mustache#

An extension of the mustache text template engine for .NET.

Download using NuGet: mustache#

Overview

Generating text has always been a chore. Either you're concatenating strings like a mad man or you're getting fancy with StringBuilder. Either way, the logic for conditionally including values or looping over a collection really obscures the intention of the code. A more declarative approach would improve your code big time. Hey, that's why server-side scripting got popular in the first place, right?

mustache is a really simple tool for generating text. .NET developers already had access to String.Format to accomplish pretty much the same thing. The only problem was that String.Format used indexes for placeholders: Hello, {0}!!!. mustache let you use meaningful names for placeholders: Hello, {{name}}!!!.

mustache is a logic-less text generator. However, almost every time I've ever needed to generate text I needed to turn some of it on or off depending on a value. Not having the ability to turn things off usually meant going back to building my text in parts.

Introducing handlebars.js... If you've needed to generate any HTML templates, handlebars.js is a really awesome tool. Not only does it support an if and each tag, it lets you define your own tags! It also makes it easy to reference nested values {{Customer.Address.ZipCode}}.

mustache# brings the power of handlebars.js to .NET and then takes it a little bit further. It is geared towards building ordinary text documents, rather than just HTML. It differs from handlebars.js in the way it handles newlines. With mustache#, you explicitly indicate when you want newlines - actual newlines are ignored.

Hello, {{Customer.Name}}
{{#newline}}
{{#newline}}
{{#with Order}}
{{#if LineItems}}
Here is a summary of your previous order:
{{#newline}}
{{#newline}}
{{#each LineItems}}
    {{ProductName}}: {{UnitPrice:C}} x {{Quantity}}
    {{#newline}}
{{/each}}
{{#newline}}
Your total was {{Total:C}}.
{{#else}}
You do not have any recent purchases.
{{/if}}
{{/with}}

Most of the lines in the previous example will never appear in the final output. This allows you to use mustache# to write templates for normal text, not just HTML/XML.

Placeholders

The placeholders can be any valid identifier. These map to the property names in your classes.

Formatting Placeholders

Each format item takes the following form and consists of the following components:

{{identifier[,alignment][:formatString]}}

The matching braces are required. Notice that they are double curly braces! The alignment and the format strings are optional and match the syntax accepted by String.Format. Refer to String.Format's documentation to learn more about the standard and custom format strings.

Placeholder Scope

The identifier is used to find a property with a matching name. If you want to print out the object itself, you can use the special identifier this.

FormatCompiler compiler = new FormatCompiler();
Generator generator = compiler.Compile("Hello, {{this}}!!!");
string result = generator.Render("Bob");
Console.Out.WriteLine(result);  // Hello, Bob!!!

Some tags, such as each and with, change which object the values will be retrieved from.

If a property with the placeholder name can't be found at the current scope, the name will be searched for at the next highest level.

mustache# will automatically detect when an object is a dictionary and search for a matching key. In this case, it still needs to be a valid identifier name.

Nested Placeholders

If you want to grab a nested property, you can separate identifiers using ..

{{Customer.Address.ZipCode}}

The 'if' tag

The if tag allows you to conditionally include a block of text.

Hello{{#if Name}}, {{Name}}{{/if}}!!!

The block will be printed if:

• The value is a non-empty string.
• The value is a non-empty collection.
• The value isn't the NUL char.
• The value is a non-zero number.
• The value evaluates to true.

The if tag has complimentary elif and else tags. There can be as many elif tags as desired but the else tag must appear only once and after all other tags.

{{#if Male}}Mr.{{#elif Married}}Mrs.{{#else}}Ms.{{/if}}

The 'each' tag

If you need to print out a block of text for each item in a collection, use the each tag.

{{#each Customers}}
Hello, {{Name}}!!
{{/each}}

Within the context of the each block, the scope changes to the current item. So, in the example above, Name would refer to a property in the Customer class.

Additionally, you can access the current index into the collection being enumerated using the index tag.

<ul>
{{#each Items}}
    <li class="list-item{{#index}}" value="{{Value}}">{{Description}}</li>
{{/each}}
</ul>

This will build an HTML list, building a list of items with Description and Value properties. Additionally, the index tag is used to create a CSS class with increasing numbers.

The 'with' tag

Within a block of text, you may refer to a same top-level placeholder over and over. You can cut down the amount of text by using the with tag.

{{#with Customer.Address}}
{{FirstName}} {{LastName}}
{{Line1}}
{{#if Line2}}
{{Line2}}
{{/if}}
{{#if Line3}}
{{Line3}}
{{/if}}
{{City}} {{State}}, {{ZipCode}}
{{/with}}

Here, the Customer.Address property will be searched first for the placeholders. If a property cannot be found in the Address object, it will be searched for in the Customer object and on up.

The 'set' tag

mustache# provides limited support for variables through use of the set tag. Once a variable is declared, it is visible to all child scopes. Multiple definitions of a variable with the same name cannot be created within the same scope. In fact, I highly recommend making variable names unique to the entire template just to prevent unexpected behavior!

The following example will print out "EvenOddEvenOdd" by toggling a variable called even:

FormatCompiler compiler = new FormatCompiler();
const string format = @"{{#set even}}
{{#each this}}
{{#if @even}}
Even
{{#else}}
Odd
{{/if}}
{{#set even}}
{{/each}}";
Generator generator = compiler.Compile(format);
generator.ValueRequested += (sender, e) =>
{
    e.Value = !(bool)(e.Value ?? false);
};
string result = generator.Render(new int[] { 0, 1, 2, 3 });

This code works by specifying a function to call whenever a value is needed for the even variable. The first time the function is called, e.Value will be null. All additional calls will hold the last known value of the variable.

Notice that when you set the variable, you don't qualify it with an @. You only need the @ when you request its value, like in the if statement above.

You should attempt to limit your use of variables within templates. Instead, perform as many up-front calculations as possible and make sure your view model closely represents its final appearance. In this case, it would make more sense to first convert the array into strings of "Even" and "Odd".

FormatCompiler compiler = new FormatCompiler();
const string format = @"{{#each this}}{{this}}{{/each}}";
Generator generator = compiler.Compile(format);
string result = generator.Render(new string[] { "Even", "Odd", "Even", "Odd" });

This code is much easier to read and understand. It is also going to run significantly faster. In cases where you also need the original value, you can create an array containing objects with properties for the original value and Even/Odd.

Defining Your Own Tags

If you need to define your own tags, mustache# has everything you need.

Once you define your own tags, you can register them with the compiler using the RegisterTag method.

FormatCompiler compiler = new FormatCompiler();
compiler.RegisterTag(myTag);

Your tag can be referenced within the template by leading its name with a #.

Custom tags can take any number of parameters. Parameters can have default values if you don't want to pass them all the time. Arguments are passed by specifying a placeholder.

Multi-line Tags

Here's an example of a tag that will make all of its content upper case:

public class UpperTagDefinition : ContentTagDefinition
{
    public UpperTagDefinition()
        : base("upper")
    {
    }
    
    public override IEnumerable<NestedContext> GetChildContext(TextWriter writer, KeyScope scope, Dictionary<string, object> arguments)
    {
        NestedContext context = new NestedContext() 
        { 
            KeyScope = scope, 
            Writer = new StringWriter(), 
            WriterNeedsConsolidated = true,
        };
        yield return context;
    }
    
    public override string ConsolidateWriter(TextWriter writer, Dictionary<string, object> arguments)
    {
        return writer.ToString().ToUpperInvariant();
    }
}

Another solution is to wrap the given TextWriter with another TextWriter that will change the case of the strings passed to it. This approach requires more work, but would be more efficient. You should attempt to wrap or reuse the text writer passed to the tag.

In-line Tags

Here's an example of a tag that will join the items of a collection:

public class JoinTagDefinition : InlineTagDefinition
{
    public JoinTagDefinition()
        : base("join")
    {
    }
    
    protected override IEnumerable<TagParameter> GetParameters()
    {
        return new TagParameter[] { new TagParameter("collection") };
    }
    
    protected override void GetText(TextWriter writer, Dictionary<string, object> arguments)
    {
        IEnumerable collection = (IEnumerable)arguments["collection"];
        string joined = String.Join(", ", collection.Cast<object>().Select(o => o.ToString()));
        writer.Write(joined);
    }
}