add property match for records

samples/CS9/Program.cs

@@ -1,34 +1,39 @@
 // ReSharper disable All
 
-namespace CS9 {
-  class Program {
+namespace CS9
+{
+  class Program
+  {
     // Relational patterns (>= operator et al.)
+
     // Logical patterns ("or", also "and" and "not")
     //   Note: parens ( and ) can be used to determine precedence
     bool IsPastMidYear(DateTime dt) => dt is { Month: >= 7 };
+
     bool IsThisYearOrLast(DateTime dt) => dt is { Year: 2022 or 2021 };
 
-    bool IsSummer(DateTime dt) => dt is
-      { Month: 7 or 8 } or
-      { Month: 6, Day: >= 21 } or
-      { Month: 9, Day: < 21 };
+    bool IsSummer(DateTime dt) =>
+      dt is { Month: 7 or 8 } or { Month: 6, Day: >= 21 } or { Month: 9, Day: < 21 };
 
     // Class with "Deconstruct" implementation
-    class Order {
+    class Order
+    {
       public int ItemCount { get; set; }
       public double ItemPrice { get; set; }
 
-      public void Deconstruct(out int itemCount, out double itemPrice) {
+      public void Deconstruct(out int itemCount, out double itemPrice)
+      {
         itemCount = ItemCount;
         itemPrice = ItemPrice;
       }
     }
 
-    public enum OrderValue {
+    public enum OrderValue
+    {
       ValuableDueToHighCount,
       ValuableDueToHighItemPrice,
       ValuableDueToHighTotal,
-      NotValuable
+      NotValuable,
     }
 
     // Positional patterns with "discard" placeholders - note that
@@ -38,7 +43,9 @@ namespace CS9 {
       {
         // Labels are optional here but can be useful
         // for readability
-        Order (itemCount: < 0, itemPrice: _) => throw new ArgumentException("Positive itemCounts please!"),
+        Order(itemCount: < 0, itemPrice: _) => throw new ArgumentException(
+          "Positive itemCounts please!"
+        ),
         (_, < 0) => throw new ArgumentException("Positive itemPrices please!"),
         (>= 100, _) => OrderValue.ValuableDueToHighCount,
         (_, >= 1000) => OrderValue.ValuableDueToHighItemPrice,
@@ -46,10 +53,58 @@ namespace CS9 {
         // Assign variable names for further processing
         var (c, p) when c * p > 1000 => OrderValue.ValuableDueToHighTotal,
 
-        _ => OrderValue.NotValuable
+        _ => OrderValue.NotValuable,
       };
 
-    static void Main(string[] args) {
+    static void Main(string[] args)
+    {
+      var order = new Order { ItemCount = 30, ItemPrice = 50 };
+      // Let's see if this order is valuable
+      var category = OrderValueCategory(order);
+      Console.WriteLine($"This order is categorized as: {category}");
+
+      // Let's see that linked list
+      LinkedList<int> list = new LinkedList<int>.Node(
+        1,
+        new LinkedList<int>.Node(2, new LinkedList<int>.Node(3, new LinkedList<int>.Empty()))
+      );
+      PrintList(list);
+      PrintListWithPropertyMatch(list);
+    }
+
+    // disable exhaustion check - we know these matches are exhaustive for our purposes, but the compiler doesn't
+#pragma warning disable CS8509
+
+    static void PrintList<T>(LinkedList<T> list)
+    {
+      string getPart(LinkedList<T> l) =>
+        l switch
+        {
+          LinkedList<T>.Node(var value, var next) => $"(N {value} {getPart(next)})",
+          LinkedList<T>.Empty => "(E)",
+        };
+      Console.WriteLine(getPart(list));
+    }
+
+    // Technically better, but more verbose - property match syntax means that we don't rely on position alone
+    static void PrintListWithPropertyMatch<T>(LinkedList<T> list)
+    {
+      string getPart(LinkedList<T> l) =>
+        l switch
+        {
+          LinkedList<T>.Node { Value: var value, Next: var next } => $"(N {value} {getPart(next)})",
+          LinkedList<T>.Empty => "(E)",
+        };
+      Console.WriteLine(getPart(list));
     }
   }
-}
+#pragma warning restore CS8509
+
+  // Linked list with discriminated unions based on records
+  public abstract record LinkedList<T>
+  {
+    public sealed record Node(T Value, LinkedList<T> Next) : LinkedList<T>;
+
+    public sealed record Empty : LinkedList<T>;
+  }
+}