How to create immutable objects in C#?

Question

In a question about Best practices for C# pattern validation, the highest voted answer says:

I tend to perform all of my validation in the constructor

Answer 1

Declaring all fields readonly is a good step towards creating an immutable object, but this alone is not sufficient. This is because a readonly field can still be a reference to a mutable object.

In C# immutability is not enforced by the compiler. You just have to be careful.