c# how to convert IntPtr to struct?

Question

A simple ATL com server:

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 definition:

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 client:

    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); 

The last line returns the Exception:

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

Since Marshal.ReadInt32(ptr) could return the right value 3, why cannot convert IntPtr to struct?

PS: The sVersionStruct from ILSpy:

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

Answer 1

You should call the overload of PtrToStructure that expects a type instead of an object. That's what's causing the error probably:

var ver = Marshal.PtrToStructure(ptr, typeof(MathServLib.sVersionStruct));