Generate exception on broken struct set op

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

@@ -331,6 +331,9 @@ namespace CppSharp.Types.Std
                 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();

tests/dotnet/CSharp/CSharp.Tests.cs

@@ -2027,6 +2027,7 @@ public unsafe class CSharpTests
     }
 
     [Test]
+    [Ignore("https://github.com/mono/CppSharp/issues/1730")]
     public void TestString()
     {
         var test = new CSharp.ValueType();

tests/dotnet/CSharp/CSharp.h

@@ -1642,8 +1642,11 @@ public:
 inline void DLL_API InstantiateOptionalTemplate(Optional<unsigned int>, Optional<std::string>,
     Optional<TestComparison>, Optional<char*>, Optional<UnionTester>) { }
 
-CS_VALUE_TYPE class DLL_API value_type {
+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;
 };