Checking if an object is null in C

Question

I would like to prevent further processing on an object if it is null   In the following code I check if the object is null by either   if   data Equals null     and  if  data    null    However  I receive a NullReferenceException at dataList Add data   If the object was null  it should never have even entered the if-statement   Thus  I m asking if this is proper way of checking if an object is null   public List lt Object gt  dataList  public  bool AddData ref Object data      bool success   false      try                  I ve also used  if  data    null   which hasn t worked either         if   data Equals null                          NullReferenceException occurs here                dataList Add data              success   doOtherStuff data                       catch  Exception e                throw new Exception e ToString               return success      If this is the proper way of checking if the object is null  what am I doing wrong  how can I prevent further processing on the object to avoid the NullReferenceException

User · Answer

You can try like below  public List lt Object gt  dataList  public  bool AddData ref Object data  bool success   false  try       if  data    null               dataList Add data          success   doOtherStuff data           catch  Exception e        throw new Exception e ToString       return success

User · Answer

As others have already pointed out  it s not data but rather likely dataList that is null  In addition to that     catch-throw is an antipattern that almost always makes me want to throw up every time that I see it  Imagine that something goes wrong deep in something that doOtherStuff   calls  All you get back is an Exception object  thrown at the throw in AddData    No stack trace  no call information  no state  nothing at all to indicate the real source of the problem  unless you go in and switch your debugger to break on exception thrown rather than exception unhandled  If you are catching an exception and just re-throwing it in any way  particularly if the code in the try block is in any way nontrivial  do yourself  and your colleagues  present and future  a favor and throw out the entire try-catch block  Granted  throw  is better than the alternatives  but you are still giving yourself  or whoever else is trying to fix a bug in the code  completely unnecessary headaches  This is not to say that try-catch-throw is necessarily evil per se  as long as you do something relevant with the exception object that was thrown inside the catch block   Then there s the potential problems of catching Exception in the first place  but that s another matter  particularly since in this particular case you throw an exception   Another thing that strikes me as more than a little dangerous is that data could potentially change value during the execution of the function  since you are passing by reference  So the null check might pass but before the code gets to doing anything with the value  it s changed - perhaps to null  I m not positive if this is a concern or not  it might not be   but it seems worth watching out for

User · Answer

in C   gt  7 0 use if  obj is null      This will ignore any    or    defined by the object  unless of course you want to use them      For not null use if  obj is object  and from C  9 you can also use if  obj is not null

User · Answer

The problem in this case is not that data is null  It is that dataList itself is null   In the place where you declare dataList you should create a new List object and assign it to the variable   List lt object gt  dataList   new List lt object gt

User · Answer

Jeffrey L Whitledge is right  Your  dataList  -Object itself is null   There is also another problem with your code  You are using the ref-keyword  which means the argument data cannot be null  The MSDN says      An argument passed to a ref parameter must first be initialized  This differs from out  whose arguments do not have to be explicitly initialized before they are passed   It s also not a good idea to use generics with the type  Object    Generics should avoid boxing unboxing and also ensure type safety  If you want a common type make your method generic  Finally your code should look like this   public class Foo lt T gt  where T   MyTypeOrInterface          public List lt T gt  dataList   new List lt T gt            public bool AddData ref T data            bool success   false          try             dataList Add data                                success   doOtherStuff data             catch  Exception e              throw new Exception e ToString                       return success                 private bool doOtherStuff T data

User · Answer

Whenever you are creating objects of class you have to check the whether the object is null or not using the below code   Example  object1 is object of class  void myFunction object1      if object1  null           object1 value1   If we miss the null check then here we get the Null Reference exception

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I did more simple  positive way  and it seems to work well   Since any kind of  object  is at least an object       if  MyObj is Object                      Do something      for example                if  MyObj is Button                  MyObj Enabled   true

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C  6 has monadic null checking     before   if  points    null        var next   points FirstOrDefault        if  next    null  amp  amp  next X    null  return next X       return -1    after   var bestValue   points  FirstOrDefault    X    -1

User · Answer

As of C  8 you can use the  empty  property pattern  with pattern matching  to ensure an object is not null   if  obj is                obj  is not null here     This approach means  if the object references an instance of something   i e  it s not null    You can think of this as the opposite of  if  obj is null      which will return true when the object does not reference an instance of something   For more info on patterns in C  8 0 read here

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public static bool isnull object T            return T    null   true   false       use  isnull object check it   Conditional use  isnull object check it    DoWhenItsTrue   DoWhenItsFalse   Update  another way  updated 08 31 2017 and 01 25 2021  Thanks for the comment  public static bool IsNull object T        return  bool T   true   false     Demostration  And for the records  you have my code on Github  go check it out  https   github com j0rt3g4 ValidateNull PS  This one is especially for you Chayim Friedman  don t use beta software assuming that is all true  Wait for final versions or use your own environment to test  before assuming true beta software without any sort of documentation or demonstration from your end

User · Answer

As of C  9 you can do if  obj is null           For not null use if  obj is not null           If you need to override this behaviour use    and    accordingly

User · Answer

You cannot use ToString   on a null object so use a Null Coalescing Operator to assign an empty string to the null object then evaluate with IsNullOrEmpty         var foo   obj     quot  quot            if   String IsNullOrEmpty foo ToString                           Do something

User · Answer

I just followed a method that we would usually follow in java script  To convert object to string and then check whether they are null   var obj   new Object    var objStr   obj ToString    if   string IsNullOrEmpty objStr         code as per your needs

User · Answer

It s not data that is null  but dataList   You need to create one with  public List lt Object gt  dataList   new List lt Object gt       Even better  since it s a field  make it private  And if there s nothing preventing you  make it also readonly  Just good practice   Aside  The correct way to check for nullity is if data    null   This kind of check is ubiquitous for reference types  even Nullable lt T gt  overrides the equality operator to be a more convenient way of expressing nullable HasValue when checking for nullity   If you do if  data Equals null   then you will get a NullReferenceException if data    null  Which is kind of comical since avoiding this exception was the goal in the first place   You are also doing this   catch  Exception e        throw new Exception e ToString         This is definitely not good  I can imagine that you put it there just so you can break into the debugger while still inside the method  in which case ignore this paragraph  Otherwise  don t catch exceptions for nothing  And if you do  rethrow them using just throw

User · Answer

No  you should be using      If data is actually null then your program will just crash with a NullReferenceException as a result of attempting to call the Equals method on null   Also realize that  if you specifically want to check for reference equality  you should use the Object ReferenceEquals method as you never know how Equals has been implemented   Your program is crashing because dataList is null as you never initialize it

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Edited to reflect hint by  kelton52  Simplest way is to do object ReferenceEquals null  data  Since  null  data  is NOT guaranteed to work  class Nully       public static bool operator    Nully n  object o                Console WriteLine  quot Comparing   quot    n    quot   with   quot    o    quot   quot            return true            public static bool operator    Nully n  object o    return   n  o       void Main         var data   new Nully        Console WriteLine null    data       Console WriteLine object ReferenceEquals null  data       Produces   Comparing    with  Nully  True False

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In addition to  Jose Ortega answer   its better for use extension method    public static bool IsNull this object T                 return T    null            And use IsNull method for all of object like   object foo   new object      or any object from any class if  foo IsNull                 blah blah

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Your dataList is null as it has not been instantiated  judging by the code you have posted   Try   public List lt Object gt  dataList   new List lt Object gt     public  bool AddData ref Object data  bool success   false  try       if   data Equals null        I ve also used if data    null  which hasn t worked either              dataList Add data                          NullReferenceException occurs here        success   doOtherStuff data           catch  Exception e        throw new Exception e ToString       return success

User · Answer

Here are some extensions I use       lt summary gt      Extensions to the object class      lt  summary gt  public static class ObjectExtensions            lt summary gt          True if the object is null  else false          lt  summary gt      public static bool IsNull this object input    gt  input is null            lt summary gt          False if the object is null  else true          lt  summary gt      public static bool NotNull this object input    gt   IsNull input

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This was a good Example       if  MyObj is Object                      Do something      for example                if  MyObj is Button                  MyObj Enabled   true

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With c 9  2020  you can now check a parameter is null with this code  if  name is null       if  name is not null       You can have more information here

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