Constant and the readonly Modifier

A class can define fields that are constant.  To define a constant field, simply apply the const modifier and
initialize the field to some value.  You must initialize a constant field to another constant value, either a literal or
a constant variable.  Of course, once initialized, a constant field value can never change.


public class MyConstants
{
  // Defining a non-constant field.
  private static string mName = "Homer";

  // Initializing constants; these are valid.
  public const int    c1 = 1;
  public const string c2 = "Another Constant";
  public const string c3 = c2;
  
  // **** These are INVALID! *****
  public const Employee c4 = new Employee();
  public const int[]    c5 = new int[]{1, 2, 3, 4};
  public const string   c6 = mName;
  public const string   c7; // Const fields must be initialized.
}


The compiler handles constants by "hard coding" the value into the CIL. Also note that constant members are
implicitly static.

























Use the readonly modifier to define a field that needs to be initialized with a non-constant value but must never
change after initialization.  The value of a read-only field is determined dynamically at runtime. In contrast, the
value of a constant field is hardcoded into the CIL at compile time.  Read-only fields are instance fields, whereas
constant fields are implicitly static.  You can combine the static and readonly modifiers.
 

public class MyConstants
{
  // Defining a non-constant field.
  private static string mName = "Homer";

  // **** These are INVALID! *****
  public const Employee c4  = new Employee();
  public const int[]    c5  = new int[]{1, 2, 3, 4};
  public const string   c6  = mName;
  public const string   c7; // const fields must be initialized.

  // All of the above work if you use readonly instead of const.
  public readonly Employee r1 = new Employee();
  public readonly int[]    r2 = new int[] {1, 2, 3, 4};
  public readonly string   r3 = mName;
  public readonly string   r4;

  public MyConstants()
  {
     // You can initialize readonly fields in a constructor but
     // not in any other method.
     r4 = "initializing a readonly field!";
  }
}
readonly and Constant
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