Visual Basic 1002005 [A Developers Notebook] [Electronic resources] نسخه متنی

2.13. Evaluate Conditions Separately with Short-Circuit Logic

In previous versions of
VB, there were two logical operators: And and
Or. Visual Basic 2005 introduces two new operators
that supplement these: AndAlso and
OrElse. These operators work in the same way as
And and Or, except they have
support for short-circuiting, which allows you to evaluate just one
part of a long conditional statement.

Note: With short-circuiting, you can combine multiple conditions
to write more compact code.

2.13.1. How do I do that?

A common programming scenario is the need to evaluate several
conditions in a row. Often, this involves checking that an object is
not null, and then examining one of its properties. In order to
handle this scenario, you need to use nested If
blocks, as shown here:

If MyObject Is Nothing Then
If MyObject.Value > 10 Then
' (Do something.)
End If
End If

It would be nice to combine both of these conditions into a single
line, as follows:

If MyObject Is Nothing And MyObject.Value > 10 Then
' (Do something.)
End If

Unfortunately, this won't work because VB always
evaluates both conditions. In other words, even if
MyObject is Nothing, VB will
evaluate the second condition and attempt to retrieve the
MyObject.Value property, which will cause a
NullReferenceException.

Visual Basic 2005 solves this problem with the
AndAlso and OrElse keywords.
When you use these keywords, Visual Basic won't
evaluate the second condition if the first condition is false.
Here's the corrected code:

If MyObject Is Nothing AndAlso MyObject.Value > 10 Then
' (Do something.)
End If

2.13.2. What about...

...other language refinements? In this chapter,
you've had a tour of the most important VB language
innovations. However, it's worth pointing out a few
of the less significant ones that I haven't included
in this chapter:

The
IsNot keyword allows you to simplify awkward
syntax slightly. Using it, you can replace syntax like If
Not x Is Nothing with the equivalent statement If
x IsNot Nothing.

The
tryCast( ) function allows you to shave a few
milliseconds off type casting code. It works like CType(
) or DirectCast( ), with one
exceptionif the object can't be converted to
the requested type a null reference is returned instead. Thus,
instead of checking an object's type and then
casting it, you can use tryCast( ) right away and
then check if you have an actual object instance.

Unsigned
integers allow you to store numeric values that
can't be negative. That restriction saves on memory
storage, allowing you to accommodate larger numbers. Unsigned numbers
have always been in the .NET Framework, but now VB 2005 includes
keywords for them (UInteger,
ULong, and UShort).