Gov.UK Design System in C#/ASP.Net Core

This is a collection of Gov.UK Design System components build in C# / ASP.Net Core.

It's the C# equivalent of the Gov.UK Prototyping Kit Nunjucks templates.

Warning: work in progress

Warning: This is very much a work in progress!
Only a subset of the components have been built and there is the possibility of breaking changes as we make bug fixes / add extra features.

Please feel free to use this in your own project. You can see our contributing guide here.

If you do, please let us know, so we can avoid making changes that will cause problems for you.
Email: James Griffiths james.griffiths@softwire.com

How to use

This library can be used in two ways:

• View components only
• View components + validation / error message generation

These features are interoperable and you can use as much or as little of them as you like in your project - it's not necessary to migrate totally to using this library.

View components

e.g. for a Gov.UK Button

On the Design System website:
https://design-system.service.gov.uk/components/button
the Nunjucks code would look like this:

{% from "govuk/components/button/macro.njk" import govukButton %}

{{ govukButton({
    text: "Save and continue"
}) }}

The C# code would look like this:

@using GovUkDesignSystem
@using GovUkDesignSystem.GovUkDesignSystemComponents

@(Html.GovUkButton(new ButtonViewModel
{
    Text = "Save and continue"
}))

e.g. for a Text Input
https://design-system.service.gov.uk/components/text-input/
the C# code would be:

@(Html.GovUkTextInput(new TextInputViewModel
{
    Label = new LabelViewModel
    {
        Text = "Your email address"
    },
    Name = "EmailAddress",
    InputMode = "email"
}))

View components + validation / error message generation

Here's an example of how to get automatic error message generation for an integer field.

On your view-model:

• Inherit from GovUkViewModel
• Use a nullable integer (int?) property
• Add the GovUkValidateRequired attribute if you want to validate that the field is required (and to specify the error message if it's missing)
• Add the GovUkDisplayNameForErrors attribute for all other error messages of the form Enter a [Field Name] and [Field Name] must be a whole number

public class MyViewModel : GovUkViewModel
{
    [GovUkDisplayNameForErrors(
        NameAtStartOfSentence = "Number of children",
        NameWithinSentence = "number of children"
    )]
    [GovUkValidateRequired(
        ErrorMessageIfMissing = "Enter the number of children you have"
    )]
    public int? NumberOfChildren
}

On the view:

@using GovUkDesignSystem
@using GovUkDesignSystem.GovUkDesignSystemComponents

@model MyViewModel

@(Html.GovUkErrorSummary())

@(Html.GovUkTextInputFor(
    m => m.NumberOfChildren,
    labelOptions: new LabelViewModel
    {
        Text = "How many children do you have?"
    }
))

On the controller:

• Take the view-model as a parameter (optional)
• Call ParseAndValidateParameters
• Call HasAnyErrors
• Add your own errors with AddErrorFor

[HttpPost("url-to-action")]
public IActionResult ActionName(MyViewModel viewModel)
{
    // Call ParseAndValidateParameters on each field you need
    viewModel.ParseAndValidateParameters(Request, m => m.NumberOfChildren);

    // Use HasAnyErrors to check if anything failed validation
    if (viewModel.HasAnyErrors())
    {
        return View("ViewName", viewModel);
    }

    // You can do your own custom validation...
    if (viewModel.NumberOfChildren > 100)
    {
        //...and add your own error messages like this
        viewModel.AddErrorFor<MyViewModel, int?>(
            m => m.NumberOfChildren,
            "The number of children you say you have is implausible");
    }

    // Then return the user back to the page if there's an error
    if (viewModel.HasAnyErrors())
    {
        return View("ViewName", viewModel);
    }
}

C