fix #195 map to target using struct

Author: chaowlert

src/Mapster.Tests/WhenMappingStruct.cs

@@ -0,0 +1,53 @@
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Shouldly;
+
+namespace Mapster.Tests
+{
+    [TestClass]
+    public class WhenMappingStruct
+    {
+        class MyClassA
+        {
+            public MyStruct MyStruct { get; set; }
+        }
+        class MyClassB
+        {
+            public MyStruct MyStruct { get; set; }
+        }
+
+        struct MyStruct
+        {
+            public MyStruct(string prop) : this()
+            {
+                Property = prop;
+            }
+
+            public string Property { get; set; }
+        }
+
+        [TestMethod]
+        public void TestMapping()
+        {
+            var a = new MyClassA();
+
+            a.MyStruct = new MyStruct("A");
+
+            var b = a.Adapt<MyClassB>();
+
+            b.MyStruct.Property.ShouldBe("A");
+        }
+
+        [TestMethod]
+        public void TestMappingToTarget()
+        {
+            var a = new MyClassA();
+            var b = new MyClassB();
+
+            a.MyStruct = new MyStruct("A");
+
+            a.Adapt(b);
+
+            b.MyStruct.Property.ShouldBe("A");
+        }
+    }
+}

src/Mapster/Adapters/BaseAdapter.cs

@@ -168,7 +168,7 @@ protected Expression CreateBlockExpressionBody(Expression source, Expression? de
 
             //new TDest();
             var set = CreateInstantiationExpression(source, destination, arg);
-            if (destination != null && this.UseTargetValue && arg.GetConstructUsing()?.Parameters.Count != 2)
+            if (destination != null && this.UseTargetValue && arg.GetConstructUsing()?.Parameters.Count != 2 && destination.CanBeNull())
             {
                 //dest ?? new TDest();
                 set = Expression.Coalesce(destination, set);

src/Mapster/Mapster.csproj

@@ -17,8 +17,8 @@
         <NetStandardImplicitPackageVersion Condition=" '$(TargetFramework)' == 'netstandard1.3' ">1.6.1</NetStandardImplicitPackageVersion>
         <GeneratePackageOnBuild>True</GeneratePackageOnBuild>
         <RootNamespace>Mapster</RootNamespace>
-        <Version>4.0.0</Version>
-        <AssemblyVersion>4.0.0.0</AssemblyVersion>
+        <Version>4.1.0</Version>
+        <AssemblyVersion>4.1.0.0</AssemblyVersion>
         <LangVersion>preview</LangVersion>
         <NullableContextOptions>enable</NullableContextOptions>
     </PropertyGroup>