const:

• Compile-time constant
• Must be initialized at declaration
• Value cannot change
• Implicitly static
• Only primitive types, null, or string literals
  readonly:
• Runtime constant
• Can be initialized at declaration or in constructor
• Value can be set once per instance
• Can be instance or static
• Can hold any type including complex objects

public class Example
{
    const int MAX_SIZE = 100;           // Compile-time constant
    readonly DateTime createdDate;      // Runtime constant
    public Example()
    {
        createdDate = DateTime.Now;     // Can set in constructor
    }
}