GeneratorWithComplexType.cs

internal static string BuildDtoProperty(
            this PropertyDeclarationSyntax pds, 
            Compilation compilation)
{
    var blah = compilation
        .GetSemanticModel(pds.SyntaxTree)
        .GetDeclaredSymbol(pds);
    var propSym = (blah as IPropertySymbol);

    if (propSym.IsOfTypeClass() || propSym.IsOfTypeStruct())
    {
        if (propSym.Type.NullableAnnotation == NullableAnnotation.Annotated)
            return $"public {propSym.Type.Name}Dto? {property.Name} {{get; set;}}";

        return $"public {propSym.Type.Name()}Dto {property.Name} {{get; set;}}";
    }
    else
        return $"public {propSym.Type.Name()} {property.Name} {{get; set;}}";
}

internal static bool IsOfTypeClass(this IPropertySymbol propSym) =>
    propSym.Type.IsReferenceType &&
    propSym.Type.TypeKind == TypeKind.Class &&
    propSym.Type.ToDisplayString().StartsWith(
        propSym.ContainingNamespace.ToDisplayString());

internal static bool IsOfTypeStruct(this IPropertySymbol propSym) =>
    propSym.Type.IsValueType &&
    propSym.Type.TypeKind == TypeKind.Struct &&
    propSym.Type.ToDisplayString().StartsWith(
        propSym.ContainingNamespace.ToDisplayString());