Conditions

If operator

9.0
If(condition > value, "True", "False")

We can use the If operator instead of If...Then...Else..End If statement blocks.

Consider the following example:

If 10 > 9 Then
     MsgBox("True")
Else
     MsgBox("False")
End If

is the same as

MsgBox(If(10 > 9, "True", "False"))

If() uses short-circuit evaluation, which means that it will only evaluate the arguments it uses. If the condition is false (or a Nullable that is Nothing), the first alternative will not be evaluated at all, and none of its side effects will be observed. This is effectively the same as C#'s ternary operator in the form of condition?a:b.

This is especially useful in avoiding exceptions:

Dim z As Integer = If(x = 0, 0, y/x)

We all know that dividing by zero will throw an exception, but If() here guards against this by short-circuiting to only the expression that the condition has already ensured is valid.

Another example:

Dim varDate as DateTime = If(varString <> "N/A", Convert.ToDateTime(varString), Now.Date)

If varString <> "N/A" evaluates to False, it will assign varDate's value as Now.Date without evaluating the first expression.

9.0

Older versions of VB do not have the If() operator and have to make do with the IIf() built-in function. As it's a function, not an operator, it does not short-circuit; all expressions are evaluated, with all possible side-effects, including performance penalties, changing state, and throwing exceptions. (Both of the above examples that avoid exceptions would throw if converted to IIf.) If any of these side effects present a problem, there's no way to use an inline conditional; instead, rely on If..Then blocks as usual.

IF...Then...Else

Dim count As Integer = 0
Dim message As String

If count = 0 Then
    message = "There are no items."
ElseIf count = 1 Then
    message = "There is 1 item."
Else
    message = "There are " & count & " items."
End If