Use HashCode package to combine primary key values

#308

src/Dommel/AutoMultiMap.cs

@@ -693,31 +693,27 @@ internal static Delegate CreateMapDelegate<T1, T2, T3, T4, T5, T6, T7, TReturn>(
     {
         var includeTypes = new[]
         {
-                typeof(T1),
-                typeof(T2),
-                typeof(T3),
-                typeof(T4),
-                typeof(T5),
-                typeof(T6),
-                typeof(T7)
-            }
+            typeof(T1),
+            typeof(T2),
+            typeof(T3),
+            typeof(T4),
+            typeof(T5),
+            typeof(T6),
+            typeof(T7)
+        }
         .Where(t => t != typeof(DontMap))
         .ToArray();
 
         var targetProperties = typeof(T1).GetProperties();
         var keyProperties = Resolvers.KeyProperties(typeof(T1)).Select(x => x.Property);
 
-        // Create unique number from two integers
-        // https://stackoverflow.com/a/14652569
-        static int Pair(int a, int b) => (int)(((a + b) * (a + b + 1) * 0.5) + b);
-
         T1 GetOrAdd(T1 target)
         {
             // Populate the dictionary with cached items keyed by the hash code of
             // the value of the primary key properties. This way multi mapping one-to-many
             // relations don't produce multiple instances of the same parent record.
             // Use paired hash codes of all key properties as cache key.
-            var cacheKey = keyProperties.Aggregate(0, (x, prop) => Pair(x, prop.GetValue(target)!.GetHashCode()));
+            var cacheKey = keyProperties.Aggregate(0, (x, prop) => HashCode.Combine(x, prop.GetValue(target)));
             if (!results.TryGetValue(cacheKey, out var cachedItem))
             {
                 cachedItem = target;

src/Dommel/Dommel.csproj

@@ -10,6 +10,7 @@
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="8.0.0" PrivateAssets="All" />
   </ItemGroup>
   <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.0'">
+    <PackageReference Include="Microsoft.Bcl.HashCode" Version="1.1.1" />
     <PackageReference Include="System.ComponentModel.Annotations" Version="5.0.0" />
   </ItemGroup>
 </Project>