Checking for empty or null List string

Question

I have a List where sometimes it is empty or null  I want to be able to check if it contains any List-item and if not then add an object to the List       I have a list  sometimes it doesn t have any data added to it     var myList   new List lt object gt          Expression is always false     if  myList    null           Console WriteLine  List is never null         if  myList 0     null           myList Add  new item           Errors encountered   Index was out of range  Must be non-negative and less than the size of the collection         Inner Exception says  null

User · Answer

I was wondering nobody suggested to create own extension method more readable name for OP s case   public static bool IsNullOrEmpty lt T gt  this IEnumerable lt T gt  source        if  source    null                return true             return source Any      false

User · Answer

This is able to solve your problem  if list Length  gt  0

User · Answer

Checkout L-Four s answer  A less-efficient answer  if myList Count    0          nothing is there  Add here    Basically new List lt T gt  will not be null but will have no elements   As is noted in the comments  the above will throw an exception if the list is uninstantiated  But as for the snippet in the question  where it is instantiated  the above will work just fine  If you need to check for null  then it would be  if myList    null  amp  amp  myList Count    0        The list is empty  Add something here    Even better would be to use  myList Any   and as is mentioned in the aforementioned L-Four s answer as short circuiting is faster than linear counting of the elements in the list

User · Answer

if  myList  Any      true                I find this the most convenient way      true  checks the value of the nullable bool implied by    Any

User · Answer

Because you initialize myList with  new   the list itself will never be null   But it can be filled with  null  values    In that case  Count  gt  0 and  Any   will be true  You can check this with the  All s   gt  s    null   var myList   new List lt object gt     if  myList Any      myList All s   gt  s    null

User · Answer

myList 0  gets the first item in the list   Since the list is empty there is no item to get and you get the IndexOutOfRangeException instead   As other answers here have shown  in order to check if the list is empty you need to get the number of elements in the list  myList Count  or use the LINQ method  Any   which will return true if there are any elements in the list

User · Answer

Most answers here focused on how to check if a collection is Empty or Null which was quite straight forward as demonstrated by them    Like many people here  I was also wondering why does not Microsoft itself provide such a basic feature which is already provided for String type  String IsNullOrEmpty     Then I encountered this guideline from Microsoft where it says       X DO NOT return null values from collection properties or from methods returning collections  Return an empty collection or an empty array instead       The general rule is that null and empty  0 item  collections or arrays   should be treated the same    So  ideally you should never have a collection which is null if you follow this guideline from Microsoft  And that will help you to remove unnecessary null checking which ultimately will make your code more readable  In this case  someone just need to check   myList Any   to find out whether there is any element present in the list   Hope this explanation will help someone who will face same problem in future and wondering why isn t there any such feature to check whether a collection is null or empty

User · Answer

We can add an extension to create an empty list       public static IEnumerable lt T gt  Nullable lt T gt  this IEnumerable lt T gt  obj                if  obj    null              return new List lt T gt             else             return obj          And use like this  foreach  model in models Nullable

User · Answer

You can use Count property of List in c   please find below code which checks list empty and null both in a single condition   if myList    null    myList Count    0          Do Something

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Try and use      if myList Any           Note  this assmumes myList is not null

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For anyone who doesn t have the guarantee that the list will not be null  you can use the null-conditional operator to safely check for null and empty lists in a single conditional statement   if  list  Any      true           Handle null or empty list

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An easy way to combine myList    null    myList Count    0 would be to use the null coalescing operator      if   myList  Count    0     0          My list is null or empty

User · Answer

Your List has no items  that s why access to non-existing 0th item   myList 0     null   throws Index was out of range exception  when you want to access n-th item check    if  myList Count  gt  n      DoSomething myList n     in your case    if  myList Count  gt  0      lt - You can safely get 0-th item     if  myList 0     null         myList Add  new item

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List in c  has a Count property   It can be used like so   if myList    null     Checks if list is null        Wasn t initialized else if myList Count    0     Checks if the list is empty     myList Add  new item    else    List is valid and has something in it        You could access the element in the list if you wanted

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Assuming that the list is never null  the following code checks if the list is empty and adds a new element if empty   if   myList Any          myList Add  new item        If it is possible that the list is null  a null check must be added before the Any   condition   if  myList    null  amp  amp   myList Any          myList Add  new item        In my opinion  using Any   instead of Count    0 is preferable since it better expresses the intent of checking if the list has any element or is empty  However  considering the performance of each approach  using Any   is generally slower than Count

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If you want a single line condition that checks both null and empty  you can use  if  list    null   true     list Any       This will work in older framework versions where the null-conditional operator is not available

User · Answer

What about using an extension method   public static bool AnyOrNotNull lt T gt  this IEnumerable lt T gt  source      if  source    null  amp  amp  source Any        return true    else     return false

User · Answer

Try the following code    if    myList   null   amp  amp    myList Any                 Add new item      myList Add  new item          A late EDIT because for these checks I now like to use the following solution  First  add a small reusable extension method called Safe     public static class IEnumerableExtension              public static IEnumerable lt T gt  Safe lt T gt  this IEnumerable lt T gt  source                if  source    null                        yield break                     foreach  var item in source                        yield return item                      And then  you can do the same like    if   myList Safe   Any                Add new item       myList Add  new item          I personally find this less verbose and easier to read  You can now safely access any collection without the need for a null check   And another EDIT  using    Null-conditional  operator  C  6 0    if   myList  Any      false           Add new item     myList Add  new item

User · Answer

We can validate like below with Extension methods  I use them for all of my projects     var myList   new List lt string gt       if  myList HasValue             Console WriteLine  List has value s                          if  myList HasValue             Console WriteLine  List is either null or empty                      if myList HasValue              if   myList 0  HasValue                       myList Add  new item                          lt summary gt      This Method will return True if List is Not Null and it s items count gt 0             lt  summary gt       lt typeparam name  T  gt  lt  typeparam gt       lt param name  items  gt  lt  param gt       lt returns gt Bool lt  returns gt  public static bool HasValue lt T gt  this IEnumerable lt T gt  items        if  items    null                if  items Count    gt  0                        return true                      return false           lt summary gt      This Method will return True if List is Not Null and it s items count gt 0             lt  summary gt       lt typeparam name  T  gt  lt  typeparam gt       lt param name  items  gt  lt  param gt       lt returns gt  lt  returns gt  public static bool HasValue lt T gt  this List lt T gt  items        if  items    null                if  items Count    gt  0                        return true                      return false           lt summary gt       This method returns true if string not null and not empty        lt  summary gt       lt param name  ObjectValue  gt  lt  param gt       lt returns gt bool lt  returns gt  public static bool HasValue this string ObjectValue        if  ObjectValue    null                if    string IsNullOrEmpty ObjectValue    amp  amp    string IsNullOrWhiteSpace ObjectValue                          return true                      return false

[c#] Checking for empty or null List<string>

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