In LINQ select all values of property X where X null

Question

Is there a shorter way to write the following   Something that would check for null without explicitly writing    null   from item in list  where item MyProperty    null  select item MyProperty

User · Answer

I tend to create a static class containing basic functions for cases like these. They allow me write expressions like

var myValues myItems.Select(x => x.Value).Where(Predicates.IsNotNull);

And the collection of predicate functions:

public static class Predicates
{
    public static bool IsNull<T>(T value) where T : class
    {
        return value == null;
    }

    public static bool IsNotNull<T>(T value) where T : class
    {
        return value != null;
    }

    public static bool IsNull<T>(T? nullableValue) where T : struct
    {
        return !nullableValue.HasValue;
    }

    public static bool IsNotNull<T>(T? nullableValue) where T : struct
    {
        return nullableValue.HasValue;
    }

    public static bool HasValue<T>(T? nullableValue) where T : struct
    {
        return nullableValue.HasValue;
    }

    public static bool HasNoValue<T>(T? nullableValue) where T : struct
    {
        return !nullableValue.HasValue;
    }
}

User · Answer

get one column in the distinct select and ignore null values    var items   db table Where p   gt  p id  null  GroupBy p   gt  p id                                   Select grp   gt  grp First   id                                   ToList

User · Answer

if you need to check if all items  MyProperty doesn t have null  if  list All x   gt  x MyProperty    null      do something      or if you need to check if at least one items  property has doesn t have null  if  list Any x   gt  x MyProperty    null      do something   But you always have to check for null

User · Answer

You can use the OfType operator  It ignores null values in the source sequence  Just use the same type as MyProperty and it won t filter out anything else      given     public T MyProperty   get    var nonNullItems   list Select x   gt  x MyProperty  OfType lt T gt       I would advise against this though  If you want to pick non-null values  what can be more explicit than saying you want  the MyProperties from the list that are not null

User · Answer

There is no way to skip a check if it exists

User · Answer

You could define your own extension method  but I wouldn t recommend that   public static IEnumerable lt TResult gt  SelectNonNull lt T  TResult gt  this IEnumerable lt T gt  sequence Func lt T  TResult gt  projection       return sequence Select projection  Where e   gt  e    null       I don t like this one because it mixes two concerns  Projecting with Select and filtering your null values are separate operations and should not be combined into one method     I d rather define an extension method that only checks if the item isn t null   public static IEnumerable lt T gt  WhereNotNull lt T gt  this IEnumerable lt T gt  sequence       return sequence Where e   gt  e    null      public static IEnumerable lt T gt  WhereNotNull lt T gt  this IEnumerable lt T  gt  sequence      where T   struct      return sequence Where e   gt  e    null  Select e   gt  e Value       This has only a single purpose  checking for null  For nullable value types it converts to the non nullable equivalent  since it s useless to preserve the nullable wrapper for values which cannot be null   With this method  your code becomes   list Select item   gt  item MyProperty  WhereNotNull

User · Answer

This is adapted from CodesInChaos s extension method  The name is shorter  NotNull  and more importantly  restricts the type  T  to reference types with where T   class       public static IEnumerable lt T gt  NotNull lt T gt  this IEnumerable lt T gt  source  where T   class               return source Where item   gt  item    null

[c#] In LINQ, select all values of property X where X != null

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