And Not In VB NET

Question

I m having the exciting task of finding out about VB NET s  lt  gt  and Not operators   Not - I m assuming by my small use of it - is the functional equivalent of   in languages such as C  and  lt  gt  being equivalent of      In VB NET a common problem is doing Boolean expressions against objects that don t have a reference  it appears   So if we do  If Request QueryString  MyQueryString    lt  gt  Nothing Then   This will actually fail if the query string doesn t exist   Why  I don t know   The way that it s done by older coders is as follows   If Not Request QueryString  MyQueryString   Is Nothing Then   And this tends to work   To me they re functionally equivalent though operators tend to do different comparisons dependent on certain factors such as operator precedence  why it doesn t work in this case however  I do not know  and neither have I found any relevant material   I ask this as I m having to write standards documentation and we re determining the use of either the Not or  lt  gt    Any ideas on which way around it should be  or you should do it

User · Answer

Just use which sounds better  I d use the first approach  though  because it seems to have fewer operations

User · Answer

If you need to know if the variable exists use Is IsNot Nothing   Using  lt   requires that the variable you re evaluating have the   lt    operator defined  Check out   Dim b As HttpContext  If b  lt  gt  Nothing Then          End If   and the resultant error  Error   1   Operator   lt  gt   is not defined for types  System Web HttpContext  and  System Web HttpContext

User · Answer

Here s the technical answer  expanding on Rowland Shaw s answer    The Is keyword checks whether the two operands are references to the same object memory  and only returns true if this is the case  I believe it is functionally equivalent to Object ReferenceEquals  The IsNot keyword is simply shorthand syntax for writing Not     Is      nothing more   The    equality  operator compares values and in this case  as in many others  is equivalent to String Equals  Now  the  lt    inequality  operator does not quite have the same analogy as the Is and IsNot keywords  since it can be overriden separately from the   operator depending on the class  I believe the case should always be that it returns the logical inverse of the   operator  and certainly is in the case of String   and just allows for a more efficient comparison to made when testing for inequality rather than equality   When dealing with strings  unless you actually mean to compare references  always use the   operator  or String Equals I suppose   In your case  because you are testing for null  Nothing   it seems you need to use the Is or IsNot keyword  the equality operator will fail because it can t compare the values of null objects   Syntactically  the IsNot keyword is a bit nicer here  so go with that

User · Answer

Is is not the same as   -- Is compares the references  whilst   will compare the values   If you re using v2 of the  Net Framework  or later   there is the IsNot operator which will do the right thing  and read more naturally

User · Answer

in fact the Is is really good  since to the developpers  you may want to override  the operator     to compare with the value  say you have a class A  operator    of A is to compare some of the field of A to the parameter  then you will be in trouble in c  to verify whether the object of A is null with following code       A a   new A            if  a    null  it will totally wrong  you always need to use if  object a    null  but in vb net you cannot write in this way  you always need to write     if not a is nothing then or     if a isnot nothing then   which just as Christian said  vb net does not  expected  anything

User · Answer

I m a total noob  I came here to figure out VB s  not equal to  syntax  so I figured I d throw it in here in case someone else needed it    lt  If Not boolean variable  gt Do this if boolean variable is false lt  End If  gt

User · Answer

The C  and VB NET compilers often generate different IL for operations that are apparently equivalent in both languages  It just so happens that C  does the  expected  thing when you write stringvar    null  but VB NET does not  To get the same effect in VB NET you have to force true reference equality with the Is operator

User · Answer

I have always used the following   If Request QueryString  MyQueryString   IsNot Nothing Then   But only because syntactically it reads better   When testing for a valid QueryString entry I also use the following   If Not String IsNullOrEmpty Request QueryString  MyQueryString    Then   These are just the methods I have always used so I could not justify their usage other than they make the most sense to me when reading back code

User · Answer

I think your question boils down to  the difference between  Is and    and also  IsNot and  lt       The answer in both cases is the same      and  lt  gt  are implicitly defined for value types and you can explicitly define them for your types   Is and IsNot are designed for comparisons between reference types to check if the two references refer to the same object   In your example  you are comparing a string object to Nothing  Null  and since the    lt  gt  operators are defined for strings  the first example works  However  it does not work when a Null is encountered because Strings are reference types and can be Null  The better way  as you guessed  is the latter version using Is IsNot

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