Understanding Program.cs in ASP.NET Core

When you start learning ASP.NET Core, the Program.cs file can feel overwhelming.
So many methods, so many configurations — authentication, middleware, services, pipelines…

Here’s the good news:

• You do NOT need to memorize everything in Program.cs
• You only need to understand how it works conceptually

This article will help you build a simple mental model so you always know what goes where — even if you forget the exact syntax.

The Big Picture

Program.cs does two main things:

1. Registers services (Dependency Injection)
2. Configures the HTTP request pipeline (middleware)

If you understand these two ideas, everything else becomes much easier.

Registering Services (Dependency Injection)

This is the part where you tell ASP.NET Core:

“My application needs these services to work.”

Examples:

• Controllers
• Entity Framework DbContext
• Authentication (JWT, cookies)
• Swagger
• CORS
• Custom application services

Example

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllers();
builder.Services.AddDbContext<AppDbContext>(options => ...);
builder.Services.AddAuthentication().AddJwtBearer(...);
builder.Services.AddSwaggerGen();

How to recognize a service

Ask yourself:

“Will I inject this into a constructor?”

If the answer is yes, it probably belongs in builder.Services.

Configuring the Middleware Pipeline

Middleware defines how an HTTP request flows through your application.

Think of it like a pipeline:

Request → Middleware → Middleware → Controller → Response

Each middleware can:

• Inspect the request
• Modify it
• Stop it
• Pass it forward

Example

var app = builder.Build();

app.UseHttpsRedirection();
app.UseAuthentication();
app.UseAuthorization();

app.MapControllers();

app.Run();

Important rule!

Order matters.

For example:

• UseAuthentication() must come before
• UseAuthorization()

The Most Common Program.cs Template (90% of APIs)

This is the structure you’ll see in most ASP.NET Core projects:

var builder = WebApplication.CreateBuilder(args);

// 1. Services
builder.Services.AddControllers();
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

var app = builder.Build();

// 2. Middleware
if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

app.UseHttpsRedirection();

app.UseAuthentication();
app.UseAuthorization();

app.MapControllers();

app.Run();

If you understand this template, you already understand most ASP.NET Core applications.

Where Do Common Features Go?

Feature	Services	Middleware
Controllers	AddControllers()	MapControllers()
Authentication	AddAuthentication()	UseAuthentication()
Authorization	—	UseAuthorization()
Swagger	AddSwaggerGen()	UseSwagger()
CORS	AddCors()	UseCors()
EF Core	AddDbContext()	—
Static files	—	UseStaticFiles()

What You SHOULD Memorize

• Services vs Middleware concept
• The idea of a request pipeline
• AddControllers() + MapControllers()
• Authentication before Authorization
• Environment.IsDevelopment()

What You Should NOT Memorize

• JWT validation parameters
• Swagger configuration details
• EF Core advanced options
• CORS policy edge cases

These are reference knowledge, not memory knowledge.

Final Advice

ASP.NET Core is modular by design.

You are not expected to remember everything —
you are expected to recognize patterns.

Once the mental model is clear:

• Reading other people’s Program.cs files becomes easy
• Debugging configuration issues becomes logical
• Adding new features feels natural

Learn the structure.
Look up the details.

That’s how real-world ASP.NET Core development works.