VB.NET 1.1 Tutorial - Conditional If...Then...Else Statement

The conditional statement If, selects one of a number of possible statements for execution based on the value of a boolean expression. The If statement comes in one of two forms

If conditional-test Then statement

or

If conditional-test Then
  multiline-statements
End If 

or

If conditional-test Then
  true-statements
Else
  false-statements
End If 

Where the conditional-test evaluates to True or False and the statements are blocks of code to be executed.

This is best shown in a simple example:

Dim Number As Integer
Number = 53   ' Initialize variable

If Number >= 0 Then
  Console.WriteLine( "Number is positive" )
Else
  Console.WriteLine( "Number is negative" )
End If 

So what does this mean? Simply put, it means that if the Integer in the variable Number is greater than or equal to zero, then the application will write the line "Number is positive" to the console. Otherwise the application will write the line "Number is negative" to the console.

Although the output is not technically correct, it shows how to use an If statement. We can fix this allowing us to show nested If statements.

Dim Number As Integer
Number = 53   ' Initialize variable

If Number >= 0 Then
  If Number > 0 Then
    Console.WriteLine( "Number is positive" )
  Else
    Console.WriteLine( "Number is zero" )
  End If
Else
  Console.WriteLine( "Number is negative" )
End If 

Alternatively, this can also be written using the Else If statement:

Dim Number As Integer
Number = 53   ' Initialize variable

If Number = 0 Then
  Console.WriteLine( "Number is zero" )
Else If Number > 0 Then
  Console.WriteLine( "Number is positive" )
Else
  Console.WriteLine( "Number is negative" )
End If 

Shorthand Notation

Be wary of using single line If statements. It is easy to get confused as to what the code is really doing. Here is an example:

If A > 10 Then A = A + 1 : B = B + A : C = C + B 

It is not hard to see what order this code will be evaluated in, the above code is equivalent to:

If A > 10 Then
  A = A + 1
  B = B + A
  C = C + B
End If 

By use of the a colon (:), we can almalgamate more than one line of code into one line of code.

References

For more information on If...Then...Else Statements, visit MSDN at microsoft here.

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