Handlebars.NET

Overview

Handlebars.NET is a .NET implementation of the Handlebars.js templating engine. It provides a logic-less template syntax with {{expression}} for output and {{#helper}} for block constructs. Templates are compiled into delegates for fast repeated rendering.

Handlebars.NET supports helpers (custom functions callable from templates), partials (reusable template fragments), and block helpers (custom block-level constructs). It is compatible with Mustache templates and extends them with helpers and more expressive iteration.

Install via NuGet:

dotnet add package Handlebars.Net

Basic Template Compilation and Rendering

Compile a template string into a reusable delegate, then invoke it with a data object.

using HandlebarsDotNet;

// Compile once, render many times
var template = Handlebars.Compile("Hello, {{name}}! You have {{count}} notifications.");

var result = template(new { name = "Alice", count = 7 });
// result: "Hello, Alice! You have 7 notifications."

// With nested objects
var orderTemplate = Handlebars.Compile(
    "Order #{{order.id}} placed by {{order.customer.name}} on {{order.date}}");

var output = orderTemplate(new
{
    order = new
    {
        id = 1042,
        customer = new { name = "Bob Smith" },
        date = "2025-01-15"
    }
});

Iteration and Conditionals

Use {{#each}} for collections and {{#if}}/{{#unless}} for conditionals.

using HandlebarsDotNet;

var reportTemplate = Handlebars.Compile(@"
<h1>{{title}}</h1>
{{#if hasItems}}
<table>
  <tr><th>Product</th><th>Price</th><th>Qty</th></tr>
  {{#each items}}
  <tr>
    <td>{{this.name}}</td>
    <td>${{this.price}}</td>
    <td>{{this.quantity}}</td>
  </tr>
  {{/each}}
</table>
{{else}}
<p>No items found.</p>
{{/if}}
<p>Generated: {{generatedAt}}</p>
");

var html = reportTemplate(new
{
    title = "Inventory Report",
    hasItems = true,
    items = new[]
    {
        new { name = "Widget A", price = 12.50, quantity = 100 },
        new { name = "Widget B", price = 8.75, quantity = 250 },
        new { name = "Gadget C", price = 45.00, quantity = 30 }
    },
    generatedAt = DateTime.UtcNow.ToString("yyyy-MM-dd HH:mm:ss")
});

Custom Helpers

Register helper functions that can be called from templates. Helpers receive the writer, context, and parameters.

using System;
using System.IO;
using HandlebarsDotNet;

// Inline helper (returns a value)
Handlebars.RegisterHelper("uppercase", (writer, context, parameters) =>
{
    writer.WriteSafeString(parameters[0]?.ToString()?.ToUpperInvariant() ?? string.Empty);
});

// Formatting helper
Handlebars.RegisterHelper("currency", (writer, context, parameters) =>
{
    if (parameters.Length > 0 && decimal.TryParse(parameters[0]?.ToString(), out var amount))
    {
        var symbol = parameters.Length > 1 ? parameters[1]?.ToString() : "$";
        writer.WriteSafeString($"{symbol}{amount:N2}");
    }
});

// Date formatting helper
Handlebars.RegisterHelper("formatDate", (writer, context, parameters) =>
{
    if (parameters.Length > 0 && DateTime.TryParse(parameters[0]?.ToString(), out var date))
    {
        var format = parameters.Length > 1 ? parameters[1]?.ToString() : "yyyy-MM-dd";
        writer.WriteSafeString(date.ToString(format));
    }
});

var template = Handlebars.Compile(
    "{{uppercase name}} owes {{currency balance}} as of {{formatDate dueDate \"MMMM dd, yyyy\"}}");

var output = template(new { name = "alice", balance = 1234.50m, dueDate = "2025-06-15" });
// Output: "ALICE owes $1,234.50 as of June 15, 2025"

Block Helpers

Block helpers wrap a section of template content and can conditionally render the block or iterate over it.

using HandlebarsDotNet;

// Conditional block helper
Handlebars.RegisterHelper("ifEqual", (output, options, context, arguments) =>
{
    if (arguments.Length >= 2 &&
        arguments[0]?.ToString() == arguments[1]?.ToString())
    {
        options.Template(output, context);
    }
    else
    {
        options.Inverse(output, context);
    }
});

var template = Handlebars.Compile(@"
{{#ifEqual status ""active""}}
  <span class=""badge-success"">Active</span>
{{else}}
  <span class=""badge-danger"">Inactive</span>
{{/ifEqual}}
");

var html = template(new { status = "active" });

Partials (Reusable Fragments)

Register reusable template fragments as partials and include them with {{> partialName}}.

using HandlebarsDotNet;

// Register partials
Handlebars.RegisterTemplate("header", @"
<header>
  <h1>{{title}}</h1>
  <nav>{{#each navItems}}<a href=""{{url}}"">{{label}}</a>{{/each}}</nav>
</header>");

Handlebars.RegisterTemplate("userCard", @"
<div class=""user-card"">
  <strong>{{name}}</strong>
  <span>{{email}}</span>
</div>");

// Use partials in a page template
var pageTemplate = Handlebars.Compile(@"
<!DOCTYPE html>
<html>
{{> header}}
<body>
  <h2>Team Members</h2>
  {{#each members}}
  {{> userCard}}
  {{/each}}
</body>
</html>");

var page = pageTemplate(new
{
    title = "Team Directory",
    navItems = new[]
    {
        new { url = "/", label = "Home" },
        new { url = "/team", label = "Team" }
    },
    members = new[]
    {
        new { name = "Alice", email = "alice@example.com" },
        new { name = "Bob", email = "bob@example.com" }
    }
});

Using IHandlebars for Isolated Environments

Create isolated Handlebars environments with their own helpers and partials instead of using the global static instance.

using HandlebarsDotNet;

public class EmailRenderer
{
    private readonly IHandlebars _handlebars;

    public EmailRenderer()
    {
        _handlebars = Handlebars.Create();

        _handlebars.RegisterHelper("nl2br", (writer, context, parameters) =>
        {
            var text = parameters[0]?.ToString() ?? string.Empty;
            writer.WriteSafeString(text.Replace("\n", "<br/>"));
        });

        _handlebars.RegisterTemplate("emailFooter", @"
<footer>
  <p>This email was sent by {{companyName}}</p>
  <p><a href=""{{unsubscribeUrl}}"">Unsubscribe</a></p>
</footer>");
    }

    public string Render(string templateSource, object data)
    {
        var compiled = _handlebars.Compile(templateSource);
        return compiled(data);
    }
}

Handlebars.NET vs Other Template Engines

Best Practices

1. Compile templates once and cache the resulting delegate -- Handlebars.Compile parses and compiles the template, so repeated compilation wastes CPU.
2. Use Handlebars.Create() for isolated environments instead of the global Handlebars static when different parts of your application need different helpers or partials.
3. Register helpers at application startup rather than per-request, since helper registration modifies shared state.
4. Use WriteSafeString in helpers for pre-escaped HTML to avoid double-encoding; use Write for values that should be HTML-escaped.
5. Extract repeated template fragments into partials with RegisterTemplate to keep templates DRY and maintainable.
6. Prefer block helpers over complex conditional nesting to keep template logic readable and testable.
7. Validate template syntax at startup by compiling all templates during initialization and failing fast if any template has syntax errors.
8. Use strongly-typed models instead of anonymous objects for production code so that property name refactoring does not silently break templates.
9. Avoid deeply nested context paths like {{../../parent.child.value}} -- flatten the data model or use helpers to simplify access.
10. Test templates with representative data including empty collections, null values, and missing properties to ensure graceful degradation.