Fixed infinite recursion error in Chapter 9

src/Chapter09.Tests/Listing09.05.EquivalentRecordClassCode.Tests.cs

@@ -1,7 +1,6 @@
 namespace AddisonWesley.Michaelis.EssentialCSharp.Chapter09.Listing09_05.Tests;
 using AddisonWesley.Michaelis.EssentialCSharp.Chapter09.Listing09_01;
-using AddisonWesley.Michaelis.EssentialCSharp.Chapter09.Listing09_04;
-using AddisonWesley.Michaelis.EssentialCSharp.Chapter09.Listing09_07;
+using AddisonWesley.Michaelis.EssentialCSharp.Chapter09.Listing09_05;
 
 [TestClass]
 public class CoordinateTests
@@ -18,16 +17,22 @@ public void With_Coordinate_IsReadOnly()
     }
 
     [TestMethod]
-    public void EqualityContract_GetType()
+    public void Equality_WithNulls_Success()
     {
-        Coordinate coordinate1 = new(
-            new Angle(180, 0, 0), new Angle(180, 0, 0));
-        NamedCoordinate geoCoordinate = new(
-            new Angle(180, 0, 0), new Angle(180, 0, 0), "GeoCoordinate");
+        Coordinate? coordinate1 = null;
+        Coordinate? coordinate2 = null;
+
+        Assert.IsTrue(coordinate1 == coordinate2);
+    }
 
-        Assert.AreEqual(coordinate1.ExternalEqualityContract, coordinate1.GetType());
+    [TestMethod]
+    public void Equality_WithLeffNull_Success()
+    {
+        Coordinate? coordinate1 = null;
+        Coordinate? coordinate2 = new(
+            new Angle(180, 0, 0), new Angle(180, 0, 0));
 
-        Assert.AreEqual(geoCoordinate.ExternalEqualityContract, geoCoordinate.GetType());
-        Assert.AreEqual(((Coordinate)geoCoordinate).ExternalEqualityContract, ((Coordinate)geoCoordinate).GetType());
+        Assert.IsFalse(coordinate1 == coordinate2);
+        Assert.IsFalse(coordinate2 == coordinate1);
     }
 }

src/Chapter09.Tests/Listing09.07.EquivalentRecordClassCode.Tests.cs

@@ -0,0 +1,23 @@
+namespace AddisonWesley.Michaelis.EssentialCSharp.Chapter09.Listing09_07.Tests;
+
+using AddisonWesley.Michaelis.EssentialCSharp.Chapter09.Listing09_01;
+using AddisonWesley.Michaelis.EssentialCSharp.Chapter09.Listing09_04;
+using AddisonWesley.Michaelis.EssentialCSharp.Chapter09.Listing09_07;
+
+[TestClass]
+public class CoordinateTests
+{
+    [TestMethod]
+    public void EqualityContract_GetType()
+    {
+        Coordinate coordinate1 = new(
+            new Angle(180, 0, 0), new Angle(180, 0, 0));
+        NamedCoordinate geoCoordinate = new(
+            new Angle(180, 0, 0), new Angle(180, 0, 0), "GeoCoordinate");
+
+        Assert.AreEqual(coordinate1.ExternalEqualityContract, coordinate1.GetType());
+
+        Assert.AreEqual(geoCoordinate.ExternalEqualityContract, geoCoordinate.GetType());
+        Assert.AreEqual(((Coordinate)geoCoordinate).ExternalEqualityContract, ((Coordinate)geoCoordinate).GetType());
+    }
+}

src/Chapter09/Listing09.05.EquivalentRecordClassCode.cs

@@ -46,7 +46,7 @@ protected virtual bool PrintMembers(StringBuilder builder)
         !(left == right);
 
     public static bool operator ==(Coordinate? left, Coordinate? right) => 
-        left == right || (left?.Equals(right) ?? false);
+        ReferenceEquals(left, right) || (left?.Equals(right) ?? false);
 
     public override int GetHashCode()
     {