Added string to byte[] conversion method to ObjectId.

libgit2sharp.Tests/ObjectIdFixture.cs

+using System;
+using NUnit.Framework;
+
+namespace libgit2sharp.Tests
+{
+    [TestFixture]
+    public class ObjectIdFixture
+    {
+        [TestCase("DDelORu/9Dw38NA3GCOlUJ7tWx0=", "0c37a5391bbff43c37f0d0371823a5509eed5b1d")]
+        [TestCase("FqASNFZ4mrze9Ld1ITwjqL109eA=", "16a0123456789abcdef4b775213c23a8bd74f5e0")]
+        public void ToString(string encoded, string expected)
+        {
+            byte[] id = Convert.FromBase64String(encoded);
+
+            string objectId = ObjectId.ToString(id);
+
+            Assert.AreEqual(expected, objectId);
+        }
+
+        [TestCase("0c37a5391bbff43c37f0d0371823a5509eed5b1d", "DDelORu/9Dw38NA3GCOlUJ7tWx0=")]
+        [TestCase("16a0123456789abcdef4b775213c23a8bd74f5e0", "FqASNFZ4mrze9Ld1ITwjqL109eA=")]
+        public void ToByteArray(string objectId, string expected)
+        {
+            byte[] id = Convert.FromBase64String(expected);
+
+            byte[] rawId = ObjectId.ToByteArray(objectId);
+
+            CollectionAssert.AreEqual(id, rawId);
+        }
+    }
+}
\ No newline at end of file

libgit2sharp.Tests/libgit2sharp.Tests.csproj

@@ -44,6 +44,7 @@
   </ItemGroup>
   <ItemGroup>
     <Compile Include="EpochHelperFixture.cs" />
+    <Compile Include="ObjectIdFixture.cs" />
     <Compile Include="RepositoryFixtures.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
   </ItemGroup>

libgit2sharp/ObjectId.cs

-using System.Diagnostics;
+using System;
+using System.Diagnostics;
 using libgit2sharp.Wrapper;
 
 namespace libgit2sharp
 {
     public static class ObjectId
     {
-        private static readonly char[] hexDigits = new []
-                                                       {
-                                                           '0', '1', '2', '3', '4', '5', '6', '7', '8', 
-                                                           '9', 'a', 'b', 'c', 'd', 'e', 'f'
-                                                       };
+        private static readonly char[] HexDigits = "0123456789abcdef".ToCharArray();
+        private static readonly byte[] ReverseHexDigits = BuildReverseHexDigits();
+
+        private static byte[] BuildReverseHexDigits()
+        {
+            var bytes = new byte['f' - '0' + 1];
+
+            for (int i = 0; i < 10; i++)
+            {
+                bytes[i] = (byte)i;
+            }
+
+            for (int i = 10; i < 16; i++)
+            {
+                bytes[i + 'a' - '0' - 0x0a] = (byte)(i);
+            }
+
+            return bytes;
+        }
 
         public static string ToString(byte[] id)
         {
@@ -17,19 +32,38 @@ public static string ToString(byte[] id)
 
             // Inspired from http://stackoverflow.com/questions/623104/c-byte-to-hex-string/3974535#3974535
 
-            var c = new char[Constants.GIT_OID_RAWSZ * 2];
+            var c = new char[Constants.GIT_OID_HEXSZ];
 
-            for (int i = 0; i < Constants.GIT_OID_RAWSZ * 2; i++)
+            for (int i = 0; i < Constants.GIT_OID_HEXSZ; i++)
             {
                 int index0 = i >> 1;
                 var b = ((byte)(id[index0] >> 4));
-                c[i++] = hexDigits[b];
+                c[i++] = HexDigits[b];
 
                 b = ((byte)(id[index0] & 0x0F));
-                c[i] = hexDigits[b];
+                c[i] = HexDigits[b];
             }
 
             return new string(c);
         }
+
+        public static byte[] ToByteArray(string id)
+        {
+            Debug.Assert(id != null && id.Length == Constants.GIT_OID_HEXSZ);
+
+            var bytes = new byte[Constants.GIT_OID_RAWSZ];
+
+            for (int i = 0; i < Constants.GIT_OID_HEXSZ; i++)
+            {
+                int c1 = ByteConverter(id[i++]) << 4;
+                int c2 = ByteConverter(id[i]);
+
+                bytes[i >> 1] = (byte)(c1 + c2);
+            }
+
+            return bytes;
+        }
+
+        private static readonly Func<int, byte> ByteConverter = i => ReverseHexDigits[i - '0'];
     }
 }
\ No newline at end of file

libgit2sharp/Wrapper/Constants.cs

@@ -5,5 +5,6 @@ public static class Constants
         //public const int GIT_PATH_MAX = 4096;
         public const int GIT_ERROR = -1;
         public const int GIT_OID_RAWSZ = 20;
+        public const int GIT_OID_HEXSZ = GIT_OID_RAWSZ * 2;
     }
 }
\ No newline at end of file