A flexible and expressive template engine for Large Language Model (LLM) prompts and structured message generation in C#.
LLT is designed to make prompt engineering and content generation as powerful and maintainable as regular C# code.
✨ Key Features
Razor-inspired DSL with @if, @foreach, expressions, metadata, and inline variables
Message-oriented syntax for LLM role structures (system, user, assistant)
Powerful metadata filtering — select the best template by language, model, or custom qualifiers
Composable templates — reuse and render nested templates inside others
Deterministic formatting — preserves whitespace and indentation, ensuring predictable output
Library-driven workflow — import templates from assemblies, files, or strings
💡 Example
var parser = new LLTParser();
var templateStr = """
@template GreetingTemplate
{
Greetings, @name!
@if age > 18
{
You are an adult.
}
else
{
You are too young!
}
Have a nice day.
}
""";
var template = parser.Parse(templateStr).First();
var adult = new { name = "Andrew", age = 20 };
var young = new { name = "Alice", age = 15 };
Console.WriteLine(template.Render(adult));
Console.WriteLine(template.Render(young));
Output:
Greetings, Andrew!
You are an adult.
Have a nice day.
Greetings, Alice!
You are too young!
Have a nice day.
🧩 Template Library Usage
Multiple templates can be stored, versioned, and retrieved by language or model ID:
Templates are resolved by metadata specificity, similar to CSS selector priority.
💬 Message Templates for LLM Chats
LLT supports special @messages syntax for structured chat prompts:
@messages template ChatBot
{
@metadata { language: 'en', version: 1 }
@system message {
You are a helpful assistant.
Here is your instructions:
@foreach instruction in instructions {
Instruction: @instruction
}
}
@foreach name in names {
@message {
@role 'user'
Hello, I am @name!
}
}
}
Rendered result: a sequence of chat messages with proper roles (system, user, assistant).
🧠 Supported Syntax
Inline expressions: @(a + b * c)
Conditionals:
@if cond {
...
} else if other {
...
} else {
...
}
Loops:
@foreach item in items {
@item
}
Variables:
@let x = 5
@x = 10
@x
Nested or external templates:
@render 'other_template'
Comments:
@// line comment
@*
block comment
*@
🧾 Formatting & Scoping Rules
Whitespace and indentation in source templates are fully preserved
