How to do integration testing for .NET 6 Web Api projects using WebApplicationFactory?

Support for this was added in .NET 6 RC1. This is the first version to be supported in production.

You can pass Program instead of Startup. Startup essentially merged into Program now. The Program class is automagically generated from the top-level statements in the Program.cs file. It's not a public class though, so you need to add InternalsVisibleTo to the application's project file to make it visible to the test project.

<ItemGroup>
  <InternalsVisibleTo Include="MinimalApiPlayground.Tests" />
</ItemGroup>

In this example Damien Edwards defines a WebApplicationFactory<Program> to demonstrate testing a Todo minimal API.

Creating a test app is essentially the same as using Startup : :

[Fact]
public async Task GetSwaggerUI_Returns_OK()
{
    await using var application = new TodoApplication();

    var client = application.CreateClient();
    var response = await client.GetAsync("/swagger/index.html");

    Assert.Equal(HttpStatusCode.OK, response.StatusCode);
}

class TodoApplication : WebApplicationFactory<Program>
{
    protected override IHost CreateHost(IHostBuilder builder)
    {
        // Add mock/test services to the builder here
        builder.ConfigureServices(services =>
        {
            services.AddScoped(sp =>
            {
                // Replace SQL Lite with test DB
                return new SqliteConnection("Data Source=testtodos.db");
            });
        });

        return base.CreateHost(builder);
    }
}

The Program class in that example defines quite a lot of endpoints :

using System.ComponentModel.DataAnnotations;
using Microsoft.Data.Sqlite;
using Dapper;

var builder = WebApplication.CreateBuilder(args);

var connectionString = builder.Configuration.GetConnectionString("TodoDb") ?? "Data Source=todos.db";
builder.Services.AddScoped(_ => new SqliteConnection(connectionString));
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

var app = builder.Build();

await EnsureDb(app.Services, app.Logger);

if (!app.Environment.IsDevelopment())
{
    app.UseExceptionHandler("/error");
}

app.MapGet("/error", () => Results.Problem("An error occurred.", statusCode: 500))
   .ExcludeFromDescription();

app.MapSwagger();
app.UseSwaggerUI();

app.MapGet("/", () => "Hello World!")
   .WithName("Hello");
...
app.MapDelete("/todos/delete-all", async (SqliteConnection db) => Results.Ok(await db.ExecuteAsync("DELETE FROM Todos")))
    .WithName("DeleteAll")
    .Produces<int>(StatusCodes.Status200OK);

app.Run();