C# 2.0 - Get and Set Visibility of Class Properties and Indexers - Asymmetric Accessor Accessibility

Asymmetric Accessor Accessibility in C# 2.0 came up in the Sarasota .NET Developer Group Meeting last night as well as a conversation I had this morning, so I thought it would be worth sharing.

C# 2.0 introduces a new concept, called Asymmetric Accessor Accessibility, which essentially means you can modify the visibility of either the get accessor or set accessor on a class property that has both a “getter” and “setter.”

This allows you to do something like below:

 

public class Customer
{
    private int _customerID;
    private string _firstname = string.Empty;
    private string _lastname = string.Empty;

    public int ID
    {
        get
        { 
            return _customerID;
        }
        internal set
        {
            _customerID = value;
        }
    }

    // ...
}

 

Here we have restricted the visibility of the set accessor to internal, which means only a class in the same assembly as Customer can set the ID of the Customer via the set accessor.

There are a few restrictions to this new asymmetric accessor accessibility feature in C# 2.0.  The most relevent are:

1.  You can only set a different visibility on 1 of the 2 accessors, not both.  Trying to restrict visibility on both get and set to internal will give the following error:

“Cannot specify accessibility modifiers for both accessors of the property or indexer”

 

2.  You cannot use the feature on properties that do not have BOTH a get and set accessor.  Removing the get accessor from the example above will give you this lovely error:

“accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor”

 

3.  The accessibility modifier used on a get or set accessor can only restrict visibility not increase it.  Hence doing something like this:

 

public class Customer
{
    private int _customerID;
    private string _firstname = string.Empty;
    private string _lastname = string.Empty;

    protected int ID
    {
        get
        {
            return _customerID;
        }

        public set
        {
            _customerID = value;
        }
    }

    // ...
}

would give you the following error because the property has a protected access modifier that is more restrictive than the public set accessor and not the other way arround:

“accessor must be more restrictive than the property or indexer“

 

I have yet to use the Asymmetric Accessor Accessibility Feature of C# 2.0, but it is good to know it is there.  For more information on new language features in C# 2.0, you can always check out the C# Programming Guide on MSDN.

 

Source:  David Hayden ( Florida .NET Developer )

 

posted on Wednesday, January 18, 2006 2:55 PM