C# Definition:

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct SHQUERYRBINFO
{
    public int cbSize;
    public long i64Size;
    public long i64NumItems;
}

Example code

using System;
using System.Runtime.InteropServices;

public class Class1
{
    [DllImport("shell32.dll")] public static extern int SHQueryRecycleBin(string pszRootPath, ref SHQUERYRBINFO pSHQueryRBInfo);

     [StructLayout(LayoutKind.Sequential, Pack = 1)]
     public struct SHQUERYRBINFO
     {
        public int cbSize;
        public long i64Size;
        public long i64NumItems;
     }

     [STAThread]
     static void Main(string[] args)
     {
        SHQUERYRBINFO shrb = new SHQUERYRBINFO();
        shrb.cbSize = Marshal.SizeOf(typeof(SHQUERYRBINFO));
        SHQueryRecycleBin(String.Empty,ref shrb);

        Console.WriteLine("Recycle bin number: " + shrb.i64NumItems.ToString());
        Console.WriteLine("Recylce bin size: " + shrb.i64Size.ToString());
     }
}