MSBuild Task to generate hash of files

Here's a handy inline MSBuild task to take a bunch of files and generate a hash. I used it to generate a hash of source files in order to determine if an exe has really been changed (given that exe's compile to distinct files each time even if the source hasn't changed).

<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  
    <UsingTask TaskName="GenerateHash" TaskFactory="CodeTaskFactory" AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.v4.0.dll">
        <ParameterGroup>
            <InputFiles ParameterType="Microsoft.Build.Framework.ITaskItem[]" Required="true" />
            <OutputFile ParameterType="System.String" Required="true" />
        </ParameterGroup>
        <Task>
            <Using Namespace="System.IO" />
            <Using Namespace="System.Linq" />
            <Using Namespace="System.Security.Cryptography" />
            <Code Type="Fragment" Language="cs"><![CDATA[
                using (var ms = new MemoryStream())
                using (var md5 = MD5.Create())
                {                
                    foreach (var item in InputFiles)
                    {
                        string path = item.ItemSpec;
                        using (FileStream stream = new FileStream(path, FileMode.Open))
                        {
                            var fileHash = md5.ComputeHash(stream);
                            ms.Write(fileHash, 0, fileHash.Length);
                        }
                    }
                    ms.Flush();
                    ms.Position = 0;
                    var dirHash = md5.ComputeHash(ms);
                    using (TextWriter w = new StreamWriter(OutputFile, false))
                    {
                        w.WriteLine(string.Join("", dirHash.Select(b => b.ToString("x2"))));
                    }
                }
            ]]></Code>
        </Task>
    </UsingTask>

    <Target Name="Demo">
        <GenerateHash InputFiles="@(SomeFiles)" OutputFile="res.txt" />
    </Target>
</Project>