What is the difference between const and readonly in C#?

Question

What is the difference between const and readonly in C#?

When would you use one over the other?

Answer 1

Constants

• Constants are static by default
• They must have a value at compilation-time (you can have e.g. 3.14 * 2, but cannot call methods)
• Could be declared within functions
• Are copied into every assembly that uses them (every assembly gets a local copy of values)
• Can be used in attributes

Readonly instance fields

• Must have set value, by the time constructor exits
• Are evaluated when instance is created

Static readonly fields

• Are evaluated when code execution hits class reference (when new instance is created or a static method is executed)
• Must have an evaluated value by the time the static constructor is done
• It's not recommended to put ThreadStaticAttribute on these (static constructors will be executed in one thread only and will set the value for its thread; all other threads will have this value uninitialized)