Question

I am currently reading Effective C# and found the following line very confusing:

Last, but certainly not least, you can
use interfaces to create const and
nonconst versions of an interface:

What are and nonconst versions of an interface? And where would we use this?

Supporting code:

public interface IConstNameValuePair
{
  object Name
  {
    get;
  }

  object Value
  {
    get;
  }
}
public interface INameValuePair
{
  object Value
  {
    get;
    set;
  }
}

// Usage:
public class Stuff : IConstNameValuePair, INameValuePair
{
  private string _name;
  private object _value;

  #region IConstNameValuePair Members
  public object Name
  {
    get
    {
      return _name;
    }
  }

  object IConstNameValuePair.Value
  {
    get
    {
      return _value;
    }
  }

  #endregion

  #region INameValuePair Members

  public object Value
  {
    get
    {
      return _value;
    }
    set
    {
     _value = value;
    }
  }
  #endregion
}

Answer 1

It is showing that, through one of those interfaces, you can’t actually modify the value (the ‘set’ property isn’t defined). It’s a little dubious to call this ‘const’, as that is actually a keyword that has special meaning. But that’s what the author means.