Program.cs

using IWebSocketManager = CoDraw.IWebSocketManager;
using WebSocketManager = CoDraw.WebSocketManager;

using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.FileProviders;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();
builder.Services.AddControllers();

builder.Services.AddSingleton<IWebSocketManager, WebSocketManager>();

builder.Services.AddCors(options =>
{
    options.AddPolicy("AllowReactApp",
        builder =>
        {
            builder.WithOrigins("http://localhost:3000")
                .AllowAnyHeader()
                .AllowAnyMethod();
        });
});

var app = builder.Build();

app.UseCors("AllowReactApp");

app.UseDefaultFiles(); // Enables default file mapping on the server.
app.UseStaticFiles(); // Enables static file serving on the server.

// Configure static files options
app.UseStaticFiles(new StaticFileOptions
{
    FileProvider = new PhysicalFileProvider(
        Path.Combine(Directory.GetCurrentDirectory(), "wwwroot")),
    RequestPath = ""
});

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

// MapFallbackToFile()
app.MapFallbackToFile("/index.html");

// options for websockets
var webSocketOptions = new WebSocketOptions
{
    KeepAliveInterval = TimeSpan.FromMinutes(2)
};
app.UseWebSockets(webSocketOptions);

app.UseHttpsRedirection();

app.MapControllers();

app.Run();