Build your first Blazor app

By Daniel Roth and Luke Latham

This tutorial shows you how to build and modify a Blazor app.

Follow the guidance in the Get started with Blazor article to create a Blazor project for this tutorial.

Build components

  1. Browse to each of the app's three pages in the Pages folder: Home, Counter, and Fetch data. These pages are implemented by the Razor component files Index.razor, Counter.razor, and FetchData.razor.

  2. On the Counter page, select the Click me button to increment the counter without a page refresh. Incrementing a counter in a webpage normally requires writing JavaScript, but Blazor provides a better approach using C#.

  3. Examine the implementation of the Counter component in the Counter.razor file.

    Pages/Counter.razor:

    @page "/counter"
    
    <h1>Counter</h1>
    
    <p>Current count: @currentCount</p>
    
    <button class="btn btn-primary" onclick="@IncrementCount">Click me</button>
    
    @functions {
        private int currentCount = 0;
    
        private void IncrementCount()
        {
            currentCount++;
        }
    }
    

    The UI of the Counter component is defined using HTML. Dynamic rendering logic (for example, loops, conditionals, expressions) is added using an embedded C# syntax called Razor. The HTML markup and C# rendering logic are converted into a component class at build time. The name of the generated .NET class matches the file name.

    Members of the component class are defined in an @functions block. In the @functions block, component state (properties, fields) and methods are specified for event handling or for defining other component logic. These members are then used as part of the component's rendering logic and for handling events.

    When the Click me button is selected:

    • The Counter component's registered onclick handler is called (the IncrementCount method).
    • The Counter component regenerates its render tree.
    • The new render tree is compared to the previous one.
    • Only modifications to the Document Object Model (DOM) are applied. The displayed count is updated.
  4. Modify the C# logic of the Counter component to make the count increment by two instead of one.

    @page "/counter"
    
    <h1>Counter</h1>
    
    <p>Current count: @currentCount</p>
    
    <button class="btn btn-primary" onclick="@IncrementCount">Click me</button>
    
    @functions {
        private int currentCount = 0;
    
        private void IncrementCount()
        {
            currentCount += 2;
        }
    }
    
  5. Rebuild and run the app to see the changes. Select the Click me button. The counter increments by two.

Use components

Include a component in another component using an HTML syntax.

  1. Add the Counter component to the app's Index component by adding a <Counter /> element to the Index component (Index.razor).

    If you're using Blazor client-side for this experience, a Survey Prompt component (<SurveyPrompt> element) is in the Index component. Replace the <SurveyPrompt> element with the <Counter> element. If you're using a Blazor server-side app for this experience, add the <Counter> element to the Index component:

    Pages/Index.razor:

    @page "/"
    
    <h1>Hello, world!</h1>
    
    Welcome to your new app.
    
    <Counter />
    
  2. Rebuild and run the app. The Index component has its own counter.

Component parameters

Components can also have parameters. Component parameters are defined using non-public properties on the component class decorated with [Parameter]. Use attributes to specify arguments for a component in markup.

  1. Update the component's @functions C# code:

    • Add a IncrementAmount property decorated with the [Parameter] attribute.
    • Change the IncrementCount method to use the IncrementAmount when increasing the value of currentCount.

    Pages/Counter.razor:

    @page "/counter"
    
    <h1>Counter</h1>
    
    <p>Current count: @currentCount</p>
    
    <button class="btn btn-primary" onclick="@IncrementCount">Click me</button>
    
    @functions {
        private int currentCount = 0;
    
        [Parameter]
        private int IncrementAmount { get; set; } = 1;
    
        private void IncrementCount()
        {
            currentCount += IncrementAmount;
        }
    }
    
  1. Specify an IncrementAmount parameter in the Index component's <Counter> element using an attribute. Set the value to increment the counter by ten.

    Pages/Index.razor:

    @page "/"
    
    <h1>Hello, world!</h1>
    
    Welcome to your new app.
    
    <Counter IncrementAmount="10" />
    
  2. Reload the Index component. The counter increments by ten each time the Click me button is selected. The counter in the Counter component continues to increment by one.

Route to components

The @page directive at the top of the Counter.razor file specifies that the Counter component is a routing endpoint. The Counter component handles requests sent to /counter. Without the @page directive, a component doesn't handle routed requests, but the component can still be used by other components.

Dependency injection

Services registered in the app's service container are available to components via dependency injection (DI). Inject services into a component using the @inject directive.

Examine the directives of the FetchData component.

If working with a Blazor server-side app, the WeatherForecastService service is registered as a singleton, so one instance of the service is available throughout the app. The @inject directive is used to inject the instance of the WeatherForecastService service into the component.

Pages/FetchData.razor:

@page "/fetchdata"
@using WebApplication1.App.Services
@inject WeatherForecastService ForecastService

The FetchData component uses the injected service, as ForecastService, to retrieve an array of WeatherForecast objects:

@functions {
    private WeatherForecast[] forecasts;

    protected override async Task OnInitAsync()
    {
        forecasts = await ForecastService.GetForecastAsync(DateTime.Now);
    }
}

If working with a Blazor client-side app, HttpClient is injected to obtain weather forecast data from the weather.json file in the wwwroot/sample-data folder:

Pages/FetchData.razor:

@inject HttpClient Http

...

protected override async Task OnInitAsync()
{
    forecasts = 
        await Http.GetJsonAsync<WeatherForecast[]>("sample-data/weather.json");
}

A @foreach loop is used to render each forecast instance as a row in the table of weather data:

<table class="table">
    <thead>
        <tr>
            <th>Date</th>
            <th>Temp. (C)</th>
            <th>Temp. (F)</th>
            <th>Summary</th>
        </tr>
    </thead>
    <tbody>
        @foreach (var forecast in forecasts)
        {
            <tr>
                <td>@forecast.Date.ToShortDateString()</td>
                <td>@forecast.TemperatureC</td>
                <td>@forecast.TemperatureF</td>
                <td>@forecast.Summary</td>
            </tr>
        }
    </tbody>
</table>

Build a todo list

Add a new component to the app that implements a simple todo list.

  1. Add an empty file named Todo.razor to the app in the Pages folder:

  2. Provide the initial markup for the component:

    @page "/todo"
    
    <h1>Todo</h1>
    
  3. Add the Todo component to the navigation bar.

    The NavMenu component (Shared/NavMenu.razor) is used in the app's layout. Layouts are components that allow you to avoid duplication of content in the app. For more information, see Blazor layouts.

    Add a <NavLink> for the Todo component by adding the following list item markup below the existing list items in the Shared/NavMenu.razor file:

    <li class="nav-item px-3">
        <NavLink class="nav-link" href="todo">
            <span class="oi oi-list-rich" aria-hidden="true"></span> Todo
        </NavLink>
    </li>
    
  4. Rebuild and run the app. Visit the new Todo page to confirm that the link to the Todo component works.

  5. If building a Blazor server-side app, add the app's namespace to the _Imports.razor file. The following @using statement assumes that the app's namespace is WebApplication:

    @using WebApplication
    

    Blazor client-side apps include the app's namespace by default in the _Imports.razor file.

  6. Add a TodoItem.cs file to the root of the project to hold a class that represents a todo item. Use the following C# code for the TodoItem class:

    public class TodoItem
    {
        public string Title { get; set; }
        public bool IsDone { get; set; }
    }
    
  7. Return to the Todo component (Pages/Todo.razor):

    • Add a field for the todo items in an @functions block. The Todo component uses this field to maintain the state of the todo list.
    • Add unordered list markup and a foreach loop to render each todo item as a list item.
    @page "/todo"
    
    <h1>Todo</h1>
    
    <ul>
        @foreach (var todo in todos)
        {
            <li>@todo.Title</li>
        }
    </ul>
    
    @functions {
        private IList<TodoItem> todos = new List<TodoItem>();
    }
    
  8. The app requires UI elements for adding todo items to the list. Add a text input and a button below the list:

    @page "/todo"
    
    <h1>Todo</h1>
    
    <ul>
        @foreach (var todo in todos)
        {
            <li>@todo.Title</li>
        }
    </ul>
    
    <input placeholder="Something todo" />
    <button>Add todo</button>
    
    @functions {
        private IList<TodoItem> todos = new List<TodoItem>();
    }
    
  9. Rebuild and run the app. When the Add todo button is selected, nothing happens because an event handler isn't wired up to the button.

  10. Add an AddTodo method to the Todo component and register it for button clicks using the onclick attribute:

    <input placeholder="Something todo" />
    <button onclick="@AddTodo">Add todo</button>
    
    @functions {
        private IList<TodoItem> todos = new List<TodoItem>();
    
        private void AddTodo()
        {
            // Todo: Add the todo
        }
    }
    

    The AddTodo C# method is called when the button is selected.

  11. To get the title of the new todo item, add a newTodo string field and bind it to the value of the text input using the bind attribute:

    private IList<TodoItem> todos = new List<TodoItem>();
    private string newTodo;
    
    <input placeholder="Something todo" bind="@newTodo">
    
  12. Update the AddTodo method to add the TodoItem with the specified title to the list. Clear the value of the text input by setting newTodo to an empty string:

    @page "/todo"
    
    <h1>Todo</h1>
    
    <ul>
        @foreach (var todo in todos)
        {
            <li>@todo.Title</li>
        }
    </ul>
    
    <input placeholder="Something todo" bind="@newTodo" />
    <button onclick="@AddTodo">Add todo</button>
    
    @functions {
        private IList<TodoItem> todos = new List<TodoItem>();
        private string newTodo;
    
        private void AddTodo()
        {
            if (!string.IsNullOrWhiteSpace(newTodo))
            {
                todos.Add(new TodoItem { Title = newTodo });
                newTodo = string.Empty;
            }
        }
    }
    
  13. Rebuild and run the app. Add some todo items to the todo list to test the new code.

  14. The title text for each todo item can be made editable and a check box can help the user keep track of completed items. Add a check box input for each todo item and bind its value to the IsDone property. Change @todo.Title to an <input> element bound to @todo.Title:

    <ul>
        @foreach (var todo in todos)
        {
            <li>
                <input type="checkbox" bind="@todo.IsDone" />
                <input bind="@todo.Title" />
            </li>
        }
    </ul>
    
  15. To verify that these values are bound, update the <h1> header to show a count of the number of todo items that aren't complete (IsDone is false).

    <h1>Todo (@todos.Count(todo => !todo.IsDone))</h1>
    
  16. The completed Todo component (Pages/Todo.razor):

    @page "/todo"
    
    <h1>Todo (@todos.Count(todo => !todo.IsDone))</h1>
    
    <ul>
        @foreach (var todo in todos)
        {
            <li>
                <input type="checkbox" bind="@todo.IsDone" />
                <input bind="@todo.Title" />
            </li>
        }
    </ul>
    
    <input placeholder="Something todo" bind="@newTodo" />
    <button onclick="@AddTodo">Add todo</button>
    
    @functions {
        private IList<TodoItem> todos = new List<TodoItem>();
        private string newTodo;
    
        private void AddTodo()
        {
            if (!string.IsNullOrWhiteSpace(newTodo))
            {
                todos.Add(new TodoItem { Title = newTodo });
                newTodo = string.Empty;
            }
        }
    }
    
  17. Rebuild and run the app. Add todo items to test the new code.

Publish and deploy the app

To publish the app, see Host and deploy Blazor.