What is the difference between And and AndAlso in VB.NET?

Question

The And operator evaluates both sides, where AndAlso evaluates the right side if and only if the left side is true.

An example:

If mystring IsNot Nothing And mystring.Contains("Foo") Then
  ' bla bla
End If

The above throws an exception if mystring = Nothing

If mystring IsNot Nothing AndAlso mystring.Contains("Foo") Then
  ' bla bla
End If

This one does not throw an exception.

So if you come from the C# world, you should use AndAlso like you would use &&.

More info here: http://www.panopticoncentral.net/2003/08/18/the-ballad-of-andalso-and-orelse/

The And operator will check all conditions in the statement before continuing, whereas the Andalso operator will stop if it knows the condition is false. For example:

if x = 5 And y = 7

Checks if x is equal to 5, and if y is equal to 7, then continues if both are true.

if x = 5 AndAlso y = 7

Checks if x is equal to 5. If it's not, it doesn't check if y is 7, because it knows that the condition is false already. (This is called short-circuiting.)

Generally people use the short-circuiting method if there's a reason to explicitly not check the second part if the first part is not true, such as if it would throw an exception if checked. For example:

If Not Object Is Nothing AndAlso Object.Load()

If that used And instead of AndAlso, it would still try to Object.Load() even if it were nothing, which would throw an exception.

Interestingly none of the answers mentioned that And and Or in VB.NET are bit operators whereas OrElse and AndAlso are strictly Boolean operators.

Dim a = 3 OR 5 ' Will set a to the value 7, 011 or 101 = 111
Dim a = 3 And 5 ' Will set a to the value 1, 011 and 101 = 001
Dim b = 3 OrElse 5 ' Will set b to the value true and not evaluate the 5
Dim b = 3 AndAlso 5 ' Will set b to the value true after evaluating the 5
Dim c = 0 AndAlso 5 ' Will set c to the value false and not evaluate the 5

Note: a non zero integer is considered true; Dim e = not 0 will set e to -1 demonstrating Not is also a bit operator.

|| and && (the C# versions of OrElse and AndAlso) return the last evaluated expression which would be 3 and 5 respectively. This lets you use the idiom v || 5 in C# to give 5 as the value of the expression when v is null or (0 and an integer) and the value of v otherwise. The difference in semantics can catch a C# programmer dabbling in VB.NET off guard as this "default value idiom" doesn't work in VB.NET.

So, to answer the question: Use Or and And for bit operations (integer or Boolean). Use OrElse and AndAlso to "short circuit" an operation to save time, or test the validity of an evaluation prior to evaluating it. If valid(evaluation) andalso evaluation then or if not (unsafe(evaluation) orelse (not evaluation)) then

Bonus: What is the value of the following?

Dim e = Not 0 And 3

If Bool1 And Bool2 Then

Evaluates both Bool1 and Bool2

If Bool1 AndAlso Bool2 Then

Evaluates Bool2 if and only if Bool1 is true.

Just for all those people who say side effects are evil: a place where having two side effects in one condition is good would be reading two file objects in tandem.

While File1.Seek_Next_Row() And File2.Seek_Next_Row()
    Str1 = File1.GetRow()
    Str2 = File2.GetRow()
End While

Using the And ensures that a row is consumed every time the condition is checked. Whereas AndAlso might read the last line of File1 and leave File2 without a consumed line.

Of course the code above wouldn't work, but I use side effects like this all the time and wouldn't consider it "bad" or "evil" code as some would lead you to believe. It's easy to read and efficient.

AndAlso is much like And, except it works like && in C#, C++, etc.

The difference is that if the first clause (the one before AndAlso) is true, the second clause is never evaluated - the compound logical expression is "short circuited".

This is sometimes very useful, e.g. in an expression such as:

If Not IsNull(myObj) AndAlso myObj.SomeProperty = 3 Then
   ...
End If

Using the old And in the above expression would throw a NullReferenceException if myObj were null.