类型为"System.Int32"的对象不能转换为类型"System.UInt32&"

Question

当我执行以下代码行时

dispatch.GetTypeInfoCount(ref typeInfoCount);

引发以下异常

"System.Int32"类型的对象无法转换为"System.UInt32&"类型

using System.Runtime.InteropServices;
using ComTypes2 = System.Runtime.InteropServices.ComTypes;
public class ComHelper
{
[ComImport(), Guid("00020400-0000-0000-c000-000000000046"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    private interface IDispatch
    {
        void GetTypeInfoCount(ref uint pctinfo);
        void GetTypeInfo(uint itinfo, uint lcid, ref IntPtr pptinfo);
        void stub_GetIDsOfNames();
        void Invoke(int dispIdMember, 
            ref Guid riid, 
            uint lcid, 
            ushort dwFlags, 
            ref ComTypes2.DISPPARAMS pDispParams, 
            ref object pVarResult, 
            ref IntPtr pExcepInfo, 
            ref uint pArgErr);
    }
   public static bool CheckIfComPropertyOrMethodExists<T1>(T1 objectToCheck, string propertyOrMethodName)
    {
        ComTypes2.ITypeInfo objectTypeInfo = null;
        var objectITypeInfo = default(IntPtr);
        var pFuncDesc = default(IntPtr);
        try
        {
            //  Convert the object to IDispatch
            var dispatch = (IDispatch)objectToCheck;
            uint typeInfoCount = 0;
            //  Attempt to get the objects TypeInfo
            dispatch.GetTypeInfoCount(ref typeInfoCount);
            if (typeInfoCount < 1) throw new ApplicationException("No type info");
            dispatch.GetTypeInfo(0, 0, ref objectITypeInfo);
            if (objectITypeInfo == IntPtr.Zero) throw new ApplicationException("No ITypeInfo");
            objectTypeInfo = (ComTypes2.ITypeInfo)Marshal.GetTypedObjectForIUnknown(objectITypeInfo, typeof(ComTypes2.ITypeInfo));
            var pTypeAttr = default(IntPtr);
            objectTypeInfo.GetTypeAttr(out pTypeAttr);
            var typeAttr = (ComTypes2.TYPEATTR)Marshal.PtrToStructure(pTypeAttr, typeof(ComTypes2.TYPEATTR));
            objectTypeInfo.ReleaseTypeAttr(pTypeAttr);
            //  Find the method we're looking for in the list of COM objects methods
            for (var iFunc = 0; iFunc <= typeAttr.cFuncs - 1; iFunc++)
            {
                objectTypeInfo.GetFuncDesc(iFunc, out pFuncDesc);
                var funcDesc = (ComTypes2.FUNCDESC)Marshal.PtrToStructure(pFuncDesc, typeof(ComTypes2.FUNCDESC));
                string[] names = { string.Empty };
                int pcNames;
                objectTypeInfo.GetNames(funcDesc.memid, names, 1, out pcNames);
                var funcName = names[0];
                if (funcName == propertyOrMethodName)
                {
                    return true;
                }
                objectTypeInfo.ReleaseFuncDesc(pFuncDesc);
            }
            return false;
        }
        finally
        {
            if (objectTypeInfo != null)
            {
                objectTypeInfo.ReleaseFuncDesc(pFuncDesc);
            }
            Marshal.Release(objectITypeInfo);
        }
    }
}

我确信答案很简单，但我现在还想不出来。GetTypeInfoCount的参数是uint。通过引用GetTypeInfoCount方法传递的局部变量typeInfoCount也是uint。为什么我会得到类型转换异常？

这个问题与以下有关

如何在不生成异常的情况下检查COM属性或方法是否存在？

到目前为止有用的链接

http://msdn.microsoft.com/en-us/library/ebbff4bc-36b2-4861-9efa-ffa45e013eb5%28VS.85%29

http://en.wikipedia.org/wiki/IDispatch

Answer 1

尝试使用out而不是ref:

void GetTypeInfoCount(out uint pctinfo);

然后：

uint typeInfoCount;
dispatch.GetTypeInfoCount(out typeInfoCount);

顺便说一下，GetTypeInfo方法也有同样的问题。您使用ref作为指针，但它应该是out:

void GetTypeInfo(uint itinfo, uint lcid, out IntPtr pptinfo);

以下是IDispatch接口的正确p/Invoke包装器：

[Guid("00020400-0000-0000-C000-000000000046")]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
internal interface IDispatch
{
    void GetTypeInfoCount(out uint pctinfo);
    void GetTypeInfo(uint iTInfo, int lcid, out IntPtr info);
    void GetIDsOfNames(ref Guid iid, [MarshalAs(UnmanagedType.LPArray, ArraySubType=UnmanagedType.LPWStr, SizeParamIndex=2)] string[] names, uint cNames, int lcid, [Out, MarshalAs(UnmanagedType.LPArray, ArraySubType=UnmanagedType.I4, SizeParamIndex=2)] int[] rgDispId);
    void Invoke(int dispIdMember, ref Guid riid, int lcid, INVOKEKIND wFlags, ref DISPPARAMS pDispParams, IntPtr pvarResult, IntPtr pExcepInfo, IntPtr puArgErr);
}