Journal for 2012

Razor Preprocessed Templates

When Miguel asked me to investigate making MonoDevelop support using Razor templates in MonoTouch and Mono for Android apps, I realized that it could be done simply and with very few dependencies by taking the same approach as T4 preprocessed templates, which I implemented for MonoDevelop a couple of years ago. Fortunately, this time the hard part was already done: I could use Microsoft's open-source Razor parser instead of writing my own parser. I also found a Visual Studio extension called Razor Generator that was very close in functionality to what I wanted, and was able to use this as a basis for my work. I was able to hook it it into the fantastic Razor editing and code completion support written by Piotr Dowgiallo in the Google Summer of Code this year.

After a few days work implementing, tweaking and tuning (and bouncing ideas off Bojan Rajković), I present Razor Preprocessed Templates in MonoDevelop.

As a basis for this demo, I created new a MonoTouch iPhone Single View application, added a UIWebView to the View's xib, and connected it to an outlet on the controller called webview. However, you can use these templates in any project type.

Just add a new Text Templating -> Preprocessed Razor Template file to the project:

Adding a new Preprocessed Razor Template

You will see that this adds a cshtml Razor C# file to the project, grouped with a C# file that contains the generated code. Like T4 files, this uses the "custom tool" extensibility mechanism. By setting the custom tool property on the cshtml file set to "RazorTemplatePreprocessor", it causes MonoDevelop to use this new custom tool to regenerate the cs file whenever the cshtml file is saved.

The files added by the Preprocessed Razor Template

I wrote a simple Razor page to demonstrate the power of Razor. It uses a simple Razor helper to demonstrate that Razor helpers work correctly. The page also demonstrates using the @model directive to specify the type for a Model property, which easily allows us to pass data into the template before running it. Since this demo is very simple, I just used an int as the model instead of defining a proper model class.

@model int
 
@helper boldtd (int i) {
<td><b>@i</b></td>
}
 
<html>
	<head>
		<title>Multiplication Table</title>
	</head>
	<body>
		<h1>Multiplication Table</h1>
		<table>
			@* Header row *@
			<tr>
				<td/>
			@for (int i = 1; i <= Model; i++) {
				@boldtd(i)
			}
			</tr>
			@* Main table *@
			@for (int i = 1; i <= Model; i++) {
			<tr>
				@boldtd(i)
				@for (int j = 1; j <= Model; j++) {
				<td>@(j * i)</td>
				}
			</tr>
			}
		</table>
	</body>
</html>

When writing this, the Razor code completion was very helpful. It has full C# completion, including locals, helpers and members from the generated class and base class, including the generated Model property:

Code completion for C# in Razor Templates

There's also HTML completion and on-the-fly underlining of HTML and Razor errors:

Code completion and error underlining for HTML in Razor Templates

After saving the cshtml file, you can look at the generated cs file. It's pretty messy, so I won't show it here, but note that it includes a well-commented generated base class. If you want, you can specify a base class using the @inherits directive, so you can pull that generated base class out and share it between multiple templates, or customize it. The template's only dependency is Action<System.IO.TextWriter>, and the default base class's dependencies are only System.Web.HttpUtility.HtmlEncode(string) and System.IO.StringWriter, so it can easily be made to run anywhere. If your target framework lacks the one potentially awkward dependency, HttpUtility.HtmlEncode(string), you can provide an alternative implementation via a custom base class.

More documentation on the generated class and the available directives can be found on the MonoDevelop website.

To use the template, I simply added two lines to my ViewDidLoad method to instantiate the template, generate a string using the Generate() method, and load it into the UIWebView:

public override void ViewDidLoad ()
{
    base.ViewDidLoad ();
 
    var template = new MultiplicationTable () { Model = 12 };
    webview.LoadHtmlString (template.GenerateString (), null);
}

Then run the app, and you can see it in action:

Razor Template running on iPhone

This is a fantastic way to generate HTML pages without pulling in the whole System.Web stack, and I hope you're as excited about it as I am. It isn't available today, unless you build MonoDevelop from source, however the code is committed to MonoDevelop master and should make it into the next 3.x release.

The State of MSBuild Support in MonoDevelop

I occasionally get questions about support for advanced MSBuild features, and my answers are usually fairly short. This post aims to be a more comprehensive description of the current state state of MSBuild support in MonoDevelop, so I can refer people to it. If you've never hand-edited custom targets for an MSBuild file, this doesn't affect you at all, but feel free to read it if you're curious.

MSBuild is the Microsoft build engine that was introduced with .NET 2.0 and Visual Studio 2005. It's the format for the project files of Visual Studio 2005 and later, and has been MonoDevelop's default/native file format since MonoDevelop 2.0. It's XML-based, so it can be handled easily and reliably by tools such as IDEs. It's essentially intended to be consumed primarily by IDEs — but it also has the power of an advanced, extensible build system that lets you do pretty much anything if you're willing to get your hands dirty. Calling it "Makefiles in XML" wouldn't be too far off the mark.

The MSBuild engine and hosting API are part of the .NET framework. Mono has its own implementation called xbuild, which runs on Mac, Linux and Windows. The core features of xbuild are solid, but it's missing some of the advanced features added to MSBuild in .NET 3.5 and 4.0, and some of the common targets such as the ASP.NET web project targets.

MSBuild is extremely complex and exposing all its features in the MonoDevelop GUI would be practically impossible. However, the projects created by the IDE by default use the "common targets" to build the project, which understand a certain number of properties (e.g. "OutputPath") and item types (e.g. "Compile", "EmbeddedResource"), and implement certain targets (e.g. "Build", "Clean") that operate on these items and are controlled by these properties. They also make very limited use of conditions, by conventionally having several groups of properties conditional upon the value of the "Configuration" and "Platform" variables. To edit projects that haven't been hand-modified, the IDE only really has to understand the item types, properties, and targets used by the common targets.

MonoDevelop has an internal project model that represents the projects and their properties and items. When loading the project, MD deserializes the MSBuild file into its internal model, and ignores the items, properties and imports it does not understand. When saving MSBuild projects, MD serializes its internal model into the MSBuild format and substitutes them for the parts of the MSBuild file it understood, thereby preserving the parts of the file that it did not understand: custom items, properties, targets, conditions, and imports.

There are a couple of things the serialization doesn't handle — when hand-edited projects use properties or items as values of any of the standard properties or items. MonoDevelop does not evaluate these — doing so would require a full MSBuild engine — and even if it did, it would not easily be able to present them cleanly in the UI and serialize back any changes to the values. It might be possible to special-case some things, but it's not something that can be fully solved in a generic way. Probably the best we could get would be to have MonoDevelop detect property/item values it cannot handle, evaluate them via the MSBuild engine when loading, make them non-editable with a warning flag in the Project Options GUI, and skip them when serializing.

Wildcards in items paths are a similar problem. For example, evaluating a wildcard in an item when deserializng would result in adding several items to MD's internal model. And in simple cases, we could match all those items back to the wildcard MSBuild item when serializing. So far so good. But what happens if a new file is added on disk outside of MD while the project is open? What happens if the user manually removed a single item in the solution tree? What happens if the user changes the metadata of a single item? In all of these cases, the items can no longer be mapped back to the single wildcard MSBuild item. It would probably be possible to remove the wildcard item and serialize all the items to MSBuild individually — but that might not be what the user expected. This is just one example of how supporting a simple MSBuild feature in the IDE might not be as simple as it looks.

For building the projects, MonoDevelop's story is much better, because it has the ability to build the projects using the actual MSBuild/xbuild engine, thereby supporting all features that MSBuild/xbuild does. There are currently two build codepaths in MonoDevelop: the old internal build engine, and the MSBuild engine. The internal build engine is very old, and predates the existence of MSBuild. It operates directly on MonoDevelop's internal project model, and is extensible via the addin system. The MSBuild engine loads the actual project files into a builder process and builds them using the MSBuild hosting API.

By default MonoDevelop uses the old engine, but the addins for individual project types can opt into using the MSBuild engine for just those projects — Mono for Android does this. Enabling the experimental option "Compile projects using MSBuild/XBuild" in MonoDevelop's preferences (and restarting MonoDevelop) causes the MSBuild engine to be used for all projects. This is marked experimental because it does not work for all project types — for example, there are no MSBuild targets for MonoTouch, MonoMac or ASP.NET projects yet. User projects that depend on MonoDevelop's old custom build command system will not build correctly with the MSBuild engine. And some of the old MonoDevelop custom project types such as translation projects and C/C++ projects would need to be migrated to a new file format before they could even have MSBuild targets.

In general, expansion of the MSBuild support in MonoDevelop is not a high priority for the MonoDevelop team, since these advanced build features are of interest to a very small subset of users, and there are other things that could be done to improve the IDE experience for a much greater number of users. However, it's an area of ongoing improvement and will likely become more important in the future.