FirstOrDefault Default value other than null

Question

As I understand it  in Linq the method FirstOrDefault   can return a Default value of something other than null   What I haven t worked out is what kind of things other than null can be returned by this  and similar  method when there are no items in the query result   Is there any particular way that this can be set up so that if there is no value for a particular query some predefined value is returned as the default value

User · Answer

From the documentation for FirstOrDefault

[Returns] default(TSource) if source is empty;

From the documentation for default(T):

the default keyword, which will return null for reference types and zero for numeric value types. For structs, it will return each member of the struct initialized to zero or null depending on whether they are value or reference types. For nullable value types, default returns a System.Nullable, which is initialized like any struct.

Therefore, the default value can be null or 0 depending on whether the type is a reference or value type, but you cannot control the default behaviour.

User · Answer

Use DefaultIfEmpty   instead of FirstOrDefault

User · Answer

General case  not just for value types   static class ExtensionsThatWillAppearOnEverything       public static T IfDefaultGiveMe lt T gt  this T value  T alternate                if  value Equals default T    return alternate          return value           var result   query FirstOrDefault   IfDefaultGiveMe otherDefaultValue     Again  this can t really tell if there was anything in your sequence  or if the first value was the default   If you care about this  you could do something like  static class ExtensionsThatWillAppearOnIEnumerables       public static T FirstOr lt T gt  this IEnumerable lt T gt  source  T alternate                foreach T t in source              return t          return alternate            and use as  var result   query FirstOr otherDefaultValue     although as Mr  Steak points out this could be done just as well by  DefaultIfEmpty      First

User · Answer

I just had a similar situation and was looking for a solution that allows me to return an alternative default value without taking care of it at the caller side every time I need it  What we usually do in case Linq does not support what we want  is to write a new extension that takes care of it  That  s what I did  Here is what I came up with  not tested though     public static class EnumerableExtensions       public static T FirstOrDefault lt T gt  this IEnumerable lt T gt  items  T defaultValue                foreach  var item in items                        return item                    return defaultValue             public static T FirstOrDefault lt T gt  this IEnumerable lt T gt  items  Func lt T  bool gt  predicate  T defaultValue                return items Where predicate  FirstOrDefault defaultValue              public static T LastOrDefault lt T gt  this IEnumerable lt T gt  items  T defaultValue                return items Reverse   FirstOrDefault defaultValue              public static T LastOrDefault lt T gt  this IEnumerable lt T gt  items  Func lt T  bool gt  predicate  T defaultValue                return items Where predicate  LastOrDefault defaultValue

User · Answer

You can use DefaultIfEmpty followed by First   T customDefault        IEnumerable lt T gt  mySequence        mySequence DefaultIfEmpty customDefault  First

User · Answer

You can also do this       Band   objects     new Band   Name    Iron Maiden           first   objects Where o   gt  o Name     Slayer            DefaultIfEmpty new Band   Name    Black Sabbath              FirstOrDefault         returns  Black Sabbath     This uses only linq - yipee

User · Answer

As I understand it  in Linq the method FirstOrDefault   can return a Default value of something other than null    No  Or rather  it always returns the default value for the element type    which is either a null reference  the null value of a nullable value type  or the natural  all zeroes  value for a non-nullable value type      Is there any particular way that this can be set up so that if there is no value for a particular query some predefined value is returned as the default value    For reference types  you can just use   var result   query FirstOrDefault      otherDefaultValue    Of course this will also give you the  other default value  if the first value is present  but is a null reference

User · Answer

I know its been a while but Ill add to this  based on the most popular answer but with a little extension Id like to share the below   static class ExtensionsThatWillAppearOnIEnumerables       public static T FirstOr lt T gt  this IEnumerable lt T gt  source  Func lt T  bool gt  predicate  Func lt T gt  alternate                var thing   source FirstOrDefault predicate           if  thing    null              return thing          return alternate              This allows me to call it inline as such with my own example I was having issues with    controlDataResolvers FirstOr x   gt  x AppliesTo item Key        gt  newDefaultResolver    GetDataAsync conn  item ToList      So for me I just wanted a default resolver to be used inline  I can do my usual check and then pass in a function so a class isn t instantiated even if unused  its a function to execute when required instead

User · Answer

Copied over from comment by  sloth  Instead of YourCollection FirstOrDefault    you could use YourCollection DefaultIfEmpty YourDefault  First   for example   Example   var viewModel   new CustomerDetailsViewModel                   MainResidenceAddressSection    MainResidenceAddressSection addresses DefaultIfEmpty new MainResidenceAddressSection    FirstOrDefault  o   gt  o is MainResidenceAddressSection               RiskAddressSection   addresses DefaultIfEmpty new RiskAddressSection    FirstOrDefault o   gt    o is MainResidenceAddressSection

User · Answer

Instead of YourCollection FirstOrDefault    you could use YourCollection DefaultIfEmpty YourDefault  First   for example

User · Answer

Actually  I use two approaches to avoid NullReferenceException when I m working with collections    public class Foo       public string Bar get  set     void Main         var list   new List lt Foo gt           before C  6 0     string barCSharp5   list DefaultIfEmpty new Foo    FirstOrDefault   Bar        C  6 0 or later     var barCSharp6   list FirstOrDefault    Bar      For C  6 0 or later   Use    or    to test if is null before perform a member access Null-conditional Operators documentation  Example  var barCSharp6   list FirstOrDefault    Bar   C  older version   Use DefaultIfEmpty   to retrieve a default value if the sequence is empty MSDN Documentation  Example  string barCSharp5   list DefaultIfEmpty new Foo    FirstOrDefault   Bar

[.net] FirstOrDefault: Default value other than null

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