ComObjectMapper.cs

using ...

static void Main(string[] args)
{
    var x = (Microsoft.Office.Interop.Excel._Application) Marshal.GetActiveObject("Excel.Application");
    x.ChartDataPointTrack = false;
    MapperRegistry.Mappers.Add(new ComObjectMapper());
    var config = new MapperConfiguration(c => c.CreateMap<Microsoft.Office.Interop.Excel._Application, App>());
    config.AssertConfigurationIsValid();
    var mapper = config.CreateMapper();
    var result = mapper.Map<App>(x);
}

class ComObjectMapper : IObjectMapper
{
    private static readonly Func<ITypeInfo, Guid> GetTypeInfoGuid = (Func<ITypeInfo, Guid>) Delegate.CreateDelegate(typeof(Func<ITypeInfo, Guid>), typeof(Marshal).GetMethod("GetTypeInfoGuid", BindingFlags.NonPublic | BindingFlags.Static), throwOnBindFailure: true);
    private static readonly Type ComObjectType = typeof(object).Assembly.GetType("System.__ComObject");
    private static readonly Dictionary<Guid, Type> typesByGuid = GetTypes();

    private static Dictionary<Guid, Type> GetTypes()
    {
        var types = 
            from assembly in AppDomain.CurrentDomain.GetAssemblies()
            from type in assembly.ExportedTypes
            where type.IsInterface && type.IsImport
            select type;
        var typesByGuid = new Dictionary<Guid, Type>();
        foreach(var type in types)
        {
            typesByGuid[type.GUID] = type;
        }
        return typesByGuid;
    }

    /// <summary>
    /// Exposes objects, methods and properties to programming tools and other
    /// applications that support Automation.
    /// </summary>
    [ComImport()]
    [Guid("00020400-0000-0000-C000-000000000046")]
    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    interface IDispatch
    {
        [PreserveSig]
        int GetTypeInfoCount(out int Count);

        ITypeInfo GetTypeInfo([MarshalAs(UnmanagedType.U4)] int iTInfo, [MarshalAs(UnmanagedType.U4)] int lcid);

        [PreserveSig]
        int GetIDsOfNames(
            ref Guid riid,
            [MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.LPWStr)]
            string[] rgsNames,
            int cNames,
            int lcid,
            [MarshalAs(UnmanagedType.LPArray)] int[] rgDispId);

        [PreserveSig]
        int Invoke(
            int dispIdMember,
            ref Guid riid,
            uint lcid,
            ushort wFlags,
            ref System.Runtime.InteropServices.ComTypes.DISPPARAMS pDispParams,
            out object pVarResult,
            ref System.Runtime.InteropServices.ComTypes.EXCEPINFO pExcepInfo,
            IntPtr[] pArgErr);
    }

    public static Type GetComObjectType(object comObject)
    {
        if(!Marshal.IsComObject(comObject))
        {
            throw new ArgumentOutOfRangeException(nameof(comObject), "Expected a COM object.");
        }
        var dispatch = (IDispatch) comObject;
        var typeInfo = dispatch.GetTypeInfo(0, 0);
        var typeGuid = GetTypeInfoGuid(typeInfo);
        return typesByGuid[typeGuid];
    }

    public bool IsMatch(TypePair context)
    {
        return context.SourceType == ComObjectType;
    }

    public static Exception MissingMapException(Type sourceType, Type destinationType)
    {
        var source = sourceType.Name;
        var destination = destinationType.Name;
        throw new InvalidOperationException($"Missing map from {source} to {destination}. Create using Mapper.CreateMap<{source}, {destination}>.");
    }


    public object Map(ResolutionContext context)
    {
        var sourceType = GetComObjectType(context.SourceValue);
        var typeMap = context.ConfigurationProvider.ResolveTypeMap(sourceType, context.DestinationType);
        if(typeMap == null)
        {
            throw MissingMapException(sourceType, context.DestinationType);
        }
        var newContext = new ResolutionContext(context.SourceValue, context.DestinationValue, new TypePair(sourceType, context.DestinationType), context);
        newContext.TypeMap = typeMap;
        return context.Mapper.Map(newContext);
    }
}