C# essentials

The C# and .NET features you meet from day one: the dotnet CLI, value and reference types, nullable references, properties, records, LINQ and async code.

The dotnet CLI

Also called .NET SDK, dotnet new, dotnet run, csproj, dotnet CLI

What: The dotnet command from the .NET SDK creates, builds, runs and tests projects. Each project is described by a .csproj file; a solution groups several projects.
Why: Knowing the CLI means you understand what Visual Studio or Rider does for you, and you can build in CI or on any machine.

Shell

dotnet new console -o Ledger
dotnet new xunit -o Ledger.Tests
dotnet add Ledger.Tests reference Ledger/Ledger.csproj
dotnet run --project Ledger
dotnet test Ledger.Tests

Watch out

Targeting an out-of-support .NET version from an old tutorial; use the current long-term support release.

Value types and reference types

Also called struct vs class, boxing

What: Value types (int, bool, struct) are copied on assignment. Reference types (class, string, arrays, List<T>) are shared: assignment copies a reference to the same object.
Why: It decides whether changing a copy changes the original, which explains most surprising mutations in C#.

var s1 = new PointS { X = 1 };
var s2 = s1;
s2.X = 9;
var c1 = new PointC { X = 1 };
var c2 = c1;
c2.X = 9;
Console.WriteLine($"{s1.X} {c1.X}"); // 1 9

struct PointS { public int X; }
class PointC { public int X; }

Watch out

Making a large or mutable struct: every assignment copies it and changes to a copy are lost.

Nullable reference types

Also called string?, null-forgiving operator, NRT

What: With nullable reference types on (the default in new projects), string means never null and string? means may be null. The compiler warns when you might dereference a null.
Why: It moves the most common .NET crash, NullReferenceException, from runtime to compile-time warnings.

string name = "Ada";
string? nickname = null;
int length = nickname?.Length ?? 0;
if (nickname is not null)
{
  Console.WriteLine(nickname.ToUpper());
}

Watch out

Silencing warnings with the ! operator instead of handling the null case.

Properties

Also called get, set, init, auto-properties

What: Properties look like fields to callers but run get and set accessors. init allows setting only during creation, and required forces callers to set it.
Why: They let a class expose data while controlling changes, such as a balance that only its own methods can modify.

public class Product
{
  public required string Name { get; init; }
  public decimal Price { get; private set; }
  public bool IsFree => Price == 0;

  public void Discount(decimal percent) =>
    Price -= Price * percent / 100;
}

Watch out

Making every property public get; set; by habit, which lets any code put the object into an invalid state.

Records

Also called record class, with expression

What: A record is a type for data with value-based equality, a readable ToString and non-destructive changes through with.
Why: Use records for data that should be compared by value, such as money, settings or messages, instead of writing Equals by hand.

var a = new Money(10m, "GBP");
var b = a with { Amount = 12m };
Console.WriteLine(a == new Money(10m, "GBP")); // True
Console.WriteLine(b); // Money { Amount = 12, Currency = GBP }

public record Money(decimal Amount, string Currency);

Watch out

Expecting a record to be deeply immutable when it holds a List<T>: the list itself can still change.

LINQ

Also called Language Integrated Query, Where, Select

What: LINQ adds query methods such as Where, Select, GroupBy and OrderBy to any collection, with lambdas for the conditions.
Why: It turns filter, group and sort logic into one readable pipeline, and the same style queries databases through Entity Framework.

var topCustomers = orders
  .Where(o => o.Total > 10)
  .GroupBy(o => o.Customer)
  .Select(g => (Customer: g.Key, Sum: g.Sum(o => o.Total)))
  .OrderByDescending(x => x.Sum)
  .Take(3)
  .ToList();

Watch out

Forgetting that LINQ is lazy: the query runs each time you enumerate it, so call ToList once if you reuse the result.

`async` and `await`

Also called Task, Task.WhenAll, asynchronous

What: An async method returns a Task and can await other tasks. While it waits for I/O, the thread is free to do other work.
Why: Web requests, database calls and file I/O in .NET are asynchronous. Task.WhenAll runs independent calls at the same time.

static async Task<string> LoadAsync(HttpClient http, string url)
{
  string body = await http.GetStringAsync(url);
  return body.Trim();
}

var tasks = urls.Select(url => LoadAsync(http, url));
string[] pages = await Task.WhenAll(tasks);

Watch out

Blocking on a task with .Result or .Wait(), which wastes threads and can deadlock; await it instead.

Go deeper

C# essentials

The dotnet CLI

Value types and reference types

Nullable reference types

Properties

Records

LINQ

async and await

`async` and `await`