ArcGIS Engine Developer Guide

THE V ISUAL BASIC 6 ENVIRONMENT
Bitwise operators
Since And, Or, and Not are bitwise operators, ensure that all conditions using
them test only for Boolean values, unless, of course, bitwise semantics are what is
intended.
If (Not pFoo Is Nothing) Then
' Valid Foo do something with it
End If
Type suffixes
Refrain from using type suffixes on variables or function names, such as myString$
or Right$(myString), unless they are needed to distinguish 16-bit from 32-bit numbers.
Ambiguous type matching
For ambiguous type matching, use explicit conversion operators, such as CSng,
CDbl, and CStr, instead of relying on VB to pick which one will be used.
Simple image display
Use an ImageControl rather than a PictureBox for simple image display. It is much
more efficient.
Recovery
Statement
Exit Sub
Raise
Resume
Resume
Next
Frequency
usually
often
rarely
very rarely
Meaning
Function failed, pass control
back to caller
Raise a new error code in
the caller's scope
Error condition removed,
reattempt offending
statement
Ignore error and continue
with next statement
Error handling
Always use On Error to ensure fault-tolerant code. For each function that does
error checking, use On Error to jump to a single error handler for the routine that
deals with all exceptional conditions that are likely to be encountered. After the
error handler processes the error—usually by displaying a message—it should
proceed by issuing one of the recovery statements shown on the table to the left.
Error handling in Visual Basic is not the same as general error handling in COM
(see the section ‘Working with HRESULTs’ in this chapter).
Recovery statements issued by error handlers
Event functions
Refrain from placing more than a few lines of code in event functions to prevent
highly fractured and unorganized code. Event functions should simply dispatch to
reusable functions elsewhere.
Memory management
To ensure efficient use of memory resources, the following points should be
considered:
• Unload forms regularly. Do not keep many forms loaded but invisible since
this consumes system resources.
• Be aware that referencing a form-scoped variable causes the form to be
loaded.
• Set unused objects to Nothing to free up their memory.
• Make use of Class_Initialize and Class_Terminate to allocate and destroy
resources.
Chapter 4 • Developer environments • 81