C＃如何将IntPtr转换为结构？

一个简单的ATL com服务器：

STDMETHODIMP CMyMath::get_Version(sVersionStruct **ppVer)
{
    sVersionStruct* pVer = reinterpret_cast<sVersionStruct*>(CoTaskMemAlloc(sizeof(sVersionStruct)));
    if (!pVer) {
        return E_OUTOFMEMORY;
    } else {
        *pVer = ver_;
        *ppVer = pVer;
        return S_OK;
    }
    return S_OK;
}

STDMETHODIMP CMyMath::put_Version(sVersionStruct* ver)
{
    ver_ = *ver;
    return S_OK;
}

idl定义：

typedef 
[
    uuid(72A4AA5B-6AD0-4249-B4CB-2FFB08301608)
]
struct tagVersionStruct {
    int majorVersion;
    int minorVersion;
} sVersionStruct;

    [propget]
    HRESULT Version([out, retval, ref]sVersionStruct** ver);
    [propput]
    HRESULT Version([in]sVersionStruct* ver)

C＃.NET客户端：

    MathServLib.sVersionStruct ver;
    ver.minorVersion = 1;
    ver.majorVersion = 3;
    math.set_Version(ver);

    ver.minorVersion = 0;
    ver.majorVersion = 0;
    IntPtr ptr = math.get_Version();
    int i = Marshal.ReadInt32(ptr); // RETURN RIGHT VALUE 3
    Marshal.PtrToStructure(ptr, ver); 

最后一行返回异常：

An unhandled exception of type 'System.ArgumentException' occurred in mscorlib.dll. This structure is not value type.

既然Marshal.ReadInt32(ptr)可以返回正确的值3，为什么不能将IntPtr转换为struct？

PS：来自ILSpy的sVersionStruct：

namespace MathServLib
{
    [Guid("72A4AA5B-6AD0-4249-B4CB-2FFB08301608")]
    [StructLayout(LayoutKind.Sequential, Pack = 4)]
    public struct sVersionStruct
    {
        public int majorVersion;
        public int minorVersion;
    }
}