ASPReactDemo/Api/Program.cs

using Api.SignalR;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
// Learn more about configuring OpenAPI at https://aka.ms/aspnet/openapi
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddOpenApi();
builder.Services.AddSignalR();
builder.Services.AddHostedService<SignalRSendService>();
builder.Services.AddOpenApiDocument(config =>
{
    config.Title = "NSwag Demo API";
});
builder.Services.AddCors(options =>
{
    options.AddDefaultPolicy(policy =>
    {
        policy.WithOrigins("http://localhost:3000") // Specify the allowed origin
              .AllowAnyHeader()
              .AllowAnyMethod()
              .AllowCredentials(); // Allow credentials
    });
});

var app = builder.Build();

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.MapOpenApi();
    app.UseOpenApi();    
    app.UseSwaggerUi();  
}

app.UseHttpsRedirection();
app.UseCors();

var summaries = new[]
{
    "Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
};

app.MapGet("/weatherforecast", async () =>
{
    var forecast =  Enumerable.Range(1, 5).Select(index =>
        new WeatherForecast
        (
            DateOnly.FromDateTime(DateTime.Now.AddDays(index)),
            Random.Shared.Next(-20, 55),
            summaries[Random.Shared.Next(summaries.Length)]
        ))
        .ToArray();

    return forecast;
})
.WithName("GetWeatherForecast");


// Add SignalR hub
app.MapHub<WeatherUpdateHub>("/WeatherUpdateHub");

app.Run();

record WeatherForecast(DateOnly Date, int TemperatureC, string? Summary)
{
    public int TemperatureF => 32 + (int)(TemperatureC / 0.5556);
}