Nullable Types

Nullable Modifier

Often it is desirable to represent values that are "missing" (unknown or unassigned)
You can declare a type to allow a null value or not, with the nullable modifier

int? number = null;

When the programmer opts to allow a variable to be null, they takes on the additional responsibility of being sure to avoid dereferencing a variable whose value is null.

For most part dereferencing a nullable value type that represents null will not throw a nullable exception. Members like HasValue, ToString()
Dereferencing a null value type throws an InvalidOperationException
Invoking GetType() when the value represents null does throw a NullReferenceException, because GetType() is not a virtual method,

Prior to C# 8.0, all reference type variables allowed null. Reference type variables are nullable by default
C# 8.0 introduced a feature known as nullable reference types.
Reference type declarations can occur with or without a nullable modifier. In C# 8.0, declaring a variable without the nullable modifier implies it is not nullable.
For backward compatibility, nullability is not enabled by default on existing projects. On new projects nullability is enabled at the start.
There are several mechanisms to configure nullability: the #nullable directive, project properties, and even the command line.

#nullable enable

<Nullable>enable</Nullable>

dotnet build /p:Nullable=enable