Let the marshal determine the size of the structure

Signed-off-by: Alin Gherman <Alin Gherman>

src/Generator/Generators/CSharp/CSharpSources.cs

@@ -2126,7 +2126,7 @@ namespace CppSharp.Generators.CSharp
                     copyCtorMethod.IsGenerated)
                 {
                     // Allocate memory for a new native object and call the ctor.
-                    WriteLine("var ret = Marshal.AllocHGlobal({0});", @class.Layout.Size);
+                    WriteLine("var ret = Marshal.AllocHGlobal(Marshal.SizeOf(sizeof({0})));", @internal);
                     TypePrinter.PushContext(TypePrinterContextKind.Native);
                     WriteLine($"{@class.Visit(TypePrinter)}.{GetFunctionNativeIdentifier(copyCtorMethod)}(ret, new global::System.IntPtr(&native));",
                         @class.Visit(TypePrinter),
@@ -2136,7 +2136,7 @@ namespace CppSharp.Generators.CSharp
                 }
                 else
                 {
-                    WriteLine("var ret = Marshal.AllocHGlobal({0});", @class.Layout.Size);
+                    WriteLine("var ret = Marshal.AllocHGlobal(Marshal.SizeOf(sizeof({0})));", @internal);
                     WriteLine("*({0}*) ret = native;", @internal);
                     WriteLine("return ret.ToPointer();");
                 }
@@ -2577,8 +2577,11 @@ namespace CppSharp.Generators.CSharp
 
         private void GenerateClassConstructor(Method method, Class @class)
         {
-            WriteLine("{0} = Marshal.AllocHGlobal({1});", Helpers.InstanceIdentifier,
+            TypePrinter.PushContext(TypePrinterContextKind.Native);
-                @class.Layout.Size);
+            var @internal = (@class.IsAbstractImpl ? @class.BaseClass : @class).Visit(TypePrinter);
+            TypePrinter.PopContext();
+            WriteLine("{0} = Marshal.AllocHGlobal(Marshal.SizeOf(sizeof({1})));", Helpers.InstanceIdentifier,
+                @internal);
             WriteLine("{0} = true;", Helpers.OwnsNativeInstanceIdentifier);
             WriteLine("NativeToManagedMap[{0}] = this;", Helpers.InstanceIdentifier);