How do I preserve COM binary compatibility for a .NET Assembly when properties are added?

Question

We have developed a .NET Assembly that stores language translation information and it needs to be consumed by a VB6 application.

We would like to be able to change the translation information without having to recompile the application.

The translation is provided by a two-file partial class called LanguageServices.

One file is non-changing library methods, the other is all auto generated properties from a resx file and the regx is generated from a database of language translation information.

This all arose from a need to have a central database of translations that could be programatically "flattened" to a format that can be consumed by each of our disparate applications.

Now, I can solve this problem by bypassing it and doing it in a different manner. In fact I could just get rid of the auto-generated list of properties and the problem would go away.

What I'm interested is how I can solve this problem, which is thus:

If we add new translation labels to the database (THIS WORD in THIS WORD becomes THAT WORD) it adds new properties to the class, which in turn adds new exposed properties to the COM interface.

The properties get added in the middle of the COM interface, thus breaking binary compatibility. They get added in the middle because the C# compiler suffixes the dynamic part of the partial class with the static part of the partial class. What I need it to do is either concatenate them the other way around or explicitly state the order in the C# files themselves. I thought setting the DispIDs explicitly on the static part of the class would do it, but it has not.

Here are the pair of IDL files generated by the build process:

Here is the IDL before I add a new property.

http://pastebin.com/qPvcUV9z

And here is the IDL after a new property has been added and compatibility is broken:

http://pastebin.com/K2MuqtYV

The exact difference is this bit gets shoved in the middle:

[id(0x60020039), propget]
HRESULT Jn_ExactCaseMatch([out, retval] VARIANT_BOOL* pRetVal);
[id(0x6002003a), propget]
HRESULT Jn_Regex([out, retval] VARIANT_BOOL* pRetVal);
[id(0x6002003b), propget]
HRESULT Jn([out, retval] BSTR* pRetVal);

And I think that's the problem, it's change the order of the methods. I thought the order could be overriden by explicitly defining the DispID (you can see that everything from HRESULT Culture([in] ICultureInfo* pRetVal); onwards has an id starting from 0.

Here's the C# code that's written/generated: ILanguageServices.cs: Auto generated interface.

[Guid("547a7f6e-eeda-4f77-94d0-2dd24f38ba58")]
public partial interface ILanguageServices
{
    /// <summary>
    /// 
    /// </summary>
    System.Boolean Offence_ExactCaseMatch { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean Offence_Regex { get; }

    /// <summary>
    /// 
    /// </summary>
    string Offence { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean Colour_ExactCaseMatch { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean Colour_Regex { get; }

    /// <summary>
    /// 
    /// </summary>
    string Colour { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean DebtManagementSystem_ExactCaseMatch { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean DebtManagementSystem_Regex { get; }

    /// <summary>
    /// 
    /// </summary>
    string DebtManagementSystem { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean DateOfContravention_ExactCaseMatch { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean DateOfContravention_Regex { get; }

    /// <summary>
    /// 
    /// </summary>
    string DateOfContravention { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean ContraventionDetails_ExactCaseMatch { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean ContraventionDetails_Regex { get; }

    /// <summary>
    /// 
    /// </summary>
    string ContraventionDetails { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean Income_ExactCaseMatch { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean Income_Regex { get; }

    /// <summary>
    /// 
    /// </summary>
    string Income { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean Hold_ExactCaseMatch { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean Hold_Regex { get; }

    /// <summary>
    /// 
    /// </summary>
    string Hold { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean CivilEnforcementOfficer_ExactCaseMatch { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean CivilEnforcementOfficer_Regex { get; }

    /// <summary>
    /// 
    /// </summary>
    string CivilEnforcementOfficer { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean PCNDebt_ExactCaseMatch { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean PCNDebt_Regex { get; }

    /// <summary>
    /// 
    /// </summary>
    string PCNDebt { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean OnHold_ExactCaseMatch { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean OnHold_Regex { get; }

    /// <summary>
    /// 
    /// </summary>
    string OnHold { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean DatePutOnHold_ExactCaseMatch { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean DatePutOnHold_Regex { get; }

    /// <summary>
    /// 
    /// </summary>
    string DatePutOnHold { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean HoldCode_ExactCaseMatch { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean HoldCode_Regex { get; }

    /// <summary>
    /// 
    /// </summary>
    string HoldCode { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean DateHoldExpires_ExactCaseMatch { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean DateHoldExpires_Regex { get; }

    /// <summary>
    /// 
    /// </summary>
    string DateHoldExpires { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean PutOnHoldByUserName_ExactCaseMatch { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean PutOnHoldByUserName_Regex { get; }

    /// <summary>
    /// 
    /// </summary>
    string PutOnHoldByUserName { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean CurrentState_ExactCaseMatch { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean CurrentState_Regex { get; }

    /// <summary>
    /// 
    /// </summary>
    string CurrentState { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean Vrm_ExactCaseMatch { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean Vrm_Regex { get; }

    /// <summary>
    /// 
    /// </summary>
    string Vrm { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean State_ExactCaseMatch { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean State_Regex { get; }

    /// <summary>
    /// 
    /// </summary>
    string State { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean CurrentStatechangedd2d2d4_ExactCaseMatch { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean CurrentStatechangedd2d2d4_Regex { get; }

    /// <summary>
    /// 
    /// </summary>
    string CurrentStatechangedd2d2d4 { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean SimonTest_ExactCaseMatch { get; }

    /// <summary>
    /// 
    /// </summary>
    System.Boolean SimonTest_Regex { get; }

    /// <summary>
    /// 
    /// </summary>
    string SimonTest { get; } 
}

ILanguageServices_Static.cs: The non-changing part of the interface

public partial interface ILanguageServices
{
    [DispId(0)]
    ICultureInfo Culture { get; set; }
    [DispId(1)]
    IResourceManager ResourceManager { get; }
    [DispId(2)]
    ICultureInfo[] GetCultures(System.Globalization.CultureTypes enCultureTypes);
    [DispId(3)]
    ICultureInfo GetCultureInfo(int LCID);
    [DispId(4)]
    ICultureInfo CurrentCulture { get; }
    [DispId(5)]
    string TranslateString(string rawString, bool searchInsideString);
    [DispId(6)]
    string TranslateString(string rawString);
}

Thinking about it, I could probably just make it not a partial class. Just change the xslt that generated the auto-generated part to include the static part. It was just neat to keep it seperate.

Regardless, can anybody tell me why it's not working and how to keep tighter control over the COM interface? Strictly ordering the methods just seems so... bleugh.

Thanks,

J1M.

Answer 1

From the C# Langauge Specification Version 4 Section 10.2.6

The ordering of members within a type is rarely significant to C# code, but may be significant when interfacing with other languages and environments. In these cases, the ordering of members within a type declared in multiple parts is undefined.

So there are no provisions in the c# laguage to control the ordering of members of a type, other than the order they are declared. In a type that is declared partially, then the order is completley undefined.

So the conclusion here is don't use partial declarations for interfaces that you are going to expose to COM. There is no way to control the interface member order, and since its undefined in the language the resulting member order could change at any time.