ITypeLib2.GetLibStatistics() always throws AccessViolationException in C#

Question

I'm trying to call the GetLibStatistics method of the ITypeLib2 interface. I've tried several variations and techniques, but they all throw System.AccessViolationException: Attempted to read or write protected memory.

I was able to successfully execute that method in native COM in C++, so I know the .tlb file I'm using is not at fault.

My guess at this point is that GetLibStatistics is not implemented in C#. Please advise.

public static void GetLibStatisticsTest()
{
    ITypeLib tlb;
    LoadTypeLibEx(@"C:\Sample.tlb", RegKind.None, out tlb);

    var tlb2 = tlb as ITypeLib2;
    if (tlb2 == null ) { return; }

    try
    {
        IntPtr pcUniqueNames = IntPtr.Zero;
        int pcchUniqueNames;

        // This always throws `System.AccessViolationException`. Why??
        tlb2.GetLibStatistics(pcUniqueNames, out pcchUniqueNames);;
    }
    catch (Exception ex)
    {
        Console.WriteLine(ex.Message);
    }
}

[DllImport("oleaut32.dll", CharSet = CharSet.Auto, SetLastError = true)]
private static extern int LoadTypeLibEx(string szFile, RegKind regKind, out ITypeLib pptlib);

Answer 1

ITypeLib2 defined by .NET is "simply" wrong. It's defined like this in C/C++:

ITypeLib2 : public ITypeLib
{
public:
    virtual HRESULT STDMETHODCALLTYPE GetCustData(REFGUID guid, VARIANT *pVarVal) = 0;
    virtual HRESULT STDMETHODCALLTYPE GetLibStatistics(ULONG *pcUniqueNames, ULONG *pcchUniqueNames) = 0;
    virtual HRESULT STDMETHODCALLTYPE GetDocumentation2(INT index, LCID lcid, BSTR *pbstrHelpString, DWORD *pdwHelpStringContext, BSTR *pbstrHelpStringDll) = 0;
    virtual HRESULT STDMETHODCALLTYPE GetAllCustData(CUSTDATA *pCustData) = 0;
}

And like this in .NET

public interface ITypeLib2 : ITypeLib
{
    ... ITypeLib ...

    void GetCustData(ref Guid guid, out object pVarVal);
    void GetDocumentation2(int index, out string pbstrHelpString, out int pdwHelpStringContext, out string pbstrHelpStringDll);
    void GetLibStatistics(IntPtr pcUniqueNames, out int pcchUniqueNames);
    void GetAllCustData(IntPtr pCustData);
}

So GetDocumentation2 and GetLibStatistics have been switched and you're calling GetDocumentation2 instead with obviously wrong parameters ().

This can be checked in a debugger:

So you must redefine ITypeLib2 in your code like this:

[ComImport, Guid("00020411-0000-0000-C000-000000000046"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface ITypeLib2 : ITypeLib
{
    [PreserveSig]
    new int GetTypeInfoCount();
    new void GetTypeInfo(int index, out ITypeInfo ppTI);
    new void GetTypeInfoType(int index, out System.Runtime.InteropServices.ComTypes.TYPEKIND pTKind);
    new void GetTypeInfoOfGuid(ref Guid guid, out ITypeInfo ppTInfo);
    new void GetLibAttr(out IntPtr ppTLibAttr);
    new void GetTypeComp(out ITypeComp ppTComp);
    new void GetDocumentation(int index, out string strName, out string strDocString, out int dwHelpContext, out string strHelpFile);
    new bool IsName([MarshalAs(UnmanagedType.LPWStr)] string szNameBuf, int lHashVal);
    new void FindName([MarshalAs(UnmanagedType.LPWStr)] string szNameBuf, int lHashVal, [Out][MarshalAs(UnmanagedType.LPArray)] ITypeInfo[] ppTInfo, [Out][MarshalAs(UnmanagedType.LPArray)] int[] rgMemId, ref short pcFound);
    [PreserveSig]
    new void ReleaseTLibAttr(IntPtr pTLibAttr);

    void GetCustData(ref Guid guid, out object pVarVal);
    void GetLibStatistics(IntPtr pcUniqueNames, out int pcchUniqueNames);
    void GetDocumentation2(int index, out string pbstrHelpString, out int pdwHelpStringContext, out string pbstrHelpStringDll);
    void GetAllCustData(IntPtr pCustData);
}

And it will work.

I've created an issue for this https://github.com/dotnet/runtime/issues/99946

Answer 2

The oaidl.idl file in the Windows SDK has the methods in the correct order, so who knows how it got imported into managed code incorrectly.

Anyway...

---

Here is the fixed ITypeLib2 interface so you can just copy/paste it into your project.

using System;
using System.Runtime.InteropServices;
using System.Runtime.InteropServices.ComTypes;

namespace MyNamespace
{
    using TYPEKIND = System.Runtime.InteropServices.ComTypes.TYPEKIND;

    //
    // Summary:
    //     Provides a managed definition of the ITypeLib2 interface.
    [ComImport]
    [Guid("00020411-0000-0000-C000-000000000046")]
    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    public interface ITypeLib2 : ITypeLib
    {
        //
        // Summary:
        //     Returns the number of type descriptions in the type library.
        //
        // Returns:
        //     The number of type descriptions in the type library.
        [PreserveSig]
        new int GetTypeInfoCount();

        //
        // Summary:
        //     Retrieves the specified type description in the library.
        //
        // Parameters:
        //   index:
        //     An index of the ITypeInfo interface to return.
        //
        //   ppTI:
        //     When this method returns, contains an ITypeInfo describing the type referenced
        //     by index. This parameter is passed uninitialized.
        new void GetTypeInfo(int index, out ITypeInfo ppTI);

        //
        // Summary:
        //     Retrieves the type of a type description.
        //
        // Parameters:
        //   index:
        //     The index of the type description within the type library.
        //
        //   pTKind:
        //     When this method returns, contains a reference to the TYPEKIND enumeration for
        //     the type description. This parameter is passed uninitialized.
        new void GetTypeInfoType(int index, out TYPEKIND pTKind);

        //
        // Summary:
        //     Retrieves the type description that corresponds to the specified GUID.
        //
        // Parameters:
        //   guid:
        //     The System.Guid, passed by reference, that represents the IID of the CLSID interface
        //     of the class whose type info is requested.
        //
        //   ppTInfo:
        //     When this method returns, contains the requested ITypeInfo interface. This parameter
        //     is passed uninitialized.
        new void GetTypeInfoOfGuid(ref Guid guid, out ITypeInfo ppTInfo);

        //
        // Summary:
        //     Retrieves the structure that contains the library's attributes.
        //
        // Parameters:
        //   ppTLibAttr:
        //     When this method returns, contains a structure that contains the library's attributes.
        //     This parameter is passed uninitialized.
        new void GetLibAttr(out IntPtr ppTLibAttr);

        //
        // Summary:
        //     Enables a client compiler to bind to a library's types, variables, constants,
        //     and global functions.
        //
        // Parameters:
        //   ppTComp:
        //     When this method returns, contains an ITypeComp instance for this ITypeLib. This
        //     parameter is passed uninitialized.
        new void GetTypeComp(out ITypeComp ppTComp);

        //
        // Summary:
        //     Retrieves the library's documentation string, the complete Help file name and
        //     path, and the context identifier for the library Help topic in the Help file.
        //
        //
        // Parameters:
        //   index:
        //     An index of the type description whose documentation is to be returned.
        //
        //   strName:
        //     When this method returns, contains a string that specifies the name of the specified
        //     item. This parameter is passed uninitialized.
        //
        //   strDocString:
        //     When this method returns, contains the documentation string for the specified
        //     item. This parameter is passed uninitialized.
        //
        //   dwHelpContext:
        //     When this method returns, contains the Help context identifier associated with
        //     the specified item. This parameter is passed uninitialized.
        //
        //   strHelpFile:
        //     When this method returns, contains a string that specifies the fully qualified
        //     name of the Help file. This parameter is passed uninitialized.
        new void GetDocumentation(int index, out string strName, out string strDocString, out int dwHelpContext, out string strHelpFile);

        //
        // Summary:
        //     Indicates whether a passed-in string contains the name of a type or member described
        //     in the library.
        //
        // Parameters:
        //   szNameBuf:
        //     The string to test.
        //
        //   lHashVal:
        //     The hash value of szNameBuf.
        //
        // Returns:
        //     true if szNameBuf was found in the type library; otherwise, false.
        [return: MarshalAs(UnmanagedType.Bool)]
        new bool IsName([MarshalAs(UnmanagedType.LPWStr)] string szNameBuf, int lHashVal);

        //
        // Summary:
        //     Finds occurrences of a type description in a type library.
        //
        // Parameters:
        //   szNameBuf:
        //     The name to search for.
        //
        //   lHashVal:
        //     A hash value to speed up the search, computed by the LHashValOfNameSys function.
        //     If lHashVal is 0, a value is computed.
        //
        //   ppTInfo:
        //     When this method returns, contains an array of pointers to the type descriptions
        //     that contain the name specified in szNameBuf. This parameter is passed uninitialized.
        //
        //
        //   rgMemId:
        //     When this method returns, contains an array of the MEMBERIDs of the found items;
        //     rgMemId [i] is the MEMBERID that indexes into the type description specified
        //     by ppTInfo [i]. This parameter cannot be null. This parameter is passed uninitialized.
        //
        //
        //   pcFound:
        //     On entry, a value, passed by reference, that indicates how many instances to
        //     look for. For example, pcFound = 1 can be called to find the first occurrence.
        //     The search stops when one instance is found. On exit, indicates the number of
        //     instances that were found. If the in and out values of pcFound are identical,
        //     there might be more type descriptions that contain the name.
        new void FindName([MarshalAs(UnmanagedType.LPWStr)] string szNameBuf, int lHashVal, [Out][MarshalAs(UnmanagedType.LPArray)] ITypeInfo[] ppTInfo, [Out][MarshalAs(UnmanagedType.LPArray)] int[] rgMemId, ref short pcFound);

        //
        // Summary:
        //     Releases the System.Runtime.InteropServices.TYPELIBATTR structure originally
        //     obtained from the System.Runtime.InteropServices.ComTypes.ITypeLib.GetLibAttr(System.IntPtr@)
        //     method.
        //
        // Parameters:
        //   pTLibAttr:
        //     The TLIBATTR structure to release.
        [PreserveSig]
        new void ReleaseTLibAttr(IntPtr pTLibAttr);

        //
        // Summary:
        //     Gets the custom data.
        //
        // Parameters:
        //   guid:
        //     A System.Guid , passed by reference, that is used to identify the data.
        //
        //   pVarVal:
        //     When this method returns, contains an object that specifies where to put the
        //     retrieved data. This parameter is passed uninitialized.
        void GetCustData(ref Guid guid, out object pVarVal);

        //
        // Summary:
        //     Returns statistics about a type library that are required for efficient sizing
        //     of hash tables.
        //
        // Parameters:
        //   pcUniqueNames:
        //     A pointer to a count of unique names. If the caller does not need this information,
        //     set to null.
        //
        //   pcchUniqueNames:
        //     When this method returns, contains a pointer to a change in the count of unique
        //     names. This parameter is passed uninitialized.
        void GetLibStatistics(IntPtr pcUniqueNames, out int pcchUniqueNames);

        //
        // Summary:
        //     Retrieves the library's documentation string, the complete Help file name and
        //     path, the localization context to use, and the context ID for the library Help
        //     topic in the Help file.
        //
        // Parameters:
        //   index:
        //     An index of the type description whose documentation is to be returned; if index
        //     is -1, the documentation for the library is returned.
        //
        //   pbstrHelpString:
        //     When this method returns, contains a BSTR that specifies the name of the specified
        //     item. If the caller does not need the item name, pbstrHelpString can be null.
        //     This parameter is passed uninitialized.
        //
        //   pdwHelpStringContext:
        //     When this method returns, contains the Help localization context. If the caller
        //     does not need the Help context, pdwHelpStringContext can be null. This parameter
        //     is passed uninitialized.
        //
        //   pbstrHelpStringDll:
        //     When this method returns, contains a BSTR that specifies the fully qualified
        //     name of the file containing the DLL used for Help file. If the caller does not
        //     need the file name, pbstrHelpStringDll can be null. This parameter is passed
        //     uninitialized.
        [LCIDConversion(1)]
        void GetDocumentation2(int index, out string pbstrHelpString, out int pdwHelpStringContext, out string pbstrHelpStringDll);

        //
        // Summary:
        //     Gets all custom data items for the library.
        //
        // Parameters:
        //   pCustData:
        //     A pointer to CUSTDATA, which holds all custom data items.
        void GetAllCustData(IntPtr pCustData);
    }
}