At its Build developer event last week, Microsoft told attendees that Blazor is its main investment in Web UI (user interface) for ASP.NET Core, despite the continuing popularity of the older ASP.NET MVC and Razor Page frameworks.

Blazor is an end-to-end stack that lets developers use .NET and C# both on the server and in a browser client. Blazor Hybrid is a further variant in which mobile and desktop applications use a web view control to run Blazor applications. Blazor Hybrid applications can be embedded in other frameworks including .NET MAUI (Multi-platform app UI), WPF (Windows Presentation Foundation) and Windows Forms.

In a Build session on ASP.NET Core and Blazor futures, principal program managers Daniel Roth and Mike Kistler described what to expect in the forthcoming .NET 10, a long-term support release expected in November, as well as outlining the company’s focus for its web platform.

One notable feature is that the Kestrel web server used by ASP.NET Core will gain the ability to trim memory for the first time. Until now, Kestrel has been unable to do this, so memory demanded by peak usage remained in use after demand cooled.

Another big feature is WebAuthN and Passkey authentication support. Microsoft will build this into .NET 10 and add scaffolding support in both the .NET CLI (command-line interface) and in Visual Studio wizards. The new support will be “inspired” by an existing third-party library called fido2-net-lib said Fistler, though it will not use this library in its entirety for reasons that are not altogether clear, but involve features that will not be supported, such as the ability to specify which authenticators a relying party will accept.

Microsoft will also add tools to update existing applications with Passkey support, for frameworks including ASP.NET Core, Blazor Hybrid and .NET MAUI.

Other areas where improvements are promised include additional metrics such as for authentication and authorization; diagnostic tools for Blazor WebAssembly; faster JSON deserialization; and support for JSON patching in the System.Text.Json library. This last feature may help developers migrate from the older but still popular Newtonsoft.Json library.

Choosing a framework for a new application is a long-term bet for developers, who are therefore keen to know which is likely to be best supported in future. The current story is that Microsoft regards Blazor as “our most comprehensive and flexible Web UI offering for .NET,” said Roth. Support for ASP.NET Core MVC and Razor Pages will continue, but Blazor is the future focus. 

On the server, Kistler said that “minimal APIs with System.Text.Json is the right place for strategic investments.” Minimal APIs are a simplified approach for building HTTP APIs and can benefit from features like ahead-of-time compilation that are not available for applications using the older controller-based Web API framework.

Another tip Kistler offered was that “All your web apps going forward should be based in Aspire.” Aspire is a project to assist development and deployment of containerised distributed applications.

Microsoft makes extensive use of ASP.NET Core internally, and uses it for Microsoft 365, Xbox services and most Azure services, said Kistler and Roth. The same is not true for Blazor, which has more limited internal use. Microsoft Office, for example, uses React and React Native, causing some to question the company’s commitment to Blazor.  

When .NET 9 was released, Roth was questioned on this subject by .NET developer and podcaster Nick Chapsas. He gave two main reasons for low use of Blazor within Microsoft, especially for public-facing applications like Office. One is historic, that when Office first adopted React Blazor was not available. The other though is more illuminating. Blazor, said Roth, is designed to solve the problem of .NET developers also needing deep knowledge of JavaScript, which he called “the cost and burden of having to bridge multiple developer ecosystems.” The .NET platform, he said, is well optimized for server use, and Blazor enables teams to use it in the browser as well. Blazor adds value, he said, “when you need to do more without a separate front-end JavaScript team.”

Companies the size of Microsoft already have front-end teams specializing in JavaScript or TypeScript, making Blazor less compelling, as it has some overhead, Roth admitted.

Blazor performance should improve in .NET 10 thanks to work on preloading WebAssembly resources and enabling more browser caching of Blazor assets.