How to null check c# 7 tuple in LINQ query?

Question

Given:

class Program {     private static readonly List<(int a, int b, int c)> Map = new List<(int a, int b, int c)>()     {         (1, 1, 2),         (1, 2, 3),         (2, 2, 4)     };      static void Main(string[] args)     {         var result = Map.FirstOrDefault(w => w.a == 4 && w.b == 4);          if (result == null)             Console.WriteLine("Not found");         else             Console.WriteLine("Found");     } } 

In the above example, a compiler error is encountered at line if (result == null).

CS0019 Operator '==' cannot be applied to operands of type '(int a, int b, int c)' and '<null>'

How would I go about checking that the tuple is found prior to proceeding in my "found" logic?

Prior to using the new c# 7 tuples, I would have this:

class Program {     private static readonly List<Tuple<int, int, int>> Map = new List<Tuple<int, int, int>>()     {         new Tuple<int, int, int> (1, 1, 2),         new Tuple<int, int, int> (1, 2, 3),         new Tuple<int, int, int> (2, 2, 4)     };      static void Main(string[] args)     {         var result = Map.FirstOrDefault(w => w.Item1 == 4 && w.Item2 == 4);          if (result == null)             Console.WriteLine("Not found");         else             Console.WriteLine("Found");     } } 

Which worked fine. I like the more easily interpreted intention of the new syntax, but am unsure on how to null check it prior to acting on what was found (or not).

Answer 1

Value tuples are value types. They can't be null, which is why the compiler complains. The old Tuple type was a reference type

The result of FirstOrDefault() in this case will be a default instance of an ValueTuple<int,int,int> - all fields will be set to their default value, 0.

If you want to check for a default, you can compare the result with the default value of ValueTuple<int,int,int>, eg:

var result=(new List<(int a, int b, int c)>()             {                 (1, 1, 2),                 (1, 2, 3),                 (2, 2, 4)             }         ).FirstOrDefault(w => w.a == 4 && w.b == 4);  if (result.Equals(default(ValueTuple<int,int,int>))) {     Console.WriteLine("Missing!");  } 

WORD OF WARNING

The method is called FirstOrDefault, not TryFirst. It's not meant to check whether a value exists or not, although we all (ab)use it this way.

Creating such an extension method in C# isn't that difficult. The classic option is to use an out parameter:

public static bool TryFirst<T>(this IEnumerable<T> seq,Func<T,bool> filter, out T result)  {     result=default(T);     foreach(var item in seq)     {         if (filter(item)) {             result=item;             return true;          }     }     return false; } 

Calling this can be simplified in C# 7 as :

if (myList.TryFirst(w => w.a == 4 && w.b == 1,out var result)) {     Console.WriteLine(result); } 

F# developers can brag that they have a Seq.tryPick that will return None if no match is found.

C# doesn't have Option types or the Maybe type (yet), but maybe (pun intended) we can build our own:

class Option<T>  {     public T Value {get;private set;}      public bool HasValue {get;private set;}      public Option(T value) { Value=value; HasValue=true;}          public static readonly Option<T> Empty=new Option<T>();      private Option(){}      public void Deconstruct(out bool hasValue,out T value)     {         hasValue=HasValue;         value=Value;     } }  public static Option<T> TryPick<T>(this IEnumerable<T> seq,Func<T,bool> filter)  {     foreach(var item in seq)     {         if (filter(item)) {             return new Option<T>(item);          }     }     return Option<T>.Empty; } 

Which allows writing the following Go-style call:

var (found,value) =myList.TryPick(w => w.a == 4 && w.b == 1); 

In addition to the more traditional :

var result=myList.TryPick(w => w.a == 4 && w.b == 1); if (result.HasValue) {...}