Introduce OrphanedHeadException

4 files changed, 66 insertions, 2 deletions

diff --git a/LibGit2Sharp.Tests/CheckoutFixture.cs b/LibGit2Sharp.Tests/CheckoutFixture.cs
index b75c7a63..0ab18740 100644
--- a/LibGit2Sharp.Tests/CheckoutFixture.cs
+++ b/LibGit2Sharp.Tests/CheckoutFixture.cs
@@ -291,6 +291,19 @@ namespace LibGit2Sharp.Tests
         }
 
         [Fact]
+        public void CheckingOutAgainstAnUnbornBranchThrows()
+        {
+            SelfCleaningDirectory scd = BuildSelfCleaningDirectory();
+
+            using (var repo = Repository.Init(scd.DirectoryPath))
+            {
+                Assert.True(repo.Info.IsHeadOrphaned);
+
+                Assert.Throws<OrphanedHeadException>(() => repo.Checkout(repo.Head));
+            }
+        }
+
+        [Fact]
         public void CheckingOutANonExistingBranchThrows()
         {
             using (var repo = new Repository(StandardTestRepoWorkingDirPath))
diff --git a/LibGit2Sharp/LibGit2Sharp.csproj b/LibGit2Sharp/LibGit2Sharp.csproj
index 4e8f1ce1..d9723b04 100644
--- a/LibGit2Sharp/LibGit2Sharp.csproj
+++ b/LibGit2Sharp/LibGit2Sharp.csproj
@@ -65,6 +65,7 @@
     <Compile Include="Changes.cs" />
     <Compile Include="CheckoutCallbacks.cs" />
     <Compile Include="CheckoutOptions.cs" />
+    <Compile Include="OrphanedHeadException.cs" />
     <Compile Include="UnmergedIndexEntriesException.cs" />
     <Compile Include="Commit.cs" />
     <Compile Include="CommitLog.cs" />
diff --git a/LibGit2Sharp/OrphanedHeadException.cs b/LibGit2Sharp/OrphanedHeadException.cs
new file mode 100644
index 00000000..015dbfc4
--- /dev/null
+++ b/LibGit2Sharp/OrphanedHeadException.cs
@@ -0,0 +1,49 @@
+using System;
+using System.Runtime.Serialization;
+
+namespace LibGit2Sharp
+{
+    /// <summary>
+    ///   The exception that is thrown when a operation requiring an existing
+    ///   branch is performed against an unborn branch.
+    /// </summary>
+    [Serializable]
+    public class OrphanedHeadException : LibGit2SharpException
+    {
+        /// <summary>
+        ///   Initializes a new instance of the <see cref = "OrphanedHeadException" /> class.
+        /// </summary>
+        public OrphanedHeadException()
+        {
+        }
+
+        /// <summary>
+        ///   Initializes a new instance of the <see cref = "OrphanedHeadException" /> class with a specified error message.
+        /// </summary>
+        /// <param name = "message">A message that describes the error. </param>
+        public OrphanedHeadException(string message)
+            : base(message)
+        {
+        }
+
+        /// <summary>
+        ///   Initializes a new instance of the <see cref = "OrphanedHeadException" /> class with a specified error message and a reference to the inner exception that is the cause of this exception.
+        /// </summary>
+        /// <param name = "message">The error message that explains the reason for the exception. </param>
+        /// <param name = "innerException">The exception that is the cause of the current exception. If the <paramref name = "innerException" /> parameter is not a null reference, the current exception is raised in a catch block that handles the inner exception.</param>
+        public OrphanedHeadException(string message, Exception innerException)
+            : base(message, innerException)
+        {
+        }
+
+        /// <summary>
+        ///   Initializes a new instance of the <see cref = "OrphanedHeadException" /> class with a serialized data.
+        /// </summary>
+        /// <param name = "info">The <see cref="SerializationInfo "/> that holds the serialized object data about the exception being thrown.</param>
+        /// <param name = "context">The <see cref="StreamingContext"/> that contains contextual information about the source or destination.</param>
+        protected OrphanedHeadException(SerializationInfo info, StreamingContext context)
+            : base(info, context)
+        {
+        }
+    }
+}
diff --git a/LibGit2Sharp/Repository.cs b/LibGit2Sharp/Repository.cs
index 27fd6ab4..323e2a63 100644
--- a/LibGit2Sharp/Repository.cs
+++ b/LibGit2Sharp/Repository.cs
@@ -530,9 +530,10 @@ namespace LibGit2Sharp
             Ensure.ArgumentNotNull(branch, "branch");
 
             // Make sure this is not an unborn branch.
-            if (branch.Tip.Tree == null)
+            if (branch.Tip == null)
             {
-                throw new Exception("branch tip is null, nothing to checkout.");
+                throw new OrphanedHeadException(
+                    string.Format("The tip of branch '{0}' is null. There's nothing to checkout.", branch.Name));
             }
 
             CheckoutTree(branch.Tip.Tree, checkoutOptions, onCheckoutProgress);