RE有两个独立的函数,一个是“有吗?”,一个是“如果有,是什么?”是的,你当然可以这么做,但你为什么要这么做呢?现在调用程序必须进行两次调用而不是一次。如果程序员没有调用“any?”而直接调用“What is it?”会发生什么?? 程序会返回一个误导的零吗?抛出异常?返回一个未定义的值?这会产生更多的代码、更多的工作和更多的潜在错误。
using System;
using System.Collections.Generic;
using System.Linq;
using NUnit.Framework;
namespace StackOverflow.EmptyCollectionUsageTests.Tests
{
/// <summary>
/// Demonstrates different approaches for empty collection results.
/// </summary>
class Container
{
/// <summary>
/// Elements list.
/// Not initialized to an empty collection here for the purpose of demonstration of usage along with <see cref="Populate"/> method.
/// </summary>
private List<Element> elements;
/// <summary>
/// Gets elements if any
/// </summary>
/// <returns>Returns elements or empty collection.</returns>
public IEnumerable<Element> GetElements()
{
return elements ?? Enumerable.Empty<Element>();
}
/// <summary>
/// Initializes the container with some results, if any.
/// </summary>
public void Populate()
{
elements = new List<Element>();
}
/// <summary>
/// Gets elements. Throws <see cref="InvalidOperationException"/> if not populated.
/// </summary>
/// <returns>Returns <see cref="IEnumerable{T}"/> of <see cref="Element"/>.</returns>
public IEnumerable<Element> GetElementsStrict()
{
if (elements == null)
{
throw new InvalidOperationException("You must call Populate before calling this method.");
}
return elements;
}
/// <summary>
/// Gets elements, empty collection or nothing.
/// </summary>
/// <returns>Returns <see cref="IEnumerable{T}"/> of <see cref="Element"/>, with zero or more elements, or null in some cases.</returns>
public IEnumerable<Element> GetElementsInconvenientCareless()
{
return elements;
}
/// <summary>
/// Gets elements or nothing.
/// </summary>
/// <returns>Returns <see cref="IEnumerable{T}"/> of <see cref="Element"/>, with elements, or null in case of empty collection.</returns>
/// <remarks>We are lucky that elements is a List, otherwise enumeration would be needed.</remarks>
public IEnumerable<Element> GetElementsInconvenientCarefull()
{
if (elements == null || elements.Count == 0)
{
return null;
}
return elements;
}
}
class Element
{
}
/// <summary>
/// http://stackoverflow.com/questions/1969993/is-it-better-to-return-null-or-empty-collection/
/// </summary>
class EmptyCollectionTests
{
private Container container;
[SetUp]
public void SetUp()
{
container = new Container();
}
/// <summary>
/// Forgiving contract - caller does not have to implement null check in addition to enumeration.
/// </summary>
[Test]
public void UseGetElements()
{
Assert.AreEqual(0, container.GetElements().Count());
}
/// <summary>
/// Forget to <see cref="Container.Populate"/> and use strict method.
/// </summary>
[Test]
[ExpectedException(typeof(InvalidOperationException))]
public void WrongUseOfStrictContract()
{
container.GetElementsStrict().Count();
}
/// <summary>
/// Call <see cref="Container.Populate"/> and use strict method.
/// </summary>
[Test]
public void CorrectUsaOfStrictContract()
{
container.Populate();
Assert.AreEqual(0, container.GetElementsStrict().Count());
}
/// <summary>
/// Inconvenient contract - needs a local variable.
/// </summary>
[Test]
public void CarefulUseOfCarelessMethod()
{
var elements = container.GetElementsInconvenientCareless();
Assert.AreEqual(0, elements == null ? 0 : elements.Count());
}
/// <summary>
/// Inconvenient contract - duplicate call in order to use in context of an single expression.
/// </summary>
[Test]
public void LameCarefulUseOfCarelessMethod()
{
Assert.AreEqual(0, container.GetElementsInconvenientCareless() == null ? 0 : container.GetElementsInconvenientCareless().Count());
}
[Test]
public void LuckyCarelessUseOfCarelessMethod()
{
// INIT
var praySomeoneCalledPopulateBefore = (Action)(()=>container.Populate());
praySomeoneCalledPopulateBefore();
// ACT //ASSERT
Assert.AreEqual(0, container.GetElementsInconvenientCareless().Count());
}
/// <summary>
/// Excercise <see cref="ArgumentNullException"/> because of null passed to <see cref="Enumerable.Count{TSource}(System.Collections.Generic.IEnumerable{TSource})"/>
/// </summary>
[Test]
[ExpectedException(typeof(ArgumentNullException))]
public void UnfortunateCarelessUseOfCarelessMethod()
{
Assert.AreEqual(0, container.GetElementsInconvenientCareless().Count());
}
/// <summary>
/// Demonstrates the client code flow relying on returning null for empty collection.
/// Exception is due to <see cref="Enumerable.First{TSource}(System.Collections.Generic.IEnumerable{TSource})"/> on an empty collection.
/// </summary>
[Test]
[ExpectedException(typeof(InvalidOperationException))]
public void UnfortunateEducatedUseOfCarelessMethod()
{
container.Populate();
var elements = container.GetElementsInconvenientCareless();
if (elements == null)
{
Assert.Inconclusive();
}
Assert.IsNotNull(elements.First());
}
/// <summary>
/// Demonstrates the client code is bloated a bit, to compensate for implementation 'cleverness'.
/// We can throw away the nullness result, because we don't know if the operation succeeded or not anyway.
/// We are unfortunate to create a new instance of an empty collection.
/// We might have already had one inside the implementation,
/// but it have been discarded then in an effort to return null for empty collection.
/// </summary>
[Test]
public void EducatedUseOfCarefullMethod()
{
Assert.AreEqual(0, (container.GetElementsInconvenientCarefull() ?? Enumerable.Empty<Element>()).Count());
}
}
}