Fix Value Type String Member Invalid Codegen (#1778)

* Fix code generation

* Generate exception on broken struct set op

* Remove redundant usings

src/Generator/Types/Std/Stdlib.CSharp.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Runtime.InteropServices;
@@ -6,6 +7,7 @@ using CppSharp.AST;
 using CppSharp.AST.Extensions;
 using CppSharp.Generators;
 using CppSharp.Generators.CSharp;
+using Type = CppSharp.AST.Type;
 
 namespace CppSharp.Types.Std
 {
@@ -145,7 +147,7 @@ namespace CppSharp.Types.Std
                 // would be really helpful to have ctx hold a Decl property representing the
                 // "appropriate" Decl when we get here. When MarshalKind == NativeField, Decl would
                 // be set to the Field we're operating on.
-                var fieldName = ctx.ReturnVarName.Substring(ctx.ReturnVarName.LastIndexOf("->") + 2);
+                var fieldName = ctx.ReturnVarName[Math.Max(ctx.ReturnVarName.LastIndexOf('.') + 1, ctx.ReturnVarName.LastIndexOf("->") + 2)..];
 
                 ctx.Before.WriteLine($"if (__{fieldName}_OwnsNativeMemory)");
                 ctx.Before.WriteLineIndent($"Marshal.FreeHGlobal({ctx.ReturnVarName});");
@@ -326,10 +328,24 @@ namespace CppSharp.Types.Std
             var assign = basicString.Methods.First(m => m.OriginalName == "assign");
             if (ctx.MarshalKind == MarshalKind.NativeField)
             {
+                string var;
+                if (ctx.ReturnVarName.LastIndexOf('.') > ctx.ReturnVarName.LastIndexOf("->"))
+                {
+                    ctx.Before.WriteLine("throw new NotImplementedException(\"This method cannot currently be called because it would " +
+                        "leave the object in an invalid state. See https://github.com/mono/CppSharp/issues/1777\");");
+
+                    var = Generator.GeneratedIdentifier(ctx.ArgName);
+                    ctx.Before.WriteLine($"fixed (void* {var} = &{ctx.ReturnVarName})");
+                    ctx.Before.WriteOpenBraceAndIndent();
+                    ctx.HasCodeBlock = true;
+                }
+                else
+                {
+                    var = $"&{ctx.ReturnVarName}";
+                }
                 ctx.Return.Write($@"{qualifiedBasicString}Extensions.{
                     Helpers.InternalStruct}.{assign.Name}(new {
-                    typePrinter.IntPtrType}(&{
-                    ctx.ReturnVarName}), ");
+                    typePrinter.IntPtrType}({var}), ");
                 if (ctx.Parameter.Type.IsTemplateParameterType())
                     ctx.Return.Write("(string) (object) ");
                 ctx.Return.Write($"{ctx.Parameter.Name})");

tests/dotnet/CSharp/CSharp.CSharp.csproj

@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
       <NoWarn>0108</NoWarn>
+      <LangVersion>10</LangVersion>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="CSharpPartialMethods.cs" />

tests/dotnet/CSharp/CSharp.Tests.cs

@@ -3,7 +3,6 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Reflection;
 using System.Runtime.InteropServices;
-using System.Text;
 using CSharp;
 using NUnit.Framework;
 
@@ -2024,4 +2023,17 @@ public unsafe class CSharpTests
         Assert.That(new CSharp.Optional<IntPtr>(IntPtr.MaxValue) == new CSharp.Optional<IntPtr>(IntPtr.MaxValue));
         Assert.That(new CSharp.Optional<IntPtr>(IntPtr.MaxValue) == IntPtr.MaxValue);
     }
+
+    [Test]
+    [Ignore("https://github.com/mono/CppSharp/issues/1730")]
+    public void TestString()
+    {
+        var test = new CSharp.ValueType();
+        Assert.AreEqual(string.Empty, test.StringMember);
+        Assert.AreEqual(null, test.CharPtrMember);
+        test.StringMember = "test";
+        test.CharPtrMember = "test2";
+        Assert.AreEqual("test", test.StringMember);
+        Assert.AreEqual("test2", test.CharPtrMember);
+    }
 }

tests/dotnet/CSharp/CSharp.h

@@ -1641,3 +1641,12 @@ public:
 // We just need a method that uses various instantiations of Optional.
 inline void DLL_API InstantiateOptionalTemplate(Optional<unsigned int>, Optional<std::string>,
     Optional<TestComparison>, Optional<char*>, Optional<UnionTester>) { }
+
+CS_VALUE_TYPE class DLL_API ValueType {
+public:
+    // Parameterless ctors are currently not generated for value types.
+    ValueType(int) { }
+
+    std::string string_member;
+    const char* char_ptr_member;
+};