Explicit clearing of variables
Code review rules
You can use Project Analyzer to require explicit clearing of object variables,
deallocation of dynamic arrays and releasing of Win API resource handles. This group of review rules, part of the problem detection feature, helps you point out possible resource leaks, memory that is kept allocated after use and objects that stay live too long.
- Object variables: require explicit clearing
- Dynamic array variables: require explicit deallocation
- Variables storing API resource handles: require explicit release call (more about API leaks)
Object variables and dynamic arrays
Visual Basic automatically clears objects and deallocates arrays.
In VB Classic, an object is destroyed when there are no more references to it.
In .NET, an object is destroyed by a process called garbage collection.
This process happens asynchronously. You can expect a .NET object to get destroyed after
is not used any more, but you cannot determine the exact time when this happens.
Whether you use .NET or VB Classic, you have two approaches to
- Rely on automatic clearing. You can leave the clearing to VB and
wait until the variables fall out of scope.
A local variable, as well as a parameter, goes out of scope when you exit the procedure.
In VB.NET, a local variable may also get garbage collected when its value is no longer used in the procedure (which may be before procedure exit).
An object-level variable goes out of scope when the object is
Other variables go out of scope when the program ends.
- This way is easy but there
is the threat that your objects are not released, your arrays are not
deallocated, and you eventually consume more system resources than
necessary. The system doesn't free an object if a variable still holds a reference to it.
Similarly, a dynamic array is not deallocated if it hasn't fallen
out of scope. If so, you may have a resource leak at hand,
and your program might even end up with an Out of memory error.
- Require explicit clearing. To be sure, you clear your
variables explicitly. This way you control the time when an object or an array
has done its duties and can go.
Here are the ways to explicitly clear your variables:
Set obj = Nothing ' VB 3-6 object variables
obj = Nothing ' VB.NET object variables
Erase myArray ' Dynamic arrays
Next follows an explanation of the various options on the Clearing
tab in Problem Options.
Selection of the variables to monitor
- Require clearing of object variables. If this option is checked,
all object variables (and arrays of object variables) are subject to the clearing requirement, except for auto-instantiated object variables declared As New in VB Classic.
- Require clearing of auto-instantiated object variables. If this
option is checked, all object variables (and arrays of object variables) declared As New in VB Classic
are subject to the scrutiny.
- Require deallocation of dynamic non-object arrays. If this option
is checked, all dynamic arrays are subject to the deallocation rule.
A dynamic array is one whose declaration doesn't define the size of the
For dynamic array allocation, the following semantics are used:
ReDim is the equivalent of assignment and Erase is the
equivalent of clearing.
- Require releasing of Win API resource handles. If this option is checked, all resource handle variables are subject to the API handle release rules. The following semantics are used: A handle create call is the equivalent of assignment and a handle release call is the equivalent of clearing.
Declare location and clearing rule
The clearing rules available are based on the scope of the variable
in question. The declare locations Standard module and
Class/form/structure are the more important ones. Procedure local
variables and ByVal parameters can also be monitored for the sake of
- No requirements. Ignore variables in this scope.
- Require clearing. Require one clearing statement for each variable.
It can be anywhere.
- Require clearing inside module/class/form/structure. Require one clearing
statement for each variable. It must be inside the same module, class, form or
structure where the variable is declared. That is, the declarer must take care of the clearing.
- Require clearing at destruction/unload. Require one clearing
statement for each variable. The clearing statement must exist either in
the destructor (.NET Finalize method), Form_Unload or Class_Terminate event (VB Classic). It can also exist in any procedure that these events call,
provided that the call tree stays inside the same class/form/structure.
- Require clear count >= assignment count. Require at least one
as many clearing statement for each variable as there are assignment statements.
The statements can be located anywhere.
Local object variables in .NET. Due to the nature of garbage collection (GC) in .NET, you may want to avoid explicit clearing of local object variables while requiring clearing for non-local ones.
A local variable is subject to garbage collection after the line where it is mentioned the last time. After this line it's dead to .NET and ready to be garbage collected. If you explicitly clear the variable at the end of a long procedure you may actually be keeping it alive for a longer time than expected. By letting .NET handle the clearing of local objects you may end up with more efficient GC.
The following variables are not monitored: Static arrays. Object fields in user-defined Types (VB Classic).
Variants in VB Classic and Objects in .NET, as they may contain
non-object values. String variables, even though it makes sense to clear
string variables to deallocate memory. Properties are not monitored
The comment directive parameter for this set of rules is CLEAR. They are also covered by the parameter STYLE.
Code review rules
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