Exposing Member Objects As Properties or Methods in .NET

Question

In .NET, if a class contains a member that is a class object, should that member be exposed as a property or with a method?

Answer 1

You should use properties for anything that conceptually represents the object's state, so long as its retrieval isn't an expensive enough operation that you should avoid using it repeatedly.

From MSDN:

Class library designers often must decide between implementing a class member as a property or a method. In general, methods represent actions and properties represent data. Use the following guidelines to help you choose between these options.

• Use a property when the member is a logical data member. In the following member declarations, Name is a property because it is a logical member of the class.

  public string Name
  get 
  {
      return name;
  }
  set 
  {
      name = value;
  }
  

• Use a method when:

  • The operation is a conversion, such as Object.ToString.
  • The operation is expensive enough that you want to communicate to the user that they should consider caching the result.
  • Obtaining a property value using the get accessor would have an observable side effect.
  • Calling the member twice in succession produces different results.
  • The order of execution is important. Note that a type's properties should be able to be set and retrieved in any order.
  • The member is static but returns a value that can be changed.

  • The member returns an array. Properties that return arrays can be very misleading. Usually it is necessary to return a copy of the internal array so that the user cannot change internal state. This, coupled with the fact that a user can easily assume it is an indexed property, leads to inefficient code. In the following code example, each call to the Methods property creates a copy of the array. As a result, 2n+1 copies of the array will be created in the following loop.

    Type type = // Get a type.
    for (int i = 0; i < type.Methods.Length; i++)
    {
       if (type.Methods[i].Name.Equals ("text"))
       {
          // Perform some operation.
       }
    }
    

The following example illustrates the correct use of properties and methods.

    class Connection
    {
       // The following three members should be properties
       // because they can be set in any order.
       string DNSName {get{};set{};}
       string UserName {get{};set{};}
       string Password {get{};set{};}

       // The following member should be a method
       // because the order of execution is important.
       // This method cannot be executed until after the 
       // properties have been set.
       bool Execute ();
    }

Answer 2

If all you are doing is exposing an object instance that is relevant to the state of the current object you should use a property.

A method should be used when you have some logic that is doing more than accessing an in memory object and returning that value or when you are performing an action that has a broad affect on the state of the current object.

Answer 3

That is irrelevant to the matter.

It should be a Property if the value is some detail about the state of the object.

It should be Method if it performs some action on the object.

Answer 4

Property. A Property is basically just a 'cheap' method. Getting or setting a reference to an object is pretty cheap.

Just to clarify, properties are usually supposed to represent the internal state of an object. However, the implementation of a member as a property or method tells the user how expensive the call is likely to be.