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Unit Testing With bUnit

This article provides information for bUnit and how it can be used with the Telerik UI for Blazor components.

What is bUnit

bUnit is a popular unit testing framework for Blazor.

bUnit covers explicitly unit testing and is not designed for e2e testing. It only runs your C# and Razor code, and not JavaScript.

Check the following resources for a general introduction to bUnit:

E2E vs. Unit Testing

The main difference between e2e tests and unit tests is that end-to-end testing focuses on the application's behavioral flow (clicking a button that populates a message) while unit testing targets functional fragments.

End-to-end tests allow you to test the application from the user's point of view. One tool you can use for end-to-end testing in Blazor is the Selenium framework.

Unit testing focuses on functional verification. You can use unit tests to ensure that the functions or calculations that generate data, a numerical value, or a text string, are working as expected.

A case that requires an interaction with the component is a task for e2e testing.

bUnit Limitations

A known limitation of bUnit is that it does not run JavaScript. So, if the components use some JSInterop, one should emulate IJSRuntime. You can find some more details in the bUnit documentation.

Testing the Telerik UI for Blazor Components with bUnit

The Telerik UI for Blazor components rely on JSInterop to support their rich UX features. This can make testing with bUnit difficult or even impossible in some scenarios due to the above-listed limitation.

You don't have to test our components, as we do that ourselves. Any unit tests for custom markup and logic have to test just that—the custom logic in an isolated scenario.

Consider e2e testing for any case that targets a complete workflow.

Common Issues

Considering the above-listed JS limitation, you may experience some issues when testing the components. The following sections in this article list these issues.

TelerikRootComponent is Missing

Subject Details
Issue When testing a DatePicker the test fails with:
System.Exception : A Telerik component on the requested view requires a TelerikRootComponent to be added to the root of the MainLayout component of the app.
Cause A possible cause for this error is that in the test the component is rendered in isolation, without a layout, so the TelerikRootComponent is missing.
Workaround Ether mock the TelerikRootComponent or use an actual TelerikRootComponent.

Attribute data-id is different in the markup

Subject Details
Issue Trying to detect an element by data-id fails as every time the data-id value in the generated markup is different.
Cause The data-id of the components is automatically generated in our components and it is unique for each instance. Thus, this is an expected difference in the output if you call RenderComponent twice.
Workaround This attribute is used for internal purposes only and should not be included in the check. You may implement a method that can strip the unique attributes from the component, or verify particular elements using their CSS selectors (for instance div.k-grid).

Scheduler Throws an Error

Subject Details
Issue Testing a component that contains a Scheduler fails with the following error:
System.NullReferenceException : Object reference not set to an instance of an object. at Telerik.Blazor.Components.Scheduler.Rendering.ContentTableBase`1.SetSlotMetrics(Dictionary`2metrics) at Telerik.Blazor.Components.Scheduler.Rendering.ContentTableBase`1.GetSlotMetrics()
Cause The root cause is that the Scheduler must render in the browser and then measure and adjust its layout with JavaScript. Then this information is sent to the .NET runtime to be used there.
Workaround Mock the component or refactor the component structure of your app, so that you can test a component that doesn't contain our Scheduler.

Cannot Find Dialog Content

Subject Details
Issue Detecting the content of a Dialog fails.
Cause The Dialog, Window and popup elements are rendered on root level and not in their place of declaration.
Workaround To detect the popup content, target the RootComponent and search inside it. See: Dialog example and Window example.

In future, UI for Blazor will support creation of an interface to easily mock the DialogFactory. Follow the request to gets status updates.

Grid OnRead Not Fired

Subject Details
Issue In test environment, OnRead is not raised after invoking Rebind.
Cause When Rebind is called, the Grid shows a loader. This loader is invoked with JS Interop, so the test fails silently.
Workaround Disable the built-in loader.


The following samples demonstrate unit testing a Blazor app with Telerik UI for Blazor components, including with bUnit:

You can also visit the Unit Testing Blazor Components with bUnit and JustMock blog post.

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