Archive for the ‘Programming’ Category

Parsing the command line with MGrammar – part 2

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In the first installment of this series we took a look at the basic grammar for parsing the command line with MGrammar. In this part I’ll show you how we can load in a compiled version of the MGrammar and parse the input (i.e. the command line) to produce a valid MGraph that we in turn can process in the backend code.

A quick reminder from part 1; the code is located here:
http://github.com/larsw/larsw.commandlineparser

You can download the code either by using git or downloading it as an archive. Once you’ve done that, open the solution LarsW.CommandLineParser.sln in Visual Studio 2008.

imageMost likely you will be presented with the following dialog box, informing you that opening the solution (or more correct the LarsW.CommandLineParser C# project inside) can pose a security risk. The reason for this is that I’ve included the a MSBuild task for compiling MGrammar files (.mg) into .mgx is that included in the Oslo SDK. Select the “Load project normally” and press OK.

We can first take a look at the extra plumbing I’ve added to the project to get the .mg file to compile. Right-click the LarsW.CommandLineParser project in the Solution Explorer, and choose Unload Project. Next, right-click it again, and choose Edit LarsW.CommandLineparser.csproj. This should bring up the project file will be shown as raw XML in the editor window.

In the first <PropertyGroup> I’ve added seven lines that I borrowed from a project created with the “M” template. They basically set’s up the path to various M-specific tools and auxiliary files.

The only line of these that really matter and that I had to tweak in order to get this right is the <MgTarget> element. Out of the box this is set to Mgx, that instructs the Mg compiler to spit out the result of the compilation as a .mgx file. As we will see later, the value needs to be set to MgResource in order to get the DynamicParser to load the .mgx as a resource.

If you navigate to the end of the project file, I’ve also added an <Import> element that imports some MGrammar specific MSBuild tasks and the most important thing; in the last <ItemGroup> section I’ve changed the element type from <None> to <MgCompile> for the cmd.mg file.

Well, we’ve been mucking around in the MSBuild plumbing too long now, haven’t we? Right-click the project again and choose Reload Project. When the project has loaded up again, build to ensure that everything is fine and dandy. Even though I haven’t stated it before, it should be obvious that the project depends on the latest (as of now that is the January 2009 CTP Refresh) Oslo SDK.

The core component is the CommandLineProcessor class.

It loads up the language (the compiled version of the cmd.mg) with DynamicParser.LoadFromResource(). The reason why we had to specify MgxResource as the MgTarget earlier is that if we don’t, and add the compiled .mgx file as a plain resource, the .LoadFromResource() method won’t find it. As of now, it seems that it will only look for resources with the .resource extension.

We then pass in the command line with a StringReader instance to the .Parse<T>() method on the DynamicParser instance. Even though it’s not specified, the T has to be object or a type that implements System.Dataflow.ISourceInfo. The internal/inner Node classes in GraphBuilder is what that will be handed out per default, but you can also create your own GraphBuilder and produce nodes from your own domain model.

So, by calling parser.Parse<object>(null, commandLineReader, ErrorReporter.Standard) we will get an instance to the root of the Abstract Syntax Tree (AST) returned if the input matches the grammar. The AST is basically a representation of the MGraph.

The next step is to traverse the AST and act upon the different node types. The grammar for this project is quite trivial and is mostly done by the private ProcessParameter() method in the CommandLineProcessor class. I suggest that you take a look at it if you’re interested in doing something similar.

So, just create an instance of the CommandLineProcessor and pass in an instance of an arbitrary class that contains method that will handle the command line arguments. To specify that a method is a argument handler, decorate it with the CommandLineArgumentHandler attribute. It will take in three parameters; short form & long form of the argument keyword and a description. For now the description isn’t used for anything but the idea is that the command line processor can auto generate a usage screen for you (typically shown with –?).

That’s about it – if you find it useful or modify the code, please let me know. With git you can push me a change set and I will try to merge it if you’ve come up with a cool feature.


Configurable PrincipalPermission attribute

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I while ago, a question came up in the WCF Forum about configuring the role and/or user name properties of the PrincipalPermission attribute. As I answered, it is possible to create a custom version of the attribute (deriving from the CodeAccessSecurityAttribute, since the PrincipalPermission attribute is sealed) and pull the property values from the {web|app}.config file.

I implemented a solution for this about a year ago and planned to put up a blog post about it, but it never made it out to the public (the main cause is probably that I experienced a blog-block period of my life :-P).

The same requirement may be a viable solution i system I’m currently working on for a customer, so I dug through my archives and found the old code.

I’ve polished it a bit made it available here under the Apache License 2.0.

The extended version, PrincipalPermissionEx can be used in two modes; either as a “normal” derivable PrincipalPermission attribute or an attribute that uses the configuration system (or a combination of both).

Instead of using the generic PrincipalPermission attribute, you’ll make derived version for each system role with a sensible name – making it more reliable and resistant to typos; e.g.

[MustHaveSuperUserPrivilegesPermission]
public void PrivilegedOperation(…)
{
}

instead of:

[PrincipalPermission(Role = "MYDOMAIN\SuperUsers")]
public void PrivilegedOperation(…)
{
}


Take a look at the supplied sample code to see how this is implemented.

The usage of PrincipalPermission-based authorization is useful in a variety of scenarios; it can be applied to WCF services, ASP.NET & Smart Client applications. Note that if you put the user name/role in the configuration file, you will need to ensure that the file is locked down with an appropriate ACL to prevent tampering by malicious users. This might not apply to solutions hosted on a locked down server (i.e. IIS-hosted web applications and services) but for smart / desktop clients where the user might have higher privileges to files on the local file system it is necessary to be aware of this.

As always, feedback is welcome :-)

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LINQ to XML: XPathSelectElement Annoyance

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It may be me – since I’m no XPath (or XSLT) pro, but the following is in my book a bug – or at least an annoyance category 3:

Given the following XML document loaded into an XDocument:

<?xml version="1.0" encoding="utf-8"?>
<Elements>
  <Element Id="1" />
  <Element Id="2" />
  <Element Id="3" />
  <Element Id="4" />
  <Element Id="5" />
</Elements>

The following XPath should  yield the first element of the list:

"//Element[@Id = '1']"


Guess what? If use the .XPathSelectElement() extension method, the result will be null – nada!

"//Element[@Id='1']"


The same query without the whitespace around the equal sign will give you the right result.

If you’re an XPath pro I would like your opinion on the matter – or else I’m turning this issue over to http://connect.microsoft.com/

Sigh.


Codename “Velocity” WF/WCF Persistence Provider

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So, it’s been a bit quiet here lately. The natural cause of it is (in no particular order):

  • A lot of work
  • Spending quality time with my son
  • Hacking on different kinds of technology bits (mainly pieces released at the PDC 2008)

I’ve also tried to get a clear picture of my “blind spots” when it comes to WCF. Even though I feel quite competent, there are still tons of stuff that I don’t touch daily so I still have to “rehearse”.

Since I have “Get to know Workflow Foundation – for real” on my TODO list I spent some time playing with durable services.

The persistence provider mechanism that is located in System.WorkflowServices is not exclusive to to Workflows / Workflow services. It can also be used with “vanilla” WCF Services.

The idea is that the framework can persist the service instance after you have invoked a method and when a future method invocation comes down the wire, it can pull it from the persistence store – revive it and pass the call to the “same” instance. A perfect fit for the scenario of long running services.

So how do you enable durable services? It is quite easy. First, you decorate your service implementation with [DurableService] and one of the mechanisms that specifies that the type is serializable (I chose [Serializable] for the sake of simplicity).

image

In this code snippet we also see that there is another attribute that can be used to tell the persistence mechanism that a call to an operation creates the instance or tears it down; [DurableOperation].

The next thing you have to do is to wire up a persistence provider using either configuration or programmatically.

Out of the box there exists only one Persistence Provider; One suited for persisting the service instances to SQL Server – System.ServiceModel.Persistence.SqlPersistenceProviderFactory. You will have to set up a SQL Server Database instance with the schema located in C:\Windows\Microsoft.NET\Framework\v3.5\SQL\EN.

But that was a digression – now back to my custom “Velocity” Persistence Provider. If you don’t know what Codename “Velocity” is, I suggest that you head over here and read more about it. The short description:

It is Microsoft’s attempt to create an in-memory, high-performance, distributed caching supporting different scenarios that can suite many needs in both a web farm or other places where caching is needed. The current version that is publicly available is CTP2. We should expect a new CTP in March (around the time of MIX’09) and the RTW/RTM in the mid of 2009.

To implement a custom persistence provider, you will have to create two classes; the persistence provider implementation and its factory. It is the fully qualified type name of the factory that is specified when you set up the configuration.

The following configuration snippet shows how a custom service behavior is set up. You will have to set the behaviorConfiguration attribute on the service element to “defaultServiceBehavior” in this case.

image

The code for the provider is available here (Licensed under the Apache License 2.0).

Cheers :-)
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Unity 1.1 Container Behavior for WCF

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Some time ago I needed a way to wire up an IoC container in a WCF service I was creating at that time. Since I’ve used ObjectBuilder-based containers a lot in both Composite UI Application Block and Enterprise Library, I wanted to take a closer look at the Unity container and the 2.0 version of ObjectBuilder.

To manage the creation of service instances, you have to implement the IInstanceProvider interface, and wire up the concrete type in a service behavior.

The code should be fairly documented, and a simple xUnit.NET test project is included. If you want to check out xUnit.NET, check out its CodePlex site.

I’ve put the code on a new resource page on code.msdn.microsoft.com that I have created WCF Resources. You can find the release here.


Fresh start

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Ok, I’ve bought some wordpress.com credits and pointed one of my domains, larswilhelmsen.com to larsw.wordpress.com. To break the no-blogging spell, I hope that starting fresh on a new site will help to get my fingers moving :-)

Currently, I’m on paternity leave, and when I start working again, it will be for Miles, Norway as a senior consultant. I will mainly do Microsoft- and Agile-related work and I am really looking forward to start.

While I’m away from work, I try to help out as much as I can in the Windows Communication Foundation (“Indigo”) MSDN Forum. If you have any questions regarding WCF, feel free to post in the group, or send me an e-mail.

In this blog I will mainly focus on Connected Systems; SOA/WCF, Smart Clients (including CAB/SCSF), synchronization and Live Mesh – when the API’s are out.

Wish me luck :-)

–larsw