C# Riddles - Part I

A few tricky riddles and corner cases for wannabe C# language gurus:

// what gets printed / happens?
Console.WriteLine(null is object);

Console.WriteLine(null is IDisposable);

Console.WriteLine(null is int);

Console.WriteLine(null is int?);

Console.WriteLine(null is Func<int>);

Console.WriteLine(null is FileMode);

Console.WriteLine(0 is FileMode);

Console.WriteLine(0 is int?);

Console.WriteLine((((short)10)/((short)5)).GetType());

Console.WriteLine(new Nullable() is bool);

Console.WriteLine(new Nullable(false) is bool);

Console.WriteLine(new Nullable(false) is Nullable);

Console.WriteLine(new Nullable() is Nullable);

Console.WriteLine(null  + (int?)100 + "");

Console.WriteLine(null  + ""  + (int?)100);

Console.WriteLine(20 + 45 + "A");

Console.WriteLine("A" + 20 + 45);

Console.WriteLine(-int.MinValue);

string x = new string(new char[0]);
string y = new string(new char[0]);
Console.WriteLine(object.ReferenceEquals(x, y));

Console.WriteLine(default(int?));

Console.WriteLine(default(int?).GetHashCode());

Console.WriteLine(default(int?).GetType());

var ii = 1;
Console.WriteLine(ii += ii += ii += ii);

var ii = 0;
Console.WriteLine(ii = ii++);

Foo(this object obj) { return DateTime.Now.Seconds; }
...
Console.WriteLine(null.Foo());

int Rec(int i) { return (i < int.MaxValue) ? Rec(i + 1) : i; }
...
Console.WriteLine(Rec(0));

var array = new string[] { "foo", "bar", "cons" };
var objs = (object[])array;
objs[0] = 1000;
Console.WriteLine(objs[0]);

enum Foo { Bar, Baz }
...
Foo f = 0.0;
Console.WriteLine(f);

void Foo(object x) { Console.WriteLine("object"); }
void Foo(params T[] x) { Console.WriteLine("params T[]"); }
...
Foo("Hello");

struct Test
{
    static Test instance;
    static public Test Instance { get { return instance; } }
    
    public int field;
}
...
Test.Instance.field = 1;
Console.WriteLine(Text.Instance.field);

for(int ii = 0; ii < 10; ++ii)
  Console.WriteLine(new Random().Next(0, Int32.MaxValue));

var nums = new List<Func<int>>();
for (int ii=0; ii < 10; ii++)
    nums.Add(() => ii);
foreach (var num in nums)
    Console.WriteLine(num());

var nums = new List<Func<int>>();
foreach(var ii in Enumerable.Range(0,10))
    nums.Add(() => ii);
foreach (var num in nums)
    Console.WriteLine(num());

abstract class Base
{
    public virtual void foo(string s = "base") { Console.WriteLine("base " + s); }
}

class Derived : Base
{
    public override void foo(string s = "derived") { Console.WriteLine("derived " + s); }
}
...
Base b = new Derived();
b.foo();

throw null;

Exception GetException() { return new Exception(); }
...
throw GetException();

throw ((() => new Exception())());

throw ((() => { throw new Exception();} )());

// can your computer do arithmetic?

Console.WriteLine(1.0 + 1.0 == 2.0);

Console.WriteLine(0.01 + 0.01 == 0.02);

Console.WriteLine(0.00000037 + 0.0000001 == 0.00000047);

Console.WriteLine(0.9999999+0.000001 == 1.0);

Console.WriteLine(1.000001-0.000001 ==1.0);

/* assume InvariantCulture */
Console.WriteLine(4.170404 == Convert.ToDouble("4.170404"));

Console.WriteLine(Convert.ToInt32(13.6) == (int)13.6);

// does this compile?

const string foo = "aaa" + "bbb";
const string bar = "aaa" + 123;
const string frob = null;
const string squawk = (string)'\0';

// riddles
When can execution leave a try { } finally { } without executing the finally code?
How can you lock across AppDomains?
How can a string.Compare() call cause an AssemblyLoadException?
How can you create an instance of System.Void?

Sources:
- my own experience
- http://dotnetgeek.tumblr.com/post/11303229042/c-nullable-gotcha
- http://stackoverflow.com/questions/194484/whats-the-strangest-corner-case-youve-seen-in-c-sharp-or-net
- http://www.yoda.arachsys.com/csharp/teasers.html
- http://ahuwanya.net/blog/post/C-Brainteasers-Part-I.aspx
- http://stackoverflow.com/questions/241134/what-is-the-worst-gotcha-in-c-sharp-or-net