From Xojo Documentation
Conditionally executes a group of statements, depending on the value of a boolean expression.
If condition Then
[ElseIf condition-n Then
If condition Then statement [Else] [statement]
|condition||Required. A boolean, numeric, or string expression that evaluates to True or False.|
|statements||Optional. One or more statements that are executed if condition is True.|
|condition-n||Optional. Same as condition.|
|elseIfStatements||Optional. One or more statements executed that are executed if the associated condition-n is True.|
|elseStatements||Optional. One or more statements executed if no previous condition or condition-n expression is True.|
For conditional compilation, see ＃If...＃Endif.
When executing an If statement, the condition is tested. If condition is True, the statements associated with the If statement following the Then statement are executed. If condition is False and an Else clause follows, its statements will be executed. If condition is False and there is no Else clause or it is preceded by an ElseIf statement, the condition following the ElseIf statement is tested. After executing the statements following Then, ElseIf or Else execution continues with the statement that follows End If.
When writing an If statement, you can use a couple of shortcuts. First, you can write only the statements that are executed if the condition is True. Then select the statements, display the Code Editor's contextual menu, and choose Wrap in If...End if. The Code Editor will then add an If statement above the selected lines and an End If statement below them. All you need to do is write the condition.
The other shortcut is this: Write the "If condition" portion of the statement and then press Ctrl+Shift+Return (Command+Shift+Return on Mac OS X). The Code Editor will then add the "'Then'" and the "'End if"' and put the cursor between them.
An If statement can be written on one line, provided the code that follows the Then and Else statements can be written on one line. Using this syntax, you omit the End if statement. For example, the following examples are valid:
If error = 123 Then MsgBox("An error occurred.") Else MsgBox("Never mind!")
The following is not valid because the Then clause requires two lines:
You can declare variables using the Dim statement inside an If statement. However, such variables go out of scope after the End If statement. For example:
Dim a As String
a = "Whoops! An error occured."
MsgBox(a) // Error: This method or property does not exist because a has gone out of scope
This example shows a simple If statement:
This example shows an If statement that includes the use of ElseIf and Else clauses:
Dim digits As Integer
theNumber = 33
If theNumber < 10 Then
digits = 1
ElseIf theNumber < 100 Then
digits = 2
ElseIf theNumber < 1000 Then
digits = 3
digits = 4
This example demonstrates using And to combine multiple conditions:
If theNumber > 10 And theNumber < 60 Then
MsgBox("Out of range")
By default, this code will display "In Range" because 50 is between 10 and 60.
This example uses Or to combine multiple conditions:
If theNumber <= 10 Or theNumber >= 60 Then
MsgBox("Out of range")
This code will display "Out of range" because 5 is less than 10.