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。

下面将打印False而不是抛出溢出异常:

Console.WriteLine("{0}", yep(int.MaxValue ));


private bool yep( int val )
{
    return ( 0 < val * 2);
}

这是我无意中遇到的最奇怪的事情:

public class DummyObject
{
    public override string ToString()
    {
        return null;
    }
}

用途如下:

DummyObject obj = new DummyObject();
Console.WriteLine("The text: " + obj.GetType() + " is " + obj);

将抛出NullReferenceException。事实证明，c#编译器将多个添加编译为对String.Concat(object[])的调用。在。net 4之前，在Concat重载中有一个错误，其中对象被检查为null，而不是ToString()的结果:

object obj2 = args[i];
string text = (obj2 != null) ? obj2.ToString() : string.Empty;
// if obj2 is non-null, but obj2.ToString() returns null, then text==null
int length = text.Length;

这是ECMA-334§14.7.4的错误:

当一个或两个操作数都是字符串类型时，binary +操作符执行字符串连接。如果字符串连接的操作数为空，则替换为空字符串。否则，任何非字符串操作数都将通过调用继承自type object的虚拟ToString方法转换为其字符串表示形式。如果ToString返回null，则替换为空字符串。

即使枚举函数重载，它们也应该使0为整数。

我知道c#核心团队将0映射到enum的基本原理，但是，它仍然没有像它应该的那样正交。来自Npgsql的例子。

测试的例子:

namespace Craft
{
    enum Symbol { Alpha = 1, Beta = 2, Gamma = 3, Delta = 4 };


   class Mate
    {
        static void Main(string[] args)
        {

            JustTest(Symbol.Alpha); // enum
            JustTest(0); // why enum
            JustTest((int)0); // why still enum

            int i = 0;

            JustTest(Convert.ToInt32(0)); // have to use Convert.ToInt32 to convince the compiler to make the call site use the object version

            JustTest(i); // it's ok from down here and below
            JustTest(1);
            JustTest("string");
            JustTest(Guid.NewGuid());
            JustTest(new DataTable());

            Console.ReadLine();
        }

        static void JustTest(Symbol a)
        {
            Console.WriteLine("Enum");
        }

        static void JustTest(object o)
        {
            Console.WriteLine("Object");
        }
    }
}

This one's pretty hard to top. I ran into it while I was trying to build a RealProxy implementation that truly supports Begin/EndInvoke (thanks MS for making this impossible to do without horrible hacks). This example is basically a bug in the CLR, the unmanaged code path for BeginInvoke doesn't validate that the return message from RealProxy.PrivateInvoke (and my Invoke override) is returning an instance of an IAsyncResult. Once it's returned, the CLR gets incredibly confused and loses any idea of whats going on, as demonstrated by the tests at the bottom.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.Remoting.Proxies;
using System.Reflection;
using System.Runtime.Remoting.Messaging;

namespace BrokenProxy
{
    class NotAnIAsyncResult
    {
        public string SomeProperty { get; set; }
    }

    class BrokenProxy : RealProxy
    {
        private void HackFlags()
        {
            var flagsField = typeof(RealProxy).GetField("_flags", BindingFlags.NonPublic | BindingFlags.Instance);
            int val = (int)flagsField.GetValue(this);
            val |= 1; // 1 = RemotingProxy, check out System.Runtime.Remoting.Proxies.RealProxyFlags
            flagsField.SetValue(this, val);
        }

        public BrokenProxy(Type t)
            : base(t)
        {
            HackFlags();
        }

        public override IMessage Invoke(IMessage msg)
        {
            var naiar = new NotAnIAsyncResult();
            naiar.SomeProperty = "o noes";
            return new ReturnMessage(naiar, null, 0, null, (IMethodCallMessage)msg);
        }
    }

    interface IRandomInterface
    {
        int DoSomething();
    }

    class Program
    {
        static void Main(string[] args)
        {
            BrokenProxy bp = new BrokenProxy(typeof(IRandomInterface));
            var instance = (IRandomInterface)bp.GetTransparentProxy();
            Func<int> doSomethingDelegate = instance.DoSomething;
            IAsyncResult notAnIAsyncResult = doSomethingDelegate.BeginInvoke(null, null);

            var interfaces = notAnIAsyncResult.GetType().GetInterfaces();
            Console.WriteLine(!interfaces.Any() ? "No interfaces on notAnIAsyncResult" : "Interfaces");
            Console.WriteLine(notAnIAsyncResult is IAsyncResult); // Should be false, is it?!
            Console.WriteLine(((NotAnIAsyncResult)notAnIAsyncResult).SomeProperty);
            Console.WriteLine(((IAsyncResult)notAnIAsyncResult).IsCompleted); // No way this works.
        }
    }
}

输出:

No interfaces on notAnIAsyncResult
True
o noes

Unhandled Exception: System.EntryPointNotFoundException: Entry point was not found.
   at System.IAsyncResult.get_IsCompleted()
   at BrokenProxy.Program.Main(String[] args) 

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。