fix #11 map to nullable enum

Author: chaowlert

src/Mapster.Tests/WhenMappingEnums.cs

@@ -87,6 +87,38 @@ public void String_Is_Mapped_To_Enum()
             dto.Department.ShouldBe(Departments.IT);
         }
 
+        [TestMethod]
+        public void String_Is_Mapped_To_Nullable_Enum()
+        {
+            var department = Departments.IT.ToString();
+            var value = TypeAdapter.Adapt<string, Departments?>(department);
+            value.ShouldBe(Departments.IT);
+        }
+
+        [TestMethod]
+        public void Null_String_Is_Mapped_To_Nullable_Enum()
+        {
+            string department = null;
+            var value = TypeAdapter.Adapt<string, Departments?>(department);
+            value.ShouldBeNull();
+        }
+
+        [TestMethod]
+        public void Nullable_Enum_Is_Mapped_To_String()
+        {
+            var department = Departments.IT;
+            var value = TypeAdapter.Adapt<Departments?, string>(department);
+            value.ShouldBe(Departments.IT.ToString());
+        }
+
+        [TestMethod]
+        public void Null_Nullable_Enum_Is_Mapped_To_String()
+        {
+            Departments? department = null;
+            var value = TypeAdapter.Adapt<Departments?, string>(department);
+            value.ShouldBeNull();
+        }
+
         [TestMethod]
         public void Enum_Is_Mapped_To_Enum()
         {

src/Mapster/Adapters/EnumAdapter.cs

@@ -12,7 +12,8 @@ internal class EnumAdapter : PrimitiveAdapter
 
         protected override bool CanMap(PreCompileArgument arg)
         {
-            return arg.SourceType.GetTypeInfo().IsEnum || arg.DestinationType.GetTypeInfo().IsEnum;
+            return arg.SourceType.UnwrapNullable().GetTypeInfo().IsEnum 
+                || arg.DestinationType.UnwrapNullable().GetTypeInfo().IsEnum;
         }
 
         protected override Expression ConvertType(Expression source, Type destinationType, CompileArgument arg)

src/Mapster/Mapster.NetCore.csproj

@@ -22,17 +22,15 @@
     <GenerateAssemblyProductAttribute>false</GenerateAssemblyProductAttribute>
     <GenerateAssemblyCopyrightAttribute>false</GenerateAssemblyCopyrightAttribute>
     <GeneratePackageOnBuild>True</GeneratePackageOnBuild>
-    <Version>3.1.4</Version>
+    <Version>3.1.5</Version>
     <RootNamespace>Mapster</RootNamespace>
   </PropertyGroup>
 
   <ItemGroup Condition=" '$(TargetFramework)' == 'net40' ">
-    <Reference Include="System" />
     <Reference Include="Microsoft.CSharp" />
   </ItemGroup>
 
   <ItemGroup Condition=" '$(TargetFramework)' == 'net45' ">
-    <Reference Include="System" />
     <Reference Include="Microsoft.CSharp" />
   </ItemGroup>