I got the following error:
System.ArgumentException: Value does not fall within the expected range.  At
System.DirectorySservcies.Interop.UnsafeNativeMethods.IAds.PutEx(Int32
InControlCode, String bstrName, Object vProp) at
System.DirecotryServices.PropertyValueCollection.set_Value(Object value)

public struct Blob
{
   public IntPtr pData;
   public int nLength;//the length of the actual data in nLength and
   public int nSize;//the length of the buffer for the data in nSize.

}
[DllImport("PaAsn.dll", CharSet = CharSet.Unicode)]
public static extern DE_ERRORS EncodeContextDefaults(ref IntPtr blob,
    [In, Out]ContextDefaults m);

// allocate memry for the blob
IntPtr pBlob = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(CDS.Blob)));
// call the function passing the pointer to the blob
byrefEncodeContextDefaults
CDS.LibWrap.EncodeContextDefaults(ref pBlob, contextDefaults);
// copy the unmanaged blob to managed struct Blob
CDS.Blob blob = (CDS.Blob)Marshal.PtrToStructure(pBlob, typeof(CDS.Blob));
// allocate byte array with size of blob.nLength
byte[] meetingBlob = new byte[blob.nLength];
// marshal the data pointed to by pData into the byte array
System.Runtime.InteropServices.Marshal.Copy(blob.pData, meetingBlob, 0,
   blob.nLength);
defaultContext.Properties["meetingBlob"].Value = ((object)(blob));
defaultContext.CommitChanges();

Thank you.