De.Loooping.Templates.Configuration 0.5.0

De.Loooping.Templates

This is a slim templating library implemented in .NET 8. It provides a simple way to generate text-based output by replacing placeholders in a template file or string with actual values.

⚠️ SECURITY WARNING ⚠️

Templates can execute arbitrary code. There is no check for malicious content inside the templates.
DO NOT ALLOW UNTRUSTED TEMPLATES TO BE EXECUTED IN YOUR ENVIRONMENT!

Features

• Easy syntax, inspired by Jinja2
• Full support of C# code inside the template
• Built-in value formatting via .NET format strings
• Error tracking, so exceptions return the error location inside your template

Installation

• Add NuGet package De.Loooping.Templates.Core.XXX.nupkg to your project, where XXX is the version number (e.g. "0.4.0")
• If you want to use the configuration extensions, also add De.Loooping.Templates.Configuration.XXX.nupkg.

Basic template syntax

A template consists of different elements that are used in turns to produce the intended output.

These elements are:

Content blocks

Syntax: {{ here.comes.an.expression }}

Content blocks begin with the opening delimiter {{ and end with the closing delimiter }}. Between these two delimiters must be a C# expression, i.e. a value or some code that represents a value.

Examples:

• Strings: "abc"
• Numbers: 42 or 12.34
• Complex objects: new DateTime()
• Code that results in some value: DateTime.Parse("2024-03-07 21:25")

Values in content blocks can be formatted by adding a : after the expression, followed by a format string.

Examples:

• {{ 12.34 :000.000}} results in the output 012.340
• {{ DateTime.Parse("2024-03-07"):dd.MM.yyyy}} results in 07.03.2024

Expression and format string can stretch over several lines, but the format string must not end with any whitespace.

Statement blocks

Syntax: {% here.comes.a.statement(); or.maybe = more.than.one; %}

Statement blocks can contain any C# code that is allowed inside a method body. You can use it for example to create loops.

Example: {% for(int i=0; i<=3; i++) { %}{{ i }}{% } %} results in 0123
The first statement block contains the for loop and the opening {.
The content block in the middle {{ i }} just outputs the current value of i. The statement block at the end finishes the loop with a closing }.

In case you need to output some values from inside a statement block, this is also possible by using yield return, but the returned value must be a string.

Example:

{%
yield return "a";
yield return "b";
%}

results in ab.

As you can see in the last example, it's also allowed to stretch statement placeholders over multiple lines.

Comment blocks

Syntax: {# this is a comment #}

The content of comment blocks is ignored in the output, so you can use them to add descriptions or explanations to your template.

Custom blocks

Syntax: {$CUSTOM_BLOCK_IDENTIFIER:content string$}

Custom blocks have a unique identifier and process a content string to provide some output. The syntax of the content string and the output created are defined by the custom blocks implementation.

Literals

Literals have no special Syntax or delimiters. Everything that is not a content, statement, comment or custom block is a literal.
Literals will be added to the output as-is, so usually the biggest part of a template consists of literals.

Example:

• Left literal -> {{ 21*2 }} <- right literal results in Left literal -> 42 <- right literal

Working with templates in your code

Building and using a template from a string is as easy as this:

public void Main()
{
  string templateString = "{{ 2*21 }}";
  TemplateBuilder templateBuilder = new TemplateBuilder(templateString);
  var template = templateBuilder.Build();
  var output = template();
  Console.WriteLine(output); // will write "42" to the console
}

Most of the time you will want to pass some data to your template. For this, you have to define a delegate with result type string and use it as a type argument when creating the TemplateBuilder:

private delegate string MyTemplate(int a, string b);

public void Main()
{
  string templateString = "{{ a }} horses go to the {{ b }}";
  TemplateBuilder templateBuilder = new TemplateBuilder<MyTemplate>(templateString);
  var template = templateBuilder.Build();
  var output = template(42, "river");
  Console.WriteLine(output); // will write "42 horses go to the river" to the console
}

Building and using custom blocks

Custom blocks must implement the interface ICustomBlock.

An example of a simple custom block is the predefined EnvironmentVariableContentBlock. It takes the name of a environment variable and returns its content:

public class EnvironmentVariableContentBlock: ICustomBlock
{
    public string DefaultIdentifier => "ENV";
    
    public string Evaluate(string content)
    {
        return Environment.GetEnvironmentVariable(content) ?? String.Empty;
    }
}

Usage:

public void Main()
{
  string templateString = "{$ENV:VariableName$}";
  TemplateBuilder templateBuilder = new TemplateBuilder(templateString);
  templateBuilder.AddCustomBlock(new EnvironmentVariableContentBlock());
  var template = templateBuilder.Build();
  var output = template();
  Console.WriteLine(output); // will write the content of environment variable 'VariableName' to the console
}

If you want to use an identifier that is different to the custom blocks default identifier, you can do so:

  templateBuilder.AddCustomBlock(new EnvironmentVariableContentBlock(), "ANOTHER_IDENTIFIER");

HINT: custom block identifiers must not contain any whitespaces!

Changing delimiters

In seldom cases you might need to output special character sequences that interfere with the templating engines default delimiters (i.e. {{, }}, {%, %}, {#, #}).

One solution to this is to return your needed output from a content or statement placeholder:

• {{ "{{ lalala }}" }} will output {{ lalala }}
• {% yield return "{% lalala %}"; %} will output {% lalala %}

but this looks nasty and is very inconvenient.

Another solution is to overwrite the default delimiters with something you don't need to output from your template:

public void Main()
{
  string templateString = "{{ 2*21 }}";
  TemplateBuilder templateBuilder = new TemplateBuilder(templateString, new TemplateProcessorConfiguration()
  {
      LeftContentDelimiter = "{{{",
      RightContentDelimiter = "}}}"
  });
  var template = templateBuilder.Build();
  var output = template();
  Console.WriteLine(output); // will write "{{ 2*21 }}" to the console
}

Adding custom types

The TemplateBuilder will only add references and using directives for the most basic types like int and string, as well as references of all types provided in the delegate type argument (TDelegate in new TemplateBuilder<TDelegate>()). If you want to use or declare other types (like a generic List<T> for example), you have to add them to the TemplateBuilder before building the template or it will throw an exception:

public void Main()
{
  string templateString = "{{ DateTime.Parse("2024-03-07"):dd.MM.yyyy}}";
  TemplateBuilder templateBuilder = new TemplateBuilder(templateString)
    .WithType<DateTime>();
  var template = templateBuilder.Build();
  var output = template();
  Console.WriteLine(output); // will write "07.03.2024" to the console
}

TemplateBuilder.WithType<T>() adds the required assembly reference to the template and also adds the types namespace as a using directive. You can also use TemplateBuilder.WithReference(Assembly reference) and/or TemplateBuilder.WithUsing(String @using) to add these separately.

Configuration extensions

The configuration extensions allow you to add templating to your JSON configurations. To use these, you have to add NuGet package De.Loooping.Templates.Configuration.XXX.nupkg.

By default, block delimiters are embedded into block comments (i.e. /* and */) inside the JSON document, so that colissions with Intellisense are kept to a minimum.

Example appsettings.template.json:

{
  "AnInteger": /*{{ 21*2 }}*/,
  "AListOfStrings": [/*{%
    for(int i=0; i<3; i++)
    {
      if (i != 0) yield return ",";
      yield return $"\"item_{i}\"";
    }
  %}*/]
}

will evaluate as

{
  "AnInteger": 42,
  "AListOfStrings": ["item_0","item_1","item_2"]
}

Add the template file to dependency injection:

public void Configure(ConfigurationManager configuration)
{
  configuration.AddJsonTemplateFile("appsettings.template.json");
}

If you need to add types, references or usings to the TemplateBuilder or change any of the delimiters:

public void Configure(ConfigurationManager configuration)
{
  configuration.AddJsonTemplateFile("appsettings.template.json",
    build: builder =>
    {
        builder.AddType(typeof(List<>));
        builder.AddReference(typeof(IDictionary<,>).Assembly);
        builder.AddUsing("Custom.Namespace");
        builder.Configuration.LeftContentDelimiter = "{{{";
    });
}

If you need to inject any data into the template:

private delegate string MyTemplate(string myString);
...
public void Configure(ConfigurationManager configuration)
{
  configuration.AddJsonTemplateFile<MyTemplate>("appsettings.template.json",
    (inject) => inject("This will be injected as variable myString into the template")
  );
}