Added C# bindings

The C# binding of the function opk_extract_file is still missing.

Author: pcercuei

bindings/csharp/Opk.cs

@@ -0,0 +1,67 @@
+using System;
+using System.Runtime.InteropServices;
+
+namespace opk
+{
+	public class Opk
+	{
+		private IntPtr opk;
+
+		[DllImport("libopk.dll", CallingConvention = CallingConvention.Cdecl)]
+		private static extern IntPtr opk_open(
+			[In()][MarshalAs(UnmanagedType.LPStr)] string filename);
+
+		[DllImport("libopk.dll", CallingConvention = CallingConvention.Cdecl)]
+		private static extern void opk_close(IntPtr opk);
+
+		[DllImport("libopk.dll", CallingConvention = CallingConvention.Cdecl)]
+		private static extern int opk_open_metadata(IntPtr opk, ref IntPtr filename);
+
+		[DllImport("libopk.dll", CallingConvention = CallingConvention.Cdecl)]
+		private static extern int opk_read_pair(IntPtr opk,
+					ref IntPtr key, ref ulong key_len,
+					ref IntPtr val, ref ulong val_len);
+
+		public Opk(string filename)
+		{
+			this.opk = opk_open(filename);
+		}
+
+		~Opk()
+		{
+			opk_close(this.opk);
+		}
+
+		public string open_metadata()
+		{
+			IntPtr ptr = (IntPtr) 0;
+
+			int ret = opk_open_metadata(this.opk, ref ptr);
+			if (ret < 0)
+				throw new Exception("Unable to open metadata: err=" + ret);
+			if (ret == 0)
+				return null;
+
+			return Marshal.PtrToStringAnsi(ptr);
+		}
+
+		public bool read_pair(out string key, out string value)
+		{
+			IntPtr kptr = (IntPtr) 0, vptr = (IntPtr) 0;
+			ulong klen = 0, vlen = 0;
+
+			int ret = opk_read_pair(this.opk, ref kptr, ref klen, ref vptr, ref vlen);
+			if (ret < 0)
+				throw new Exception("Unable to read key/value pair: err=" + ret);
+			if (ret == 0) {
+				key = null;
+				value = null;
+				return false;
+			}
+
+			value = Marshal.PtrToStringAnsi(vptr, (int) vlen);
+			key = Marshal.PtrToStringAnsi(kptr, (int) klen);
+			return true;
+		}
+	}
+}