diff --git a/GitVersionExe.Tests/AssemblyInfoFileUpdateTests.cs b/GitVersionExe.Tests/AssemblyInfoFileUpdateTests.cs
new file mode 100644
index 0000000000..4e838cac68
--- /dev/null
+++ b/GitVersionExe.Tests/AssemblyInfoFileUpdateTests.cs
@@ -0,0 +1,23 @@
+namespace GitVersionExe.Tests
+{
+    using System.Collections.Generic;
+    using System.IO;
+    using GitVersion;
+    using NSubstitute;
+    using NUnit.Framework;
+
+    [TestFixture]
+    public class AssemblyInfoFileUpdateTests
+    {
+        [Test]
+        public void ShouldStartSearchFromWorkingDirectory()
+        {
+            var fileSystem = Substitute.For<IFileSystem>();
+            const string workingDir = "C:\\Testing";
+            using (new AssemblyInfoFileUpdate(new Arguments{ UpdateAssemblyInfo = true }, workingDir, new Dictionary<string, string>(), fileSystem))
+            {
+                fileSystem.Received().DirectoryGetFiles(Arg.Is(workingDir), Arg.Any<string>(), Arg.Any<SearchOption>());
+            }
+        }
+    }
+}
\ No newline at end of file
diff --git a/GitVersionExe.Tests/GitVersionExe.Tests.csproj b/GitVersionExe.Tests/GitVersionExe.Tests.csproj
index 8c1982e21f..62dcc0c050 100644
--- a/GitVersionExe.Tests/GitVersionExe.Tests.csproj
+++ b/GitVersionExe.Tests/GitVersionExe.Tests.csproj
@@ -39,6 +39,9 @@
     <Reference Include="LibGit2Sharp">
       <HintPath>..\packages\LibGit2Sharp.0.19.0.0\lib\net40\LibGit2Sharp.dll</HintPath>
     </Reference>
+    <Reference Include="NSubstitute">
+      <HintPath>..\packages\NSubstitute.1.8.0.0\lib\net45\NSubstitute.dll</HintPath>
+    </Reference>
     <Reference Include="nunit.framework">
       <HintPath>..\packages\NUnit.2.6.3\lib\nunit.framework.dll</HintPath>
     </Reference>
@@ -80,6 +83,7 @@
     <Compile Include="GitVersionHelper.cs" />
     <Compile Include="MsBuildProjectArgTest.cs" />
     <Compile Include="PullRequestInTeamCityTest.cs" />
+    <Compile Include="AssemblyInfoFileUpdateTests.cs" />
   </ItemGroup>
   <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
   <PropertyGroup>
diff --git a/GitVersionExe.Tests/packages.config b/GitVersionExe.Tests/packages.config
index f4578de3f1..b98d178f1b 100644
--- a/GitVersionExe.Tests/packages.config
+++ b/GitVersionExe.Tests/packages.config
@@ -3,6 +3,7 @@
   <package id="ApprovalTests" version="3.0.7" targetFramework="net45" />
   <package id="ApprovalUtilities" version="3.0.7" targetFramework="net45" />
   <package id="LibGit2Sharp" version="0.19.0.0" targetFramework="net45" />
+  <package id="NSubstitute" version="1.8.0.0" targetFramework="net45" />
   <package id="NUnit" version="2.6.3" targetFramework="net45" />
   <package id="Shouldly" version="2.2.0" targetFramework="net45" />
 </packages>
\ No newline at end of file
diff --git a/GitVersionExe/AssemblyInfoFileUpdate.cs b/GitVersionExe/AssemblyInfoFileUpdate.cs
index 1110d67c99..72c376b440 100644
--- a/GitVersionExe/AssemblyInfoFileUpdate.cs
+++ b/GitVersionExe/AssemblyInfoFileUpdate.cs
@@ -9,51 +9,53 @@ class AssemblyInfoFileUpdate : IDisposable
         List<Action> restoreBackupTasks = new List<Action>();
         List<Action> cleanupBackupTasks = new List<Action>();
 
-        public AssemblyInfoFileUpdate(Arguments args, string workingDirectory, Dictionary<string, string> variables)
+        public AssemblyInfoFileUpdate(Arguments args, string workingDirectory, Dictionary<string, string> variables, IFileSystem fileSystem)
         {
             if (!args.UpdateAssemblyInfo) return;
 
             if (args.Output != OutputType.Json)
                 Console.WriteLine("Updating assembly info files");
 
-            var assemblyInfoFiles = GetAssemblyInfoFiles(workingDirectory, args);
+            var assemblyInfoFiles = GetAssemblyInfoFiles(workingDirectory, args, fileSystem);
 
             foreach (var assemblyInfoFile in assemblyInfoFiles)
             {
                 var backupAssemblyInfo = assemblyInfoFile + ".bak";
                 var localAssemblyInfo = assemblyInfoFile;
-                File.Copy(assemblyInfoFile, backupAssemblyInfo, true);
+                fileSystem.Copy(assemblyInfoFile, backupAssemblyInfo, true);
                 restoreBackupTasks.Add(() =>
                 {
-                    if (File.Exists(localAssemblyInfo))
-                        File.Delete(localAssemblyInfo);
-                    File.Move(backupAssemblyInfo, localAssemblyInfo);
+                    if (fileSystem.Exists(localAssemblyInfo))
+                        fileSystem.Delete(localAssemblyInfo);
+                    fileSystem.Move(backupAssemblyInfo, localAssemblyInfo);
                 });
-                cleanupBackupTasks.Add(() => File.Delete(backupAssemblyInfo));
+                cleanupBackupTasks.Add(() => fileSystem.Delete(backupAssemblyInfo));
 
                 var assemblyVersion = string.Format("{0}.{1}.0.0", variables[VariableProvider.Major], variables[VariableProvider.Minor]);
                 var assemblyInfoVersion = variables[VariableProvider.InformationalVersion];
                 var assemblyFileVersion = variables[VariableProvider.AssemblySemVer];
-                var fileContents = File.ReadAllText(assemblyInfoFile)
+                var fileContents = fileSystem.ReadAllText(assemblyInfoFile)
                     .RegexReplace(@"AssemblyVersion\(""\d+.\d+.\d+(.\d+|\*)?""\)", string.Format("AssemblyVersion(\"{0}\")", assemblyVersion))
                     .RegexReplace(@"AssemblyInformationalVersion\(""\d+.\d+.\d+(.\d+|\*)?""\)", string.Format("AssemblyInformationalVersion(\"{0}\")", assemblyInfoVersion))
                     .RegexReplace(@"AssemblyFileVersion\(""\d+.\d+.\d+(.\d+|\*)?""\)", string.Format("AssemblyFileVersion(\"{0}\")", assemblyFileVersion));
 
-                File.WriteAllText(assemblyInfoFile, fileContents);
+                fileSystem.WriteAllText(assemblyInfoFile, fileContents);
             }
         }
 
-        static IEnumerable<string> GetAssemblyInfoFiles(string workingDirectory, Arguments args)
+        static IEnumerable<string> GetAssemblyInfoFiles(string workingDirectory, Arguments args, IFileSystem fileSystem)
         {
             if (args.UpdateAssemblyInfoFileName != null)
             {
-                if (File.Exists(args.UpdateAssemblyInfoFileName))
+                var fullPath = Path.Combine(workingDirectory, args.UpdateAssemblyInfoFileName);
+
+                if (fileSystem.Exists(fullPath))
                 {
-                    return new[] { Path.GetFullPath(args.UpdateAssemblyInfoFileName) };
+                    return new[] { fullPath };
                 }
             }
 
-            return Directory.GetFiles(workingDirectory, "AssemblyInfo.cs", SearchOption.AllDirectories);
+            return fileSystem.DirectoryGetFiles(workingDirectory, "AssemblyInfo.cs", SearchOption.AllDirectories);
         }
 
         public void Dispose()
diff --git a/GitVersionExe/FileSystem.cs b/GitVersionExe/FileSystem.cs
new file mode 100644
index 0000000000..4ddd2f09bc
--- /dev/null
+++ b/GitVersionExe/FileSystem.cs
@@ -0,0 +1,43 @@
+namespace GitVersion
+{
+    using System.Collections.Generic;
+    using System.IO;
+
+    class FileSystem : IFileSystem
+    {
+        public void Copy(string @from, string to, bool overwrite)
+        {
+            File.Copy(from, to, overwrite);
+        }
+
+        public void Move(string @from, string to)
+        {
+            File.Move(from, to);
+        }
+
+        public bool Exists(string file)
+        {
+            return File.Exists(file);
+        }
+
+        public void Delete(string path)
+        {
+            File.Delete(path);
+        }
+
+        public string ReadAllText(string path)
+        {
+            return File.ReadAllText(path);
+        }
+
+        public void WriteAllText(string file, string fileContents)
+        {
+            File.WriteAllText(file, fileContents);
+        }
+
+        public IEnumerable<string> DirectoryGetFiles(string directory, string searchPattern, SearchOption searchOption)
+        {
+            return Directory.GetFiles(directory, searchPattern, searchOption);
+        }
+    }
+}
\ No newline at end of file
diff --git a/GitVersionExe/GitVersionExe.csproj b/GitVersionExe/GitVersionExe.csproj
index eb23e12659..3a029c4cb9 100644
--- a/GitVersionExe/GitVersionExe.csproj
+++ b/GitVersionExe/GitVersionExe.csproj
@@ -55,8 +55,10 @@
     <Compile Include="ArgumentParser.cs" />
     <Compile Include="Arguments.cs" />
     <Compile Include="ExtensionMethods.cs" />
+    <Compile Include="FileSystem.cs" />
     <Compile Include="GitPreparer.cs" />
     <Compile Include="HelpWriter.cs" />
+    <Compile Include="IFileSystem.cs" />
     <Compile Include="ProcessHelper.cs" />
     <Compile Include="Program.cs" />
     <Compile Include="AssemblyInfo.cs" />
diff --git a/GitVersionExe/IFileSystem.cs b/GitVersionExe/IFileSystem.cs
new file mode 100644
index 0000000000..9e2a8637af
--- /dev/null
+++ b/GitVersionExe/IFileSystem.cs
@@ -0,0 +1,16 @@
+namespace GitVersion
+{
+    using System.Collections.Generic;
+    using System.IO;
+
+    public interface IFileSystem
+    {
+        void Copy(string from, string to, bool overwrite);
+        void Move(string from, string to);
+        bool Exists(string file);
+        void Delete(string path);
+        string ReadAllText(string path);
+        void WriteAllText(string file, string fileContents);
+        IEnumerable<string> DirectoryGetFiles(string directory, string searchPattern, SearchOption searchOption);
+    }
+}
\ No newline at end of file
diff --git a/GitVersionExe/Program.cs b/GitVersionExe/Program.cs
index c8b9c63b89..b0aac61c1f 100644
--- a/GitVersionExe/Program.cs
+++ b/GitVersionExe/Program.cs
@@ -114,7 +114,7 @@ static int Run()
                     }
                 }
 
-                using (var assemblyInfoUpdate = new AssemblyInfoFileUpdate(arguments, workingDirectory, variables))
+                using (var assemblyInfoUpdate = new AssemblyInfoFileUpdate(arguments, workingDirectory, variables, new FileSystem()))
                 {
                     var execRun = RunExecCommandIfNeeded(arguments, workingDirectory, variables);
                     var msbuildRun = RunMsBuildIfNeeded(arguments, workingDirectory, variables);