(OrElse and Or) and (AndAlso and And) - When to use?

Question

Or/And will always evaluate both¹ the expressions and then return a result. They are not short-circuiting.

OrElse/AndAlso are short-circuiting. The right expression is only evaluated if the outcome cannot be determined from the evaluation of the left expression alone. (That means: OrElse will only evaluate the right expression if the left expression is false, and AndAlso will only evaluate the right expression if the left expression is true.)

Assuming that no side effects occur in the expressions and the expressions are not dependent (and any execution overhead is ignored), then they are the same.

However, in many cases it is that the expressions are dependent. For instance, we want to do something when a List is not-Nothing and has more than one element:

If list IsNot Nothing AndAlso list.Length > 0 Then .. 'list has stuff

This can also be used to avoid an "expensive" computation (or side-effects, ick!):

If Not Validate(x) OrElse Not ExpensiveValidate(x) Then .. 'not valid

Personally, I find that AndAlso and OrElse are the correct operators to use in all but the 1% - or less, hopefully! - of the cases where a side-effect is desired.

Happy coding.

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¹ An Exception thrown in the first expression will prevent the second expression from being evaluated, but this should hardly be surprising ..

Besides the short-circuiting mentioned in the other answers, Or/And are usable as bitwise operators where OrElse/AndAlso are not. Bitwise operations include combining values of Flags enums, such as the FileAttributes enumeration where you might indicate a file is both read only and hidden by FileAttributes.ReadOnly Or FileAttributes.Hidden

The difference is that OrElse and AndAlso will short-circuit based on the first condition, meaning that if the first condition doesn't pass, the second (or more) conditions will not be evaluated. This is particularly useful when one of the conditions might be more intensive than the other.

Example where Or is fine (both conditions evaluated):

If Name = "Fred" Or Name = "Sam" Then

It really doesn't matter which way around they are evaluated

The following AndAlso is useful because the second condition might fail

If Not SomeObject Is Nothing AndAlso CheckObjectExistsInDatabase(SomeObject) Then

This allows for the first condition to check whether the object has been set and only if it has been set will go and check the database (or some other task). If this had been a plain And keyword, both would be evaluated.

@Gideon - glad someone pointed that out. Here is a simple test that shows the dramatic impact of AndAlso:

    Dim tm As New Stopwatch
    Const tries As Integer = 123456
    Dim z As Integer = 0
    Dim s() As String = New String() {"0", "one"}

    Debug.WriteLine("AndAlso")
    For x As Integer = 0 To s.Length - 1
        z = 0
        tm.Restart() 'restart the stopwatch
        For y As Integer = 0 To tries
            If s(x) = x.ToString AndAlso s(x) = y.ToString Then '<<<<<<<<<<
                z += 1
            End If
        Next
        tm.Stop()
        Debug.WriteLine(x.ToString.PadRight(3, " "c) & z.ToString.PadRight(10, " "c) & tm.Elapsed.ToString)
    Next

    Debug.WriteLine("And")
    For x As Integer = 0 To s.Length - 1
        z = 0
        tm.Restart() 'restart the stopwatch
        For y As Integer = 0 To tries
            If s(x) = x.ToString And s(x) = y.ToString Then '<<<<<<<<<<
                z += 1
            End If
        Next
        tm.Stop()
        Debug.WriteLine(x.ToString.PadRight(3, " "c) & z.ToString.PadRight(10, " "c) & tm.Elapsed.ToString)
    Next