Checking if a list is empty with LINQ

Question

What s the  best   taking both speed and readability into account  way to determine if a list is empty  Even if the list is of type IEnumerable lt T gt  and doesn t have a Count property   Right now I m tossing up between this   if  myList Count      0            and this   if   myList Any              My guess is that the second option is faster  since it ll come back with a result as soon as it sees the first item  whereas the second option  for an IEnumerable  will need to visit every item to return the count   That being said  does the second option look as readable to you  Which would you prefer  Or can you think of a better way to test for an empty list   Edit  lassevk s response seems to be the most logical  coupled with a bit of runtime checking to use a cached count if possible  like this   public static bool IsEmpty lt T gt  this IEnumerable lt T gt  list        if  list is ICollection lt T gt   return   ICollection lt T gt  list  Count    0       return  list Any

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Konrad what surprises me is that in my tests  I m passing the list into a method that accepts IEnumerable lt T gt   so the runtime can t optimize it by calling the Count   extension method for IList lt T gt    I can only assume that the Count   extension method for IEnumerable is doing something like this   public static int Count lt T gt  this IEnumerable lt T gt  list        if  list is IList lt T gt   return   IList lt T gt  list  Count       int i   0      foreach  var t in list  i        return i          in other words  a bit of runtime optimization for the special case of IList lt T gt     EDIT  Konrad  1 mate - you re right about it more likely being on ICollection lt T gt

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myList ToList   Count    0  That s all

User · Answer

List lt T gt  li   new List lt T gt      li First   DefaultValue HasValue    string Format   0 yyyy MM dd    sender First   DefaultValue Value    string Empty

User · Answer

Another idea   if enumerable FirstOrDefault      null    However I like the Any   approach more

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You could do this   public static Boolean IsEmpty lt T gt  this IEnumerable lt T gt  source        if  source    null          return true     or throw an exception     return  source Any        Edit  Note that simply using the  Count method will be fast if the underlying source actually has a fast Count property  A valid optimization above would be to detect a few base types and simply use the  Count property of those  instead of the  Any   approach  but then fall back to  Any   if no guarantee can be made

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I just wrote up a quick test  try this    IEnumerable lt Object gt  myList   new List lt Object gt       Stopwatch watch   new Stopwatch      int x    watch Start     for  var i   0  i  lt   1000000  i           if  myList Count      0  x   i       watch Stop      Stopwatch watch2   new Stopwatch      watch2 Start     for  var i   0  i  lt   1000000  i            if   myList Any    x   i      watch2 Stop      Console WriteLine  myList Count         watch ElapsedMilliseconds ToString      Console WriteLine  myList Any         watch2 ElapsedMilliseconds ToString      Console ReadLine      The second is almost three times slower     Trying the stopwatch test again with a Stack or array or other scenarios it really depends on the type of list it seems - because they prove Count to be slower   So I guess it depends on the type of list you re using    Just to point out  I put 2000  objects in the List and count was still faster  opposite with other types

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I would make one small addition to the code you seem to have settled on  check also for ICollection  as this is implemented even by some non-obsolete generic classes as well  i e   Queue lt T gt  and Stack lt T gt    I would also use as instead of is as it s more idiomatic and has been shown to be faster   public static bool IsEmpty lt T gt  this IEnumerable lt T gt  list        if  list    null                throw new ArgumentNullException  list               var genericCollection   list as ICollection lt T gt       if  genericCollection    null                return genericCollection Count    0             var nonGenericCollection   list as ICollection      if  nonGenericCollection    null                return nonGenericCollection Count    0             return  list Any

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private bool NullTest lt T gt  T   list  string attribute                 bool status   false          if  list    null                        int flag   0              var property   GetProperty list FirstOrDefault    attribute               foreach  T obj in list                                if  property GetValue obj  null     null                      flag                              status   flag    0   true   false                    return status          public PropertyInfo GetProperty lt T gt  T obj  string str                 Expression lt Func lt T  string  PropertyInfo gt  gt  GetProperty    TypeObj  Column    gt  TypeObj GetType   GetProperty TypeObj              GetType   GetProperties   ToList                Find property   gt  property Name              ToLower      Column              ToLower    Name ToString             return GetProperty Compile   obj  str

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This extension method works for me   public static bool IsEmpty lt T gt  this IEnumerable lt T gt  enumerable        try               enumerable First            return false            catch  InvalidOperationException                return true

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Here s my implementation of Dan Tao s answer  allowing for a predicate   public static bool IsEmpty lt TSource gt  this IEnumerable lt TSource gt  source  Func lt TSource  bool gt  predicate        if  source    null  throw new ArgumentNullException        if  IsCollectionAndEmpty source   return true      return  source Any predicate      public static bool IsEmpty lt TSource gt  this IEnumerable lt TSource gt  source        if  source    null  throw new ArgumentNullException        if  IsCollectionAndEmpty source   return true      return  source Any       private static bool IsCollectionAndEmpty lt TSource gt  IEnumerable lt TSource gt  source        var genericCollection   source as ICollection lt TSource gt       if  genericCollection    null  return genericCollection Count    0      var nonGenericCollection   source as ICollection      if  nonGenericCollection    null  return nonGenericCollection Count    0      return false

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If I check with Count   Linq executes a  SELECT COUNT       in the database  but I need to check if the results contains data  I resolved to introducing FirstOrDefault   instead of Count     Before  var cfop   from tabelaCFOPs in ERPDAOManager GetTable lt TabelaCFOPs gt     if  cfop Count    gt  0        var itemCfop   cfop First                   After  var cfop   from tabelaCFOPs in ERPDAOManager GetTable lt TabelaCFOPs gt     var itemCfop   cfop FirstOrDefault     if  itemCfop    null

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This was critical to get this to work with Entity Framework   var genericCollection   list as ICollection lt T gt    if  genericCollection    null         your code

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Ok  so what about this one   public static bool IsEmpty lt T gt  this IEnumerable lt T gt  enumerable        return  enumerable GetEnumerator   MoveNext        EDIT  I ve just realized that someone has sketched this solution already  It was mentioned that the Any   method will do this  but why not do it yourself  Regards

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The second option is much quicker if you have multiple items    Any   returns as soon as 1 item is found  Count   has to keep going through the entire list    For instance suppose the enumeration had 1000 items    Any   would check the first one  then return true  Count   would return 1000 after traversing the entire enumeration    This is potentially worse if you use one of the predicate overrides - Count   still has to check every single item  even it there is only one match   You get used to using the Any one - it does make sense and is readable   One caveat - if you have a List  rather than just an IEnumerable then use that list s Count property

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List Count is O 1  according to Microsoft s documentation  http   msdn microsoft com en-us library 27b47ht3 aspx    so just use List Count    0 it s much faster than a query  This is because it has a data member called Count which is updated any time something is added or removed from the list  so when you call List Count it doesn t have to iterate through every element to get it  it just returns the data member

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LINQ itself must be doing some serious optimization around the Count   method somehow    Does this surprise you  I imagine that for IList implementations  Count simply reads the number of elements directly while Any has to query the IEnumerable GetEnumerator method  create an instance and call MoveNext at least once    EDIT  Matt      I can only assume that the Count   extension method for IEnumerable is doing something like this    Yes  of course it does  This is what I meant  Actually  it uses ICollection instead of IList but the result is the same

[c#] Checking if a list is empty with LINQ

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