What is the difference between const and static readonly?

The difference is that the value of a static readonly field is set at run time, and can thus be modified by the containing class, whereas the value of a const field is set to a compile time constant.

In the static readonly case, the containing class is allowed to modify it only

• in the variable declaration (through a variable initializer)
• in the static constructor (instance constructors, if it's not static)

static readonly is typically used if the type of the field is not allowed in a const declaration (array of constants), or when the value is not known at compile time.