Problems with Marshal.PtrToStructure and String

I've the following class

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
public class xy11Data开发者_开发百科set : SZLDataset
{
    public short Index { get; set; }

    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
    private string _mlfB;
    public string MlfB
    {
        get { return _mlfB; }
        set { _mlfB = value; }
    }

    public UInt16 BGTyp { get; set; }

    public UInt16 Ausbg1 { get; set; }

    public UInt16 Ausbg2 { get; set; }
}

and I fill it with the following code:

byte[] objBuffer = new byte[retVal.Size];
Array.Copy(buffer, (n*retVal.Size) + 8, objBuffer, 0, retVal.Size);
GCHandle handle = GCHandle.Alloc(objBuffer, GCHandleType.Pinned);
datsets.Add((xy11Dataset)Marshal.PtrToStructure(handle.AddrOfPinnedObject(), typeof(xy11Dataset)));
handle.Free();

I've Values in objBuffer at Position 2 (which should be the start of the string), but the string stays empty!

You need to give up on using auto properties here. They generate a private backing field that is not sequential with the property, it's added to the end. You can see them with ildasm.exe, they have a name like <Index>k_Backingfield. You need to make this expression return the correct value:

        int offs = (int)Marshal.OffsetOf(typeof(xy11Dataset), "_mlfB");

I cannot see what SZLDataSet contains but without it this returns 0 right now. Not correct, you'd want 2. Best thing to do is to declare a struct with public fields whose layout is an exact match with the data in the buffer. Initialize the class object from the value.