Difference between const and readonly in C#

'const':

• Can't be static.
• Value is evaluated at compile time.
• Initiailized at declaration only.

'readonly':

• Can be either instance-level or static.
• Value is evaluated at run time.
• Can be initialized in declaration or by code in the constructor.

The 'const' modifier is applied to a field or local variable whose value can never change. It  is evaluated at compile time and can only be one of the built-in types such as int, bool, char, double, string etc which support literals.

In contrast, the 'readonly' modifier can only be applied to a static or instance field - it can't be applied to a local variable. Also, it can only be given a value either where it's defined or within the enclosing type's constructor and cannot be changed after that. However, it's value is not determined until runtime and can therefore be of any type, not just the built-in types.