Fluid is an open-source .NET template engine that is as close as possible to the Liquid template language. It's a secure template language that is also very accessible for non-programmer audiences. It also contains an ASP.NET Core MVC View Engine.

• Parses and renders Liquid templates.
• Supports async filters, templates can execute database queries more efficiently under load.
• Parses templates in an intermediate AST that lets you analyze and alter the templates before they are rendered. You can also cache them to get even better performance.
• Register any .NET types and properties, or define custom handlers to intercept when a named variable is accessed.
• Secure by white-listing all the available properties in the template.

• Features
• Differences with Liquid
• Using Fluid in your project
• White-listing object members
• Execution limits
• Converting CLR types
• Encoding
• Localization
• Customizing tags and blocks
• ASP.NET MVC View Engine
• Custom filters
• Performance
• Used by

In Fluid a Filter doesn't need to have a default parameter, you can name all of them.

{% assign customers = 'allcustomers' | query: limit:10 %}

Fluid will automatically maintain the whitespaces from the original template and won't inject extra lines where tags are used. This means that templates don't need to add extra -%} to the end of their tags to maintain consistent whitespaces. However it's supported and will be applied on output tags when used.

<ul id="products">
  {% for product in products %}
    <li>
      <h2>{{product.name}}</h2>
      Only {{product.price | price }}

      {{product.description | prettyprint | paragraph }}
    </li>
  {% endfor %}
</ul>

<ul id="products">
    <li>
      <h2>Apple</h2>
      $329

      Flat-out fun.
    </li>
    <li>
      <h2>Orange</h2>
      $25

      Colorful. 
    </li>
    <li>
      <h2>Banana</h2>
      $99

      Peel it.
    </li>
</ul>

Notice

• The <li> tags are at the same index as in the template, even though the {% for } tag had some leading spaces
• The <ul> and <li> tags are on contiguous lines even though the {% for } is taking a full line.

You can directly reference the Nuget package.

var model = new { Firstname = "Bill", Lastname = "Gates" };
var source = "Hello {{ p.Firstname }} {{ p.Lastname }}";

if (FluidTemplate.TryParse(source, out var template))
{   
    var context = new TemplateContext(model);

    context.SetValue("p", model);

    Console.WriteLine(template.Render(context));
}

Hello Bill Gates

Filters can be async or not. They are defined as a delegate that accepts an input, a set of arguments and the current context of the rendering process.

Here is the downcase filter as defined in Fluid.

public static FluidValue Downcase(FluidValue input, FilterArguments arguments, TemplateContext context)
{
    return new StringValue(input.ToStringValue().ToLower());
}

Filters can be registered globally for the lifetime of the application, or for each usage of a template.

TemplateContext.GlobalFilters.AddFilter('downcase', Downcase);

// Or for a specific context

var context = new TemplateContext();
context.Filters.AddFilter('downcase', Downcase);

To create an async filter use the AddAsyncFilter method instead.

Liquid is a secure template language which will only allow a predefined set of members to be accessed. Like filters, this can be done globally to the application with GlobalMemberAccessStrategy, or for each context with MemberAccessStrategy. Even if a member is white-listed its value won't be able to be changed.

Warning: To prevent concurrency issues you should always register global filters and members in a static constructor. Local ones can be defined at the time of usage.

This will allow any public field or property to be read from a template.

TemplateContext.GlobalMemberAccessStrategy.Register<Person>();

This will only allow the specific fields or properties to be read from a template.

TemplateContext.GlobalMemberAccessStrategy.Register<Person>("Firstname", "Lastname");

This will provide a method to intercept when a member is accessed and either return a custom value or prevent it.

This example demonstrates how to intercept calls to a JObject and return the corresponding property.

TemplateContext.GlobalMemberAccessStrategy.Register<JObject, object>((obj, name) => obj[name]);

All the members of the class hierarchy are registered. Besides, all inherited classes will be correctly evaluated when a base class is registered and a member of the base class is accessed.

By default, the properties of a registered object are case sensitive and registered as they are in their source code. For instance, the property FirstName would be access using the {{ p.FirstName }} tag.

However it can be necessary to register these properties with different cases, like Camel case (firstName), or Snake case (first_name).

The following example configures the templates to use Camel casing.

TemplateContext.GlobalMemberAccessStrategy.MemberNameStrategy = MemberNameStrategies.CamelCase;

When invoking {% include 'sub-template' %} statements it is possible that some templates create an infinite recursion that could block the server. To prevent this the TemplateContext class defines a default DefaultMaxRecursion = 100 that prevents templates from being have a depth greater than 100. This can be defined globally with this static member, or on an individual TemplateContext instance on its MaxRecursion property.

Template can inadvertently create infinite loop that could block the server by running indefinitely. To prevent this the TemplateContext class defines a default DefaultMaxSteps. By default this value is not set. This can be defined globally with this static member, or on an individual TemplateContext instance on its MaxSteps property.

Whenever an object is manipulated in a template it is converted to a specific FluidValue instance that provides a dynamic type system somehow similar to the one in JavaScript.

In Liquid they can be Number, String, Boolean, Array, or Dictionary. Fluid will automatically convert the CLR types to the corresponding Liquid ones, and also provides specialized ones.

To be able to customize this conversion you can either add type mappings or value converters.

The following example shows how to support JObject and JValue types to map their values to FluidValue instances.

First is solves the issue that a JObject implements IEnumerable and would be converted to an ArrayValue instead of an ObjectValue. Then we use FluidValue.Create to automatically convert the CLR value of the JValue object.

FluidValue.SetTypeMapping<JObject>(o => new ObjectValue(o));
FluidValue.SetTypeMapping<JValue>(o => FluidValue.Create(o.Value));

Note: Type mapping are defined globally for the application.

When the conversion logic is not directly inferred from the type of an object, a value converter can be used.

Value converters can return:

• null to indicate that the value couldn't be converted
• a FluidValue instance to stop any further conversion and use this value
• another object instance to continue the conversion using custom and internal type mappings

The following example shows how to convert any instance implementing an interface to a custom string value:

FluidValue.ValueConverters.Add((value) => value is IUser user ? user.Name : null);

Note: Type mapping are defined globally for the application.

The classes that are used in Json.NET don't have direct named properties like classes, which makes them unusable out of the box in a Liquid template.

To remedy that we can configure Fluid to map names to JObject properties, and convert JValue objects to the ones used by Fluid.

// When a property of a JObject value is accessed, try to look into its properties
TemplateContext.GlobalMemberAccessStrategy.Register<JObject, object>((source, name) => source[name]);

// Convert JToken to FluidValue
FluidValue.TypeMappings.Add(typeof(JObject), o => new ObjectValue(o));
FluidValue.TypeMappings.Add(typeof(JValue), o => FluidValue.Create(((JValue)o).Value));

var expression = "{{ Model.Name }}";
var model = JObject.Parse("{\"Name\": \"Bill\"}");

if (FluidTemplate.TryParse(expression, out var template))
{
    var context = new TemplateContext();
    context.SetValue("Model", model);

    Console.WriteLine(template.Render(context));
}

By default Fluid doesn't encode the output. Encoders can be specified when calling Render() or RenderAsync() on the template.

To render a template with HTML encoding use the System.Text.Encodings.Web.HtmlEncoder.Default instance.

This encoder is used by default for the MVC View engine.

When an encoder is defined you can use a special raw filter or {% raw %} ... {% endraw %} tag to prevent a value from being encoded, for instance if you know that the content is HTML and is safe.

{% assign html = '<em>This is some html</em>' %}

Encoded: {{ html }}
Not encoded: {{ html | raw }

<em%gt;This is some html</em%gt;
<em>This is some html</em>

When using capture blocks, the inner content is flagged as pre-encoded and won't be double-encoded if used in a {{ }} tag.

{% capture breaktag %}<br />{% endcapture %}

{{ breaktag }}

<br />

By default templates are rendered using an invariant culture so that the results are consistent across systems. This is important for instance when rendering dates, times and numbers.

However it is possible to define a specific culture to use when rendering a template using the TemplateContext.CultureInfo property.

var context = new TemplateContext();
context.CultureInfo = new CultureInfo("en-US");
var result = template.Render(context);

{{ 1234.56 }}
{{ "now" | date: "%v" }}

1234.56
Tuesday, August 1, 2017

Fluid's grammar can be modified to accept any new tags and blocks with any custom parameters. It is even possible to use different grammars in the same application.

Unlike blocks, tags don't have a closing element (e.g., cycle, increment). A closing element will match the name of the opening tag with and end suffix, like endfor. Blocks are useful when manipulating a section of a a template as a set of statements.

To create a custom tag or block it is necessary to create a class implementing the ITag interface, or for most common cases to just inherit from some of the available base classes.

Custom tags can use these base types:

• SimpleTag: Tag with no parameter, like {% renderbody %}
• IdentifierTag: Tag taking an identifier as parameter, like {% increment my_variable %}
• ExpressionTag: Tag taking an expression as parameter, like {% layout template | default: 'layout' %}
• ArgumentsTag: Tag taking a list of arguments as parameter, like {% display 'default', arg1: 1 + 1 %}
• ITag: Tag that can define any custom grammar.

Here are some examples:

public class QuoteTag : ExpressionTag
{
  public override async ValueTask<Completion> WriteToAsync(TextWriter writer, TextEncoder encoder, TemplateContext context, Expression expression)
  {
    var value = (await expression.EvaluateAsync(context)).ToStringValue();
    await writer.WriteAsync("'" + value + "'");
    
    return Completion.Normal;
  }
}

{% quote 5 + 11 %}

'16'

Blocks are created the same way as tags, with these classes: SimpleBlock, IdentifierBlock, ExpressionBlock, ArgumentsBlock or ITag.

public class RepeatBlock : ExpressionBlock
{
  public override async ValueTask<Completion> WriteToAsync(TextWriter writer, TextEncoder encoder, TemplateContext context, Expression expression, IList<Statements> statements)
  {
    var value = (await expression.EvaluateAsync(context)).ToNumberValue();
    for (var i=0; i < value; i++)
    {
      await RenderStatementsAsync(writer, encoder, context, statements);
    }

    return Completion.Normal;
  }
}

{% repeat 1 + 2 %}Hi! {% endrepeat %}

Hi! Hi! Hi!

To prevent your customization from altering the default Liquid syntax, it is recommended to create a custom template type.

using Fluid;

public class MyFluidTemplate : BaseFluidTemplate<MyFluidTemplate>
{
  static MyFluidTemplate()
  {
      Factory.RegisterTag<QuoteTag>("quote");
      Factory.RegisterBlock<RepeatBlock>("repeat");
  }
}

MyFluidTemplate.TryParse(source, out var template);

To see a complete example of a customized Fluid grammar, look at this class: CustomGrammarTests

To provide a convenient view engine implementation for ASP.NET Core MVC the grammar is extended as described in Customizing tags by adding these new tags:

1- Reference the Fluid.MvcViewEngine NuGet package 2- Add a using statement on FluidMvcViewEngine 3- Call AddFluid() in your Startup.cs.

using FluidMvcViewEngine;

public class Startup
{
  public void ConfigureServices(IServiceCollection services)
    {
        services.AddMvc().AddFluid();
    }
}

Because the Liquid language only accepts known members to be accessed, the View Model classes need to be registered in Fluid. Usually from a static constructor such that the code is run only once for the application.

public class Startup
{
    static Startup()
    {
        TemplateContext.GlobalMemberAccessStrategy.Register<Person>();
    }
}

More way to register types and members can be found in the White-listing object members section.

When using the MVC View engine, custom tags can be added to the FluidViewTemplate class. Refer to this section on how to create custom tags.

public class Startup
{
    static Startup()
    {
        FluidViewTemplate.Factory.RegisterTag<MyTag>("mytag");
    }
}

{% layout '_layout.liquid' %}

This is is the home page

The {% layout [template] %} tag accepts one argument which can be any expression that return the relative location of a liquid template that will be used as the master template.

The layout tag is optional in a view. It can also be defined multiple times or conditionally.

From a layout template the {% renderbody %} tag is used to depict the location of the view's content inside the layout itself.

<html>
  <body>
    <div class="menu"></div>
    
    <div class="content">
      {% renderbody %}
    </div>
    
    <div class="footer"></div>
  </body>
</html>

Sections are defined in a layout as for views to render content in specific locations. For instance a view can render some content in a menu or a footer section.

{% layout '_layout.liquid' %}

This is is the home page

{% section menu %}
  <a href="h#">This link goes in the menu</a>
{% endsection %}

{% section footer %}
  This text will go in the footer
{% endsection %}

<html>
  <body>
    <div class="menu">
      {% rendersection menu %}
    </div>
    
    <div class="content">
      {% renderbody %}
    </div>
    
    <div class="footer">
      {% rendersection footer %}
    </div>
  </body>
</html>

Defining the layout template in each view might me cumbersome and make it difficult to change it globally. To prevent that it can be defined in a _ViewStart.liquid file.

When a view is rendered all _ViewStart.liquid files from its current and parent directories are executed before. This means multiple files can be defined to defined settings for a group of views.

{% layout '_layout.liquid' %}
{% assign background = 'ffffff' }

You can also define other variables or render some content.

It is possible to add custom file locations containing views by adding them to FluidViewEngineOptions.ViewLocationFormats.

The default ones are:

• Views/{1}/{0}
• Views/Shared/{0}

Where {0} is the view name, and {1} is the controller name.

The content of a view is parsed once and kept in memory until the file or one of its dependencies changes. Once parsed, the tag are executed every time the view is called. To compare this with Razor, where views are first compiled then instantiated every time they are rendered. This means that on startup or when the view is changed, views with Fluid will run faster than those in Razor, unless you are using precompiled Razor views. In all cases Razor views will be faster on subsequent calls as they are compiled directly to C#.

This difference makes Fluid very adapted for rapid development cycles where the views can be deployed and updated frequently. And because the Liquid language is secure, developers give access to them with more confidence.

Some non-standard filters are provided by default

Formats date and times using standard .NET date and time formats. It uses the current culture of the system.

Input

"now" | format_date: "G"

Output

6/15/2009 1:45:30 PM

Documentation: https://docs.microsoft.com/en-us/dotnet/standard/base-types/standard-date-and-time-format-strings

Formats numbers using standard .NET number formats.

Input

123 | format_number: "N"

Output

123.00

Documentation: https://docs.microsoft.com/en-us/dotnet/standard/base-types/standard-numeric-format-strings

Formats custom string using standard .NET format strings.

Input

"hello {0} {1:C}" | format_string: "world" 123

Output

hello world $123.00

Documentation: https://docs.microsoft.com/en-us/dotnet/api/system.string.format

Some performance boost can be gained in your application if you decide to cache the parsed templates before they are rendered. Even though parsing is memory-safe as it won't induce any compilation (meaning all the memory can be collected if you decide to parse a lot of templates), you can skip the parsing step by storing and reusing the FluidTemplate instance.

These object are thread-safe as long as each call to Render() uses a dedicated TemplateContext instance.

A performance benchmark application is provided in the source code to compare Fluid, DotLiquid and Liquid .NET. Run it locally to analyze the time it takes to execute specific templates.

• Parse sample: Parses a sample HTML template containing filters and properties
• Render sample: Renders a parsed HTML template containing filters and properties
• Parse and render sample: Parses and renders a sample HTML template containing filters and properties
• Render simple output tag: Renders a single tag outputting a string property
• Parse 8KB Lorem Ipsum: Parses 8KB of Lorem Ipsum text containing no tags to exercise the parser on long and simple texts (Not displayed for Liquid.NET as it takes too long to fit in the chart).
• Render 1KB Lorem Simple Output: Parses and render 1KB of Lorem Ipsum text containing no tags

Fluid is known to be used in the following projects:

• Orchard Core CMS
• MaltReport OpenDocument/OfficeOpenXML powered reporting engine for .NET and Mono
• Elsa Workflows .NET Workflows Library

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