Question

is it possible to define a custom UnmanagedType for the MarshalAs attribute class?
Specifically I want to convert a long int unix time into a DateTime type. Something like this:

[MarshalAs(UnmanagedType.LongTimeUnix)]
public DateTime Time;

Where do I have to put the custom LongTimeUnix enumeration type and where to put the time conversion code:

public static DateTime ConvertUnix2DateTime(long timeStamp)
{
        DateTime DT = new DateTime(1970, 1, 1, 0, 0, 0, 0);
        DT = DT.AddSeconds(timeStamp);
        return DT;
}

When transferring the data with

(SomeStruct)Marshal.PtrToStructure(
 IntPtr,
 typeof(SomeStruct));

I want that the long time unix is automatically converted with the code sinppet above.
Do I have to inherit from the MarshalAs class and write the conversion into this class?
Thanks, Juergen

Update
Here is the custom marshaller:

class MarshalTest : ICustomMarshaler
{
    public void CleanUpManagedData(object ManagedObj)
    {
        throw new NotImplementedException();
    }

    public void CleanUpNativeData(IntPtr pNativeData)
    {
        throw new NotImplementedException();
    }

    public int GetNativeDataSize()
    {
        return 8;
    }

    public IntPtr MarshalManagedToNative(object ManagedObj)
    {
        throw new NotImplementedException();
    }

    public object MarshalNativeToManaged(IntPtr pNativeData)
    {
        long UnixTime = 0;
        try
        {
            UnixTime = Marshal.ReadInt64(pNativeData);
        }
        catch (Exception e)
        {

           QFXLogger.Error(e, "MarshalNativeToManaged");
        }
        DateTime DT = new DateTime(1970, 1, 1, 0, 0, 0, 0);
        DT = DT.AddSeconds(UnixTime);
        return DT;
    }
 }

Here is the class definition:

unsafe public struct MT5ServerAttributes
{
    /// <summary>
    /// Last known server time.
    /// </summary>
    [MarshalAs(UnmanagedType.CustomMarshaler, MarshalTypeRef = typeof(MarshalTest))]
    public DateTime CurrentTime;

    //[MarshalAs(UnmanagedType.U8)]
    [MarshalAs(UnmanagedType.CustomMarshaler, MarshalTypeRef = typeof(MarshalTest))]
    public DateTime TradeTime;

 }

And finally the code to marshal the data from unmanaged memory:

try
{
   MT5ServerAttributes MT5SrvAttributes = (MT5ServerAttributes)Marshal.PtrToStructure(mMT5Proxy.MT5InformationProxy.ServerData,
                                                                    typeof(MT5ServerAttributes));
}
catch (Exception e)
{

QFXLogger.Error(e, "ConsumeCommand inner");
}

When running this the following excpetion is thrown(which is not a direct exception ot PtrToStructure!)
Cannot marshal field ‘CurrentTime’ of type ‘QFX_DLL.MT5ServerAttributes’: Invalid managed/unmanaged type combination (the DateTime class must be paired with Struct).
Any ideas?

Answer 1

You cannot add your own to the enumeration, but you can use UnmanagedType.CustomMarshaler. To specify that you want to marshal it using a custom type.

MSDN has an entire section dedicated to this.

You would end up doing something along these lines:

[MarshalAs(UnmanagedType.CustomMarshaler, MarshalTypeRef = typeof(MyCustomMarshaler))]
public DateTime Time;

Then implement MyCustomMarshaler as ICustomMarshaler.