# Types, var and Nullability — .NET Core: Build, Test and Ship Web APIs

Source: https://www.geekswithgeeks.com/en/dotnet-core/cs-types-nullability

> Declare variables, interpolate strings and handle nullable reference types.

## Static types, inferred

C# is statically typed: every variable has a type known at compile time. `var` lets the compiler infer it, but the type is still fixed. `var n = 5;` is an `int`, not a dynamic value.

## Data, behaviour, async

Most .NET code models data, transforms collections, and awaits slow I/O.

![Three stages: types, collections, async work.](assets/figures/dotnet-core/section-2-map.svg) — Figure 2.1 — Types, collections and async I/O.

## Nullable reference types

With `Nullable` enabled, `string` means "never null" and `string?` means "may be null". The compiler warns before you dereference a possible null.

```csharp
string name = "Asha";
string? nickname = null;

Console.WriteLine($"Hi {name}");
Console.WriteLine(nickname?.Length ?? 0);
```

Output:

```
Hi Asha
0
```

## Do not silence warnings with !

The `!` operator tells the compiler "trust me, not null". If you are wrong you still get a `NullReferenceException` at runtime. Prefer a null check or `?.` instead.

**Quiz:** What does `string?` mean with Nullable enabled?

- [ ] The string is always empty
- [x] The string may be null
- [ ] The string is read-only
- [ ] The string is a number

*Answer:* The string may be null. The `?` marks a reference type as allowed to be null, so the compiler makes you handle that case.