Teach LambdaEqualityHelper to deal with null equality contribution

LibGit2Sharp.Tests/EqualityFixture.cs

@@ -0,0 +1,52 @@
+using LibGit2Sharp.Tests.TestHelpers;
+using Xunit;
+
+namespace LibGit2Sharp.Tests
+{
+    public class EqualityFixture : BaseFixture
+    {
+        [Fact]
+        public void EqualityHelperCanTestNullInEquals()
+        {
+            var one = new ObjectWithEquality();
+            var two = new ObjectWithEquality();
+            var three = new ObjectWithEquality(ObjectId.Zero);
+            var four = new ObjectWithEquality(ObjectId.Zero);
+
+            Assert.True(one.Equals(one));
+            Assert.True(two.Equals(two));
+            Assert.True(three.Equals(four));
+            Assert.True(four.Equals(three));
+            Assert.False(one.Equals(three));
+            Assert.False(three.Equals(one));
+        }
+
+        [Fact]
+        public void EqualityHelperCanTestNullInHashCode()
+        {
+            var one = new ObjectWithEquality();
+            var two = new ObjectWithEquality();
+            var three = new ObjectWithEquality(ObjectId.Zero);
+            var four = new ObjectWithEquality(ObjectId.Zero);
+
+            Assert.Equal(one.GetHashCode(), two.GetHashCode());
+            Assert.Equal(three.GetHashCode(), four.GetHashCode());
+            Assert.NotEqual(one.GetHashCode(), three.GetHashCode());
+        }
+
+        private class ObjectWithEquality : GitObject
+        {
+            private readonly ObjectId id;
+
+            public ObjectWithEquality(ObjectId id = null)
+            {
+                this.id = id;
+            }
+
+            public override ObjectId Id
+            {
+                get { return id; }
+            }
+        }
+    }
+}

LibGit2Sharp.Tests/LibGit2Sharp.Tests.csproj

@@ -59,6 +59,7 @@
   </ItemGroup>
   <ItemGroup>
     <Compile Include="CheckoutFixture.cs" />
+    <Compile Include="EqualityFixture.cs" />
     <Compile Include="SignatureFixture.cs" />
     <Compile Include="FilterBranchFixture.cs" />
     <Compile Include="RemoveFixture.cs" />

LibGit2Sharp/Core/LambdaEqualityHelper.cs

@@ -47,7 +47,8 @@ public int GetHashCode(T instance)
             {
                 foreach (Func<T, object> accessor in equalityContributorAccessors)
                 {
-                    hashCode = (hashCode*397) ^ accessor(instance).GetHashCode();
+                    object item = accessor(instance);
+                    hashCode = (hashCode*397) ^ (item != null ? item.GetHashCode() : 0);
                 }
             }