我收集了一些极端案例和脑筋急转弯，总是想听到更多。这个页面只涵盖了c#语言的一些细节，但我也发现了。net核心的东西也很有趣。例如，这里有一个没有在页面上，但我觉得不可思议:
string x = new string(new char[0]);
string y = new string(new char[0]);
Console.WriteLine(object.ReferenceEquals(x, y));
我希望输出False -毕竟，“new”(具有引用类型)总是创建一个新对象，不是吗?c#和CLI的规范都表明应该这样做。嗯，在这个特殊情况下不是这样。它输出True，并且在我测试过的框架的每个版本上都是这样。(不可否认，我还没有在Mono上尝试过……)
只是为了澄清，这只是我正在寻找的事情的一个例子-我并不是特别寻找对这个奇怪现象的讨论/解释。(这和普通的弦乐实习不一样;特别地，当调用构造函数时，字符串实习通常不会发生。)我真的是在要求类似的奇怪行为。
还有其他的宝藏吗?
当前回答
这一点很简单，但我还是觉得很有趣。调用Foo后x的值是多少?
static int x = 0;
public static void Foo()
{
try { return; }
finally { x = 1; }
}
static void Main() { Foo(); }
其他回答
c#中的作用域有时真的很奇怪。让我给你们举个例子:
if (true)
{
OleDbCommand command = SQLServer.CreateCommand();
}
OleDbCommand command = SQLServer.CreateCommand();
编译失败，因为命令被重新声明?在stackoverflow的这个帖子和我的博客中，有一些有趣的猜测，为什么它会这样工作。
我想我之前向您展示过这个，但我喜欢这里的乐趣——这需要一些调试才能跟踪!(原来的代码显然更加复杂和微妙……)
static void Foo<T>() where T : new()
{
T t = new T();
Console.WriteLine(t.ToString()); // works fine
Console.WriteLine(t.GetHashCode()); // works fine
Console.WriteLine(t.Equals(t)); // works fine
// so it looks like an object and smells like an object...
// but this throws a NullReferenceException...
Console.WriteLine(t.GetType());
}
那么T是什么?
答:任何可空<T> -如int?。所有的方法都被重写，除了GetType()不能;因此它被强制转换为object(因此为null)来调用object. gettype()…哪个调用null;-p
更新:情节变得越来越复杂……Ayende Rahien在他的博客上提出了类似的挑战，但使用了where T: class, new():
private static void Main() {
CanThisHappen<MyFunnyType>();
}
public static void CanThisHappen<T>() where T : class, new() {
var instance = new T(); // new() on a ref-type; should be non-null, then
Debug.Assert(instance != null, "How did we break the CLR?");
}
但它是可以被打败的!使用与远程处理一样的间接方式;警告-以下是纯粹的邪恶:
class MyFunnyProxyAttribute : ProxyAttribute {
public override MarshalByRefObject CreateInstance(Type serverType) {
return null;
}
}
[MyFunnyProxy]
class MyFunnyType : ContextBoundObject { }
在此之后，new()调用被重定向到代理(MyFunnyProxyAttribute)，该代理返回null。现在去洗眼睛!
VB。NET、可空值和三元操作符:
Dim i As Integer? = If(True, Nothing, 5)
这花了我一些时间来调试，因为我希望I包含Nothing。
i到底包含了什么?0.
这是令人惊讶的，但实际上是“正确的”行为:在VB中没有。NET与CLR中的null并不完全相同:对于值类型T，没有值可以表示null或默认值(T)，这取决于上下文。在上面的例子中，If将Integer推断为Nothing和5的常见类型，因此，在这种情况下，Nothing意味着0。
这个让我很困惑(我很抱歉篇幅太长，但它是WinForm)。我之前在新闻组里发的。
I've come across an interesting bug. I have workarounds but i'd like to know the root of the problem. I've stripped it down into a short file and hope someone might have an idea about what's going on. It's a simple program that loads a control onto a form and binds "Foo" against a combobox ("SelectedItem") for it's "Bar" property and a datetimepicker ("Value") for it's "DateTime" property. The DateTimePicker.Visible value is set to false. Once it's loaded up, select the combobox and then attempt to deselect it by selecting the checkbox. This is rendered impossible by the combobox retaining the focus, you cannot even close the form, such is it's grasp on the focus. I have found three ways of fixing this problem. a) Remove the binding to Bar (a bit obvious) b) Remove the binding to DateTime c) Make the DateTimePicker visible !?! I'm currently running Win2k. And .NET 2.00, I think 1.1 has the same problem. Code is below.
using System;
using System.Collections;
using System.Windows.Forms;
namespace WindowsApplication6
{
public class Bar
{
public Bar()
{
}
}
public class Foo
{
private Bar m_Bar = new Bar();
private DateTime m_DateTime = DateTime.Now;
public Foo()
{
}
public Bar Bar
{
get
{
return m_Bar;
}
set
{
m_Bar = value;
}
}
public DateTime DateTime
{
get
{
return m_DateTime;
}
set
{
m_DateTime = value;
}
}
}
public class TestBugControl : UserControl
{
public TestBugControl()
{
InitializeComponent();
}
public void InitializeData(IList types)
{
this.cBoxType.DataSource = types;
}
public void BindFoo(Foo foo)
{
this.cBoxType.DataBindings.Add("SelectedItem", foo, "Bar");
this.dtStart.DataBindings.Add("Value", foo, "DateTime");
}
/// <summary>
/// Required designer variable.
/// </summary>
private System.ComponentModel.IContainer components = null;
/// <summary>
/// Clean up any resources being used.
/// </summary>
/// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
protected override void Dispose(bool disposing)
{
if (disposing && (components != null))
{
components.Dispose();
}
base.Dispose(disposing);
}
#region Component Designer generated code
/// <summary>
/// Required method for Designer support - do not modify
/// the contents of this method with the code editor.
/// </summary>
private void InitializeComponent()
{
this.checkBox1 = new System.Windows.Forms.CheckBox();
this.cBoxType = new System.Windows.Forms.ComboBox();
this.dtStart = new System.Windows.Forms.DateTimePicker();
this.SuspendLayout();
//
// checkBox1
//
this.checkBox1.AutoSize = true;
this.checkBox1.Location = new System.Drawing.Point(14, 5);
this.checkBox1.Name = "checkBox1";
this.checkBox1.Size = new System.Drawing.Size(97, 20);
this.checkBox1.TabIndex = 0;
this.checkBox1.Text = "checkBox1";
this.checkBox1.UseVisualStyleBackColor = true;
//
// cBoxType
//
this.cBoxType.FormattingEnabled = true;
this.cBoxType.Location = new System.Drawing.Point(117, 3);
this.cBoxType.Name = "cBoxType";
this.cBoxType.Size = new System.Drawing.Size(165, 24);
this.cBoxType.TabIndex = 1;
//
// dtStart
//
this.dtStart.Location = new System.Drawing.Point(117, 40);
this.dtStart.Name = "dtStart";
this.dtStart.Size = new System.Drawing.Size(165, 23);
this.dtStart.TabIndex = 2;
this.dtStart.Visible = false;
//
// TestBugControl
//
this.AutoScaleDimensions = new System.Drawing.SizeF(8F, 16F);
this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
this.Controls.Add(this.dtStart);
this.Controls.Add(this.cBoxType);
this.Controls.Add(this.checkBox1);
this.Font = new System.Drawing.Font("Verdana", 9.75F,
System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point,
((byte)(0)));
this.Margin = new System.Windows.Forms.Padding(4);
this.Name = "TestBugControl";
this.Size = new System.Drawing.Size(285, 66);
this.ResumeLayout(false);
this.PerformLayout();
}
#endregion
private System.Windows.Forms.CheckBox checkBox1;
private System.Windows.Forms.ComboBox cBoxType;
private System.Windows.Forms.DateTimePicker dtStart;
}
public class Form1 : Form
{
public Form1()
{
InitializeComponent();
this.Load += new EventHandler(Form1_Load);
}
void Form1_Load(object sender, EventArgs e)
{
InitializeControl();
}
public void InitializeControl()
{
TestBugControl control = new TestBugControl();
IList list = new ArrayList();
for (int i = 0; i < 10; i++)
{
list.Add(new Bar());
}
control.InitializeData(list);
control.BindFoo(new Foo());
this.Controls.Add(control);
}
/// <summary>
/// Required designer variable.
/// </summary>
private System.ComponentModel.IContainer components = null;
/// <summary>
/// Clean up any resources being used.
/// </summary>
/// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
protected override void Dispose(bool disposing)
{
if (disposing && (components != null))
{
components.Dispose();
}
base.Dispose(disposing);
}
#region Windows Form Designer generated code
/// <summary>
/// Required method for Designer support - do not modify
/// the contents of this method with the code editor.
/// </summary>
private void InitializeComponent()
{
this.components = new System.ComponentModel.Container();
this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
this.Text = "Form1";
}
#endregion
}
static class Program
{
/// <summary>
/// The main entry point for the application.
/// </summary>
[STAThread]
static void Main()
{
Application.EnableVisualStyles();
Application.SetCompatibleTextRenderingDefault(false);
Application.Run(new Form1());
}
}
}
如果你有扩展方法:
public static bool? ToBoolean(this string s)
{
bool result;
if (bool.TryParse(s, out result))
return result;
else
return null;
}
这段代码:
string nullStr = null;
var res = nullStr.ToBoolean();
这不会抛出异常，因为它是一个扩展方法(实际上是HelperClass.ToBoolean(null))，而不是一个实例方法。这可能会令人困惑。
