Constant & Static Readonly fields

Constant
const field can never be modified and must be initialized where it is declared
class Company
{
    public const string CompanyName = "RK Inc";
}

Static Readonly
class Company
{
    public static readonly string CompanyName = "RK Inc";
    staticCompany()
    {
        CompanyName = "My Company";
    }
}
Assigning the value to a readonly field outside the class will throw the following error:
Company.CompanyName = "My Company";

A static readonly field cannot be assigned to (except in a static constructor or a variable initializer)