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

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.

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.

Also see Stack Overflow question: Should I always use the AndAlso and OrElse operators?.

Also: A comment for those who mentioned using And if the right side of the expression has a side-effect you need:

If the right side has a side effect you need, just move it to the left side rather than using "And". You only really need "And" if both sides have side effects. And if you have that many side effects going on you're probably doing something else wrong. In general, you really should prefer AndAlso.

Use And and Or for logical bit operations, e.g. x% = y% Or 3

AndAlso and OrElse are for If statements:

If x > 3 AndAlso x <= 5 Then

is same as

If (x > 3) And (x <= 5) Then

The way I see it...

Also it's worth noting that it's recommended (by me) that you enclose equations with logical operators in If statements, so they don't get misinterpreted by the compiler, for example:

If x = (y And 3) AndAlso ...

To understand with words not cods:

Use Case:
With “And” the compiler will check all conditions so if you are checking that an object could be “Nothing” and then you are checking one of it’s properties you will have a run time error.
But with AndAlso with the first “false” in the conditions it will checking the next one so you will not have an error.

The And / AndAlso Or / OrElse are actually quite useful. Consider this code, where the function DoSomethingWihtAandB may not set A and/or B if it returns False:

Dim A,B,C,D As Object
If DoSomethingWithAandB(A,B)=True And A=1 And B=2 Then C=3

It will crash if DoSomethingWithAandB returns False, because it will continue to evaluate after the And and A and B will equal Nothing

Dim A,B,C,D As Object
If DoSomethingWithAandB(A,B)=True AndAlso A=1 AndAlso B=2 Then C=3

This won't crash if DoSomethingWithAandB returns False because it will stop evaluating at DoSomethingWithAandB(A,B)=True, because False is returned. The AndAlso prevents any further evaluation of the conditions (as the first condition failed). OrElse works the same way. The first True evaluated in the logic chain will stop any further evaluation.