.NET 8 ComVisible对象通过CreateObject()创建时出现‘Object required’错误

问题场景 #

用.NET 8和C#创建了一个COM可见的计数器类，代码如下：

SimpleCounter.cs #

using System.Runtime.InteropServices;

namespace Counters
{
    [ComVisible(true)]
    [Guid("01FB5599-3CD1-462E-98AA-C29614C0AFEC")]
    [ClassInterface(ClassInterfaceType.None)]
    [ProgId("Counters.SimpleCounter")]
    public class SimpleCounter : ISimpleCounter
    {
        private int _counter;

        public SimpleCounter()
        {
            _counter = 0;
        }

        public int Counter
        {
            get { return _counter; }
            set { _counter = value; }
        }

        public void IncrementCounter()
        {
            _counter++;
        }
    }
}

ISimpleCounter.cs（初始版本） #

using System.Runtime.InteropServices;

namespace Counters
{
    [ComVisible(true)]
    [Guid("D65B22DD-4E59-4C11-88B3-67A2535D472E")]
    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    public interface ISimpleCounter
    {
        public int Counter { get; set; }

        public void IncrementCounter();
    }
}

Counters.csproj #

<Project Sdk="Microsoft.NET.Sdk">

    <PropertyGroup>
        <TargetFramework>net8.0-windows</TargetFramework>
        <ImplicitUsings>enable</ImplicitUsings>
        <Nullable>enable</Nullable>
        <PlatformTarget>x86</PlatformTarget>
        <EnableComHosting>true</EnableComHosting>
        <Platforms>AnyCPU;x86</Platforms>
    </PropertyGroup>

</Project>

项目构建后生成Counters.dll和Counters.comhost.dll，通过dscom32生成并注册TLB，再用regsvr32注册comhost.dll，操作符合.NET 8 COM规范。

在VB6中测试时，早期绑定（直接声明类型）可以正常运行：

Private Sub Command1_Click()

    Dim obj As New Counters.SimpleCounter
    MsgBox "Counter is " & obj.Counter
    obj.IncrementCounter
    MsgBox "Counter now is " & obj.Counter

End Sub

但使用后期绑定（CreateObject）创建对象时，即使obj不为Nothing，调用obj.Counter仍会触发“Object required”错误：

' 报错的代码示例
Private Sub Command1_Click()

    Dim obj As Object
    Set obj = CreateObject("Counters.SimpleCounter")
    MsgBox "Counter is " & obj.Counter ' 此处触发错误

End Sub

原因分析 #

VB6的后期绑定依赖IDispatch接口来动态解析对象的属性和方法。当接口标记为InterfaceType(ComInterfaceType.InterfaceIsIUnknown)时，.NET仅向COM暴露IUnknown接口，不提供IDispatch支持，导致后期绑定无法识别Counter属性和IncrementCounter方法，从而抛出错误。

而InterfaceType(ComInterfaceType.InterfaceIsDual)会让接口同时暴露IUnknown和IDispatch，既支持早期绑定，也兼容VB6的后期绑定机制。

解决方法 #

将接口的InterfaceType属性修改为ComInterfaceType.InterfaceIsDual即可解决问题：

修改后的ISimpleCounter.cs #

using System.Runtime.InteropServices;

namespace Counters
{
    [ComVisible(true)]
    [Guid("D65B22DD-4E59-4C11-88B3-67A2535D472E")]
    [InterfaceType(ComInterfaceType.InterfaceIsDual)]
    public interface ISimpleCounter
    {
        public int Counter { get; set; }

        public void IncrementCounter();
    }
}

重新构建并注册COM对象后，使用CreateObject的后期绑定代码即可正常运行。

内容的提问来源于stack exchange，提问作者Étienne Laneville