Fix wrong [MarshalAs(UnmanagedType)] for strings (#1438)

src/Generator/Types/Std/Stdlib.cs

@@ -155,18 +155,54 @@ namespace CppSharp.Types.Std
                 return new CustomType(typePrinter.IntPtrType);
             }
 
-            if (Context.Options.Encoding == Encoding.ASCII ||
-                Context.Options.Encoding == Encoding.UTF8)
+            Type type = Type.Desugar();
+
+            uint charWidth = 0;
+            if (type is PointerType pointerType)
+                charWidth = GetCharPtrWidth(pointerType);
+            else if (type.GetPointee()?.Desugar() is PointerType pointeePointerType)
+                charWidth = GetCharPtrWidth(pointeePointerType);
+
+            if (charWidth == 8)
+            { 
+                if (Context.Options.Encoding == Encoding.ASCII)
+                    return new CustomType("[MarshalAs(UnmanagedType.LPStr)] string");
                 return new CustomType("[MarshalAs(UnmanagedType.LPUTF8Str)] string");
+            }
 
             if (Context.Options.Encoding == Encoding.Unicode ||
                 Context.Options.Encoding == Encoding.BigEndianUnicode)
+            {
+                // NOTE: This will break if charWidth is not 16 bit which may
+                // happen if someone uses wchar_t on platforms where it's size is 32 bit.
+                // TODO: Create a custom marshaller to support that scenario.
+                // Once that's done consider supporting char32_t as well
                 return new CustomType("[MarshalAs(UnmanagedType.LPWStr)] string");
+            }
+
+            if (Context.Options.Encoding == Encoding.UTF8 && charWidth == 16)
+                return new CustomType("[MarshalAs(UnmanagedType.LPWStr)] string"); // fallback;
 
             throw new System.NotSupportedException(
                 $"{Context.Options.Encoding.EncodingName} is not supported yet.");
         }
 
+        public uint GetCharPtrWidth(PointerType pointer)
+        {
+            var pointee = pointer?.Pointee?.Desugar();
+            if (pointee != null)
+            {
+                if (pointee.IsPrimitiveType(PrimitiveType.Char))
+                    return Context.TargetInfo.CharWidth;
+                if (pointee.IsPrimitiveType(PrimitiveType.WideChar))
+                    return Context.TargetInfo.WCharWidth;
+                if (pointee.IsPrimitiveType(PrimitiveType.Char16))
+                    return Context.TargetInfo.Char16Width;
+            }
+
+            return 0;
+        }
+
         public override void CSharpMarshalToNative(CSharpMarshalContext ctx)
         {
             string param = ctx.Parameter.Name;

tests/CSharp/CSharp.Tests.cs

@@ -1479,4 +1479,11 @@ public unsafe class CSharpTests : GeneratorTestFixture
     {
         StringAssert.EndsWith(nameof(CSharp.TestAnonymousMemberNameCollision._0.__0), "__0");
     }
+
+    [Test]
+    public void TestStringMarshall()
+    {
+        Assert.IsTrue(StringMarshall.CSharpString8(StringMarshall.CSharpString));
+        Assert.IsTrue(StringMarshall.CSharpString16(StringMarshall.CSharpString));
+    }
 }

tests/CSharp/CSharp.cpp

@@ -1777,3 +1777,13 @@ boolean_t takeTypemapTypedefParam(boolean_t b)
 {
     return b;
 }
+
+bool StringMarshall::CSharpString8(const char* in)
+{
+    return in[0] == 'C' && in[1] == '#';
+}
+
+bool StringMarshall::CSharpString16(const char16_t* in)
+{
+    return in[0] == 'C' && in[1] == '#';
+}

tests/CSharp/CSharp.h

@@ -1481,3 +1481,11 @@ struct TestVariableWithoutType
 
     static constexpr auto variable = create(n...);
 };
+
+class DLL_API StringMarshall
+{
+public:
+    static constexpr const char* CSharpString = "C#";
+    static bool CSharpString8(const char* in);
+    static bool CSharpString16(const char16_t* in);
+};