caller. For that you use Property Get the same as you would for returning any other class property (there is no object-specific version for returning class properties).

[MarkJ]

+1 Correct that Property Set defines a property that can be used on the left hand side of a Set statement. I do think the mention of WithEvents is a bit confusing though, it's not really related to explaining the difference. Property Set can be useful for 1:1 relationships between objects that can be changed dynamically at runtime, i.e. beyond WithEvents.

[Bob77]

Yes, I had that exactly backwards, it's for assigning to an object property - duh, really bad goof there.

[Bob77]

Yes, for "setting" a property - I had this 180 degrees backwards, mea culpa. In the end it's as simple as reading the manual as pointed out above.

[mwolfe02]

@Bob: +1 for admitting a mistake. I apologize if my tone came across as harsh, but I wanted to clear up the technical issue. And while we're issuing mea culpas, I'll admit my original wording unnecessarily confused the issue of Property Set and WithEvents. When I was first learning about classes in VB6/VBA I had trouble understanding when you would want to use the Property Set statement. I simply wanted to provide the OP with a common example. To suggest it was the 'only' situation where Property Set would be useful was a mistake on my part. Hopefully the new wording is clearer.

[Panzercrisis]

Assuming you have both Property Set and Property Let for an object type, and assuming the implementing code is the same (like in your example), does VB6 have any built-in difference in what it'll actually do when the developer calls either property? For instance, will VB6 implicitly decide to suddenly pass the object by value, instead of by reference, if the developer calls the Let property, or are there any other little nuances like that? (I know you said the ones you like to write have the separate semantics, but I'm not sure if the language does anything different on its own.)

[wqw]

In VB6 there is no "pass the object by value" option for params. Using ByVal oValue As StdPicture vs ByRef oValue As StdPicture changes the way the object reference is passed. It's like StdPicture *oValue vs StdPicture **oValue in C/C++ -- always pointers but in the second case one has the ability to change the reference (e.g. assign ref to a new object) inside the procedure and the caller can receive this new reference. So there is no "copy" option per se and I'm usually using Clone method (e.g. on IFont interface) inside Property Let to manually impl "deep copy" semantics.

[Panzercrisis]

For reference types, I mean passing the reference by value, like in VB.NET/C#.

[Marcelo Scofano Diniz]

I adopted @wqw's two last paragraphs as my personal coding rule since I read it and had the need of deep copy Collections and Class instances; it facilitates too much on a glance know if it's only a pointer or a clone...