These notes are specific to Blazor WASM because Blazor Server uses environment configuration of ASP.NET Core.

When run locally, app defaults to Development.
When app is published, app defaults to Production.
In a hosted Blazor WASM solution, the Server app sets the Blazor-Environment variable to its value. The Client app reads it and sets the environment when WebAssemblyHost is created in Program.cs.

Set Environment via Blazor Start Configuration

In wwwroot/index.html inside the closing </body> tag:

<script src="_framework/blazor.webassembly.js" autostart="false"></script>
  if (window.location.hostname.includes("localhost")) {
      environment: "Staging"
  } else {
      environment: "Production"

This overrides the Blazor-Environment header.

Set Environment via Environment Header

For IIS, in web.config:

<?xml version="1.0" encoding="UTF-8"?>
        <add name="Blazor-Environment" value="Staging" />

Read the Environment in a Component

Inject IWebAssemblyHostEnvironment and read the Environment property:

@page "/read-environment"
@using Microsoft.AspNetCore.Components.WebAssembly.Hosting
@inject IWebAssemblyHostEnvironment HostEnvironment

<h1>Environment example</h1>

<p>Environment: @HostEnvironment.Environment</p>

Or, during startup, in Program.cs:

if (builder.HostEnvironment.Environment == "Custom")

WebAssemblyHostEnvironmentExtensions contains IsDevelopment, IsStaging, IsProduction, and IsEnvironment("environment-name").