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POIs it posible to perform QueryInterface on behalf of VBScript
 

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  1. POIs it posible to perform QueryInterface on behalf of VBScript
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    <p>I'm trying to create a COM class with one method that will cast an object to a specific interface on behalf of VBScript.</p> <p>This is the method signature I'm using:<br /> <code>public object GetInterface(object unknown, string iid)</code></p> <p>I thought this would be possible because if the method explicitly declares the return type as : <br /> <code>public IRequestedInterface GetInterface(object unknown, string iid)</code> </p> <p>Then VBScript gets the reference to the desired interface.</p> <p>So I tried just casting to the interface<br /> <code>return (IRequestedInterface)unknown;</code> </p> <p>Unfortunately, VBScript gets a reference to the default interface instead of the requested interface.</p> <p>I have tried getting round this by creating a custom marshaller using <code>ICustomMarshaler</code>.<br /> I thought this would work because the method <code>MarshalManagedToNative</code> returns a <code>IntPtr</code>.</p> <p>Because of this I thought that if i just returned the <code>IntPtr</code> to the interface <br /> <code>return Marshal.GetComInterfaceForObject(unknown, typeof(IRequestedInterface));</code><br /> it would work. But, obviously, it didn't have the desired effect :(</p> <p>So does anybody know if it is posible and how you would do it?</p> <p>EDIT:</p> <p>I thought it would be helpful to add a concrete example (although it is contrived) to explain why I haven't accepted that VBScript will always get the default interface. I'm still clinging to my hope.</p> <p>Below you will find the contents of 3 files, 'TestLib.cs', 'Build.cmd' and 'Test.vbs'. These hopefully demonstrate why I still think it 'should' be possible.</p> <p>Note: I have tested this on Windows XP SP3 (x86).</p> <p>TestLib.cs</p> <pre><code>using System; using System.Reflection; using System.Runtime.InteropServices; using System.Windows.Forms; [assembly: ComVisible(false)] [assembly: Guid("64e20009-c664-4883-a6e5-1e36a31a0fd8")] [assembly: AssemblyVersion("2012.06.*")] [ComVisible(true)] [Guid("EB77C7B1-D1B9-4BB3-9D63-FBFBD56C9ABA")] [InterfaceType(ComInterfaceType.InterfaceIsDual)] public interface IPerformQi { [DispId(1000)] object GetInterface(object unknown, string iid); [DispId(2000)] IRequested GetIRequested(object unknown); } [ComVisible(true)] [Guid("7742BC0A-8719-483E-B1DF-AE9CD9A958DC")] [InterfaceType(ComInterfaceType.InterfaceIsDual)] public interface IDefault { [DispId(1000)] void SayHello(string name); } [ComVisible(true)] [Guid("FFF34296-2A06-47D4-B09C-B93B63D5CC53")] [InterfaceType(ComInterfaceType.InterfaceIsDual)] public interface IRequested { [DispId(1000)] void SayGoodbye(string name); } [ComVisible(true)] [ClassInterface(ClassInterfaceType.None)] [ComDefaultInterface(typeof(IPerformQi))] [Guid("222BB88D-B9FA-4F23-8DB3-BA998F4E668B")] [ProgId("TestLib.PerformQi")] public class PerformQi : IPerformQi { object IPerformQi.GetInterface(object unknown, string iid) { if(iid == "FFF34296-2A06-47D4-B09C-B93B63D5CC53") return (IRequested)unknown; throw new Exception("Unable to find inteface"); } IRequested IPerformQi.GetIRequested(object unknown) { return (IRequested)unknown; } } [ComVisible(true)] [ClassInterface(ClassInterfaceType.None)] [ComDefaultInterface(typeof(IDefault))] [Guid("174ABED6-3325-4878-89E3-BF8BD1107488")] [ProgId("TestLib.Test")] public class Test : IDefault, IRequested { void IDefault.SayHello(string name) { MessageBox.Show(string.Format("Hello '{0}'", name)); } void IRequested.SayGoodbye(string name) { MessageBox.Show(string.Format("Goodbye '{0}'", name)); } }</code></pre> <p>Build.cmd</p> <pre><code>"%windir%\Microsoft.Net\Framework\v4.0.30319\csc.exe" /out:TestLib.dll /target:library /r:System.Windows.Forms.dll TestLib.cs "%windir%\Microsoft.NET\Framework\v4.0.30319\RegAsm.exe" TestLib.dll /codebase /tlb:TestLib.tlb PAUSE</code></pre> <p>Test.vbs</p> <pre><code>Dim oPerformQi 'As TestLib.PerformQi Dim oTest 'As TestLib.Test Dim oTest2 'As IRequested Dim oTest3 'As IRequested Set oPerformQi = CreateObject("TestLib.PerformQi") Set oTest = CreateObject("TestLib.Test") Call oTest.SayHello("Robert") Set oTest2 = oPerformQi.GetIRequested(oTest) 'Note: This works Call oTest2.SayGoodbye("Robert") Set oTest3 = oPerformQi.GetInterface(oTest, "FFF34296-2A06-47D4-B09C-B93B63D5CC53") 'Note: This does not work Call oTest3.SayGoodbye("Robert")</code></pre> <p>Using the call <code>oPerformQi.GetIRequested(oTest)</code> makes the call to <code>oTest3.SayGoodbye("Robert")</code> work. This makes me think you are not limited to just the default interface in VBS.</p> <p>Perhaps .Net is not capable of returning the specified interface because of an implicit cast on the return value? Ideally I would use generics for this, but as we all know COM does not support genrics.</p> <p>Under this restriction is there any other way that you can think of to achieve this?</p> <p>EDIT 2:</p> <p>I have found that I can achieve this using VB6, below is the code for the class.</p> <pre><code>Option Explicit Public Function GetInterface(ByVal oUnknown As Object, ByVal IID As String) As Variant Dim oIRequested As IRequested If IID = "FFF34296-2A06-47D4-B09C-B93B63D5CC53" Then Set oIRequested = oUnknown Set GetInterface = oIRequested Else Err.Raise 1, , "Unable to find inteface" End If End Function </code></pre> <p>I would still like to find a C# version if anybody can shed some light on the subject i would appreciate it.</p>
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    1. COThere's a high likelihood that this won't work if IRequestedInterface is a non-default interface of the same coclass. The cast will work but the VBScript interpreter will just QI for IDispatch again and get back the default interface.
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      1. USHans Passant
    2. COHI. Thanks for your comment. I understand what you are saying, however VBScript can use a non-default interface when the method returning it explicitly declares that interface as the return value. I’ve edited my question due to the feedback I have received so far. I’d really appreciate it if you could have another look in light of my edit. Thanks
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    3. COIt appears that fact VBScript can use a non default interface was unintentional http://blogs.msdn.com/b/ericlippert/archive/2003/10/10/53188.aspx.
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