Fix for equality operator when both left and right are null

Author: tmilnthorp

UnitsNet.Tests/BaseDimensionsTests.cs

@@ -80,6 +80,11 @@ public void EqualityOperatorsWorkAsExpected()
 
             Assert.False(baseDimensions2 == null);
             Assert.False(null == baseDimensions2);
+
+            BaseDimensions nullBaseDimensions1 = null;
+            BaseDimensions nullBaseDimensions2 = null;
+
+            Assert.True(nullBaseDimensions1 == nullBaseDimensions2);
         }
 
         [Fact]

UnitsNet.Tests/BaseUnitsTests.cs

@@ -102,6 +102,11 @@ public void EqualityOperatorIsImplementedCorrectly()
 
             Assert.False(baseUnits1 == null);
             Assert.False(null == baseUnits1);
+
+            BaseUnits nullBaseUnits1 = null;
+            BaseUnits nullBaseUnits2 = null;
+
+            Assert.True(nullBaseUnits1 == nullBaseUnits2);
         }
 
         [Fact]

UnitsNet.Tests/UnitSystemTests.cs

@@ -119,6 +119,11 @@ public void EqualityOperatorIsImplementedCorrectly()
 
             Assert.False(unitSystem1 == null);
             Assert.False(null == unitSystem1);
+
+            UnitSystem nullUnitSystem1 = null;
+            UnitSystem nullUnitSystem2 = null;
+
+            Assert.True(nullUnitSystem1 == nullUnitSystem2);
         }
 
         [Fact]

UnitsNet/BaseDimensions.cs

@@ -127,7 +127,7 @@ public BaseDimensions Divide(BaseDimensions right)
         /// <returns>True if equal.</returns>
         public static bool operator ==(BaseDimensions left, BaseDimensions right)
         {
-            return left?.Equals(right) == true;
+            return left is null ? right is null : left.Equals(right);
         }
 
         /// <summary>

UnitsNet/BaseUnits.cs

@@ -106,7 +106,7 @@ public override int GetHashCode()
         /// <seealso cref="Equals(BaseUnits)"/>
         public static bool operator ==(BaseUnits left, BaseUnits right)
         {
-            return left?.Equals(right) == true;
+            return left is null ? right is null : left.Equals(right);
         }
 
         /// <summary>

UnitsNet/UnitSystem.cs

@@ -89,7 +89,7 @@ public bool Equals(UnitSystem other)
         /// <seealso cref="Equals(UnitSystem)"/>
         public static bool operator ==(UnitSystem left, UnitSystem right)
         {
-            return left?.Equals(right) == true;
+            return left is null ? right is null : left.Equals(right);
         }
 
         /// <summary>