When to use properties instead of functions

问题:

This is probably a matter of personal preference, but when do you use properties instead of functions in your code

For instance to get an error log I could say

string GetErrorLog()
{
      return m_ErrorLog;
}

or I could

string ErrorLog
{
     get { return m_ErrorLog; }
}

How do you decide which one to use? I seem to be inconsistent in my usage and I'm looking for a good general rule of thumb. Thanks.

回答1:

I tend to use properties if the following are true:

• The property will return a single, logic value
• Little or no logic is involved (typically just return a value, or do a small check/return value)

I tend to use methods if the following are true:

• There is going to be significant work involved in returning the value - ie: it'll get fetched from a DB, or something that may take "time"
• There is quite a bit of logic involved, either in getting or setting the value

---

In addition, I'd recommend looking at Microsoft's Design Guidelines for Property Usage. They suggest:

Use a property when the member is a logical data member.

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.

回答2:

Here are Microsoft's guidelines:

Choosing Between Properties and Methods

• Consider using a property if the member represents a logical attribute of the type.

• Do use a property, rather than a method, if the value of the property is stored in the process memory and the property would just provide access to the value.

• Do use a method, rather than a property, in the following situations.

  • The operation is orders of magnitude slower than a field set would be. If you are even considering providing an asynchronous version of an operation to avoid blocking the thread, it is very likely that the operation is too expensive to be a property. In particular, operations that access the network or the file system (other than once for initialization) should most likely be methods, not properties.

  • The operation is a conversion, such as the Object.ToString method.

  • The operation returns a different result each time it is called, even if the parameters do not change. For example, the NewGuid method returns a different value each time it is called.

  • The operation has a significant and observable side effect. Note that populating an internal cache is not generally considered an observable side effect.

  • The operation returns a copy of an internal state (this does not include copies of value type objects returned on the stack).

  • The operation returns an array.

回答3:

I use properties when its clear the semantic is "Get somevalue from the object". However using a method is a good way to communicate "this may take a bit more than a trivial effort to return".

For example a collection could have a Count property. Its reasonable to assume a collection object knows how many items are currently held without it actually having to loop through them and count them.

On the hand this hypothetical collection could have GetSum() method which returns the total of the set of items held. The collection just a easily have a Sum property instead but by using a method it communicates the idea that the collection will have to do some real work to get an answer.

回答4:

I'd never use a property if I could be affecting more than one field - I'd always use a method.

Generally, I just use the public string ErrorLog { get; private set; } syntax for Properties and use Methods for everything else.

回答5:

In addition to Reed's answer when the property is only going to be a getter like getting a resource such as an Event Log might be. I try and only use properties when the property will be side effect free.

回答6:

If there is more than something trivial happening in a property, then it should be a method. For example, if your ErrorLog getter property was actually going and reading files, then it should be a method. Accessing a property should be fast, and if it is doing much processing, it should be a method. If there are side affects of accessing a property that the user of the class might not expect, then it should probably be a method.

There is .NET Framework Design Guidelines book that covers this kind of stuff in great detail.