.NET - When should I use a property vs. variable + accessor function?

Question

Is there ever a situation where I should do the following in .NET instead of using a property with read/write capability?

private S as string

public function GetS() as string
     return S
end function

public sub SetS(byval NewS as string)
    S = NewS
end function

Do properties simply provide a more efficient way for doing the same thing?

Will properties be any slower than the above accessor functions in a high performance application?

Answer 1

Properties, internally, are nothing but a pair of methods. They basically evaluate to a get and set accessor method.

You should use properties, unless the property is going to cause some unexpected, potentially long running side effect, or there is some other good reason to use a method.

For details, I suggest reading the Property Usage Guidelines on MSDN. In particular, 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.

Otherwise, I'd use a property. Brad Abram's blogged some other details, including good reasons why certain API functions use methods (mostly because they could cause cross-computer communication, which would fall into the "side effect" category).