Puzles C#

#dotNETSpain2015

Leo Antoli @lantoliPuzzles C#

.NET Conference 2015

Y

AX B

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Equals & ==

string s1 = "hello";

string s2 = "HELLO".ToLower();

object o1 = s1;

object o2 = s2;

1) s1 == s2 2) o1 == o2 3) s1 == o2

4) s1.Equals(s2) 5) s1.Equals(o2) 6) o1.Equals(s2)

A) All true

B) All == false, All Equals true

C) 1 & 4 true, else false

D) Other

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Equals & ==

D) Other

1) s1 == s2 true

2) o1 == o2 false

3) s1 == o2 false

4) s1.Equals(s2) true

5) s1.Equals(o2) true

6) o1.Equals(s2) true

Equals is overridden (runtime)

Static methods can only be overloaded (compile time)

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Equals semantics

double n = double.NaN;

Console.WriteLine(n == n);

Console.WriteLine(n.Equals(n));

A) False False

B) False True

C) True False

D) True True

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Equals semantics

B) False True

== semantics can be less strict, and it’s normally used for referential

equality.

Equals semantics is expected to be an equivalence relationship:

- Reflexive (a equals a)

- Commutative (a equals b implies b equals a)

- Transitive (a equals b and b equals c implies a equals c)

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Enumerable equals

IEnumerable<string> list1 = new List<String> {"a", "b"};

IEnumerable<string> list2 = new List<String> { "b", "a" };

IEnumerable<string> list3 = new List<String> { "a", "b" };

Console.WriteLine(list1.Equals(list2));

Console.WriteLine(list1.SequenceEqual(list2));

Console.WriteLine(list1.Equals(list3));

Console.WriteLine(list1.SequenceEqual(list3));

A) False False False True

B) False False True True

C) True False True True

D) Other

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Enumerable equals

A) False False False True

Equals in lists is not overridden, so it only compares whether list

references are equals.

SequenceEqual check the enumerables have the same count and the

same elements in the same order.

If order is not important, you can sort the lists, create sets and use

SetEquals (if duplicates are not important), etc.

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Inheritance override

interface Inter { void Message(); }

class A : Inter {

public void Message() { Console.WriteLine("A"); }}

class B : A {

public new void Message() { Console.WriteLine("B"); }}

B b = new B();

((Inter)b).Message();

((A)b).Message();

A) Displays A A

B) Displays A B

C) Displays B A

D) Displays B B

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Inheritance override

A) Displays A A

A.Message is sealed (default modifier)

B.Message is a new method (no override)

Inter.Message is implemented by A.Message because B does not

explicitly implements Inter

Avoid "new" to redefine methods.

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Overload choice 1

class A { public void Print(int n) {

Console.WriteLine("A"); }

class B : A { public void Print(double n) {

Console.WriteLine("B"); }

...

var b = new B();

b.Print(10);

A) Displays A

B) Displays B

C) Compilation error

D) Other

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Overload choice 1

B) Displays B

First methods in the current class are tried to be called. As an implicit conversion from int to double

exists, it is called.

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Overload choice 2

void Hello(double a) { Console.WriteLine("double"); }

void Hello<T>(T a) { Console.WriteLine("T"); }

...

Hello(10);

A) Displays double

B) Displays T


D) Other

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Overload choice 2

B) Displays T

Overload with existing methods in the class is tried before implicit casting.

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Enum

enum Quarter { Q1 = 1, Q2 = 2, Q3 = 3, Q4 = 4 };

class Invoice {

public Quarter CurrentQuarter { get; set; }

}

Invoice invoice = new Invoice();

Console.WriteLine(invoice.CurrentQuarter);

A) Displays Q1

B) Displays 0

C) Runtime exception

D) Other

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Enum

B) Displays 0

Value types are always initialised to 0

Provide always a 0 enum element.

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Assignment compatibility

uint[] foo = new uint[10];

object bar = foo;

foo is uint[] // True

foo is int[] // ???

bar is uint[] // True

bar is int[] // ???

A) False False

B) False True

C) True False

D) True True

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Assignment compatibility

B) False True

The CLI has the concept of "assignment compatibility"... The assignment from source value to

target variable has to be "representation preserving".

One of the rules of the CLI is "if X is assignment compatible with Y then X[] is assignment

compatible with Y[]".

That is not a rule of C#. C#'s array covariance rule is "if X is a reference type implicitly

convertible to reference type Y (via a reference or identity conversion) then X[] is implicitly

convertible to Y[]". That is a subtly different rule!

http://blogs.msdn.com/b/ericlippert/archive/2009/09/24/why-is-covariance-of-value-typed-

arrays-inconsistent.aspx

http://blogs.msdn.com/ericlippert/archive/2009/03/19/representation-and-identity.aspx

http://blogs.msdn.com/b/ericlippert/archive/2009/09/24/why-is-covariance-of-value-typed-arrays-inconsistent.aspx

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Unboxing

short s = 123;

object o = s;

int i = (int)o;

A) i = 123

B) Compilation error


D) Other

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int i = (int)(short)o is OK

InvalidCastException. A boxed T can only be unboxed to Thttp://blogs.msdn.com/b/ericlippert/archive/2009/03/19/representation-and-identity.aspx

Unboxing

http://blogs.msdn.com/b/ericlippert/archive/2009/03/19/representation-and-identity.aspx

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Tryparse

int res;

bool success =

int.TryParse("1", (NumberStyles)1111, null, out res);

Console.WriteLine(success);

Console.WriteLine(res);

A) Displays True 1

B) Displays False 0


D) Other

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Tryparse


ArgumentException: An undefined NumberStyles value is being

used.

Tryparse doesn't throw FormatException,

ArgumentNullException or OverflowException, but it

can throw ArgumentException.

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Checked / Unchecked

int Square(int n) { return n*n; }

...

int a = 1000000;

checked {

Console.WriteLine(Square(a));

}

A) OverflowException

B) Displays a positive number

C) Displays a negative number

D) Other

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Checked / Unchecked for methods

C) Displays a negative number

The effect is in the code used to emit IL instructions for integer add / subtract / multiply / data conversions inside the block.

Use settings to enable "checked" by default

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Using

using System;

using System.Collections.Generic;

using MyStringList = System.Collections.Generic.List<string>;

var a = new List<String>();

var b = new MyStringList();

Console.WriteLine(a.GetType() == b.GetType());

A) Displays True

B) Displays False


D) Other

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Using

A) Displays True

Using is an alias. In CLR using or namespaces don't

exist. Full-qualified names are always used in CLR.

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Basic numbers

Int32 int1 = 10;

Int64 long1 = 10;

int int2 = 10;

long long2 = 10;

Console.WriteLine(int2.GetType());

Console.WriteLine((int1 is Int32) && (long1 is Int64));

Console.WriteLine((int1 is Int64) || (long1 is Int32));

A) Displays int True False

B) Displays System.Int32 True False

C) Displays System.Int32 True True

D) Other

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Basic numbers

B) Displays System.Int32 True False

int is an alias for System.Int32, long for System.Int64

Int32 and Int64 are not inheritance-related

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Internal classes

namespace MyNamespace {

class A {

private int a;

class Internal {

void DoSomething(A arg) {

arg.a = 10;

} } }

class B {

void DoSomething(A arg) {

arg.a = 10;

} } }

A) Compiles OK

B) Compilation error in Internal

C) Compilation error in B

D) Both compilation errors

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Internal classes

C) Compilation error in B

Internal classes have access to private members

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Interface member modifiers

interface IExample

{

public void Do1();

internal void Do2();

}

A) Compiles OK

B) Compilation error in Do1

C) Compilation error in Do2


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Interface member modifiers


All interface members are public. Modifiers can

not be used.

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Overriding member modifiers

class A { protected virtual void Do1() {} }

class B : A { public override void Do1() { } }

class C : A { private override void Do1() { } }

A) Compiles OK

B) Compilation error in B

C) Compilation error in C


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Overriding member modifiers


The original member and the overriding

member must have the same accesiblity.

CLR allows to increase visibility but not C#

compiler.

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Virtual methods in ctor 1

class A {

public A() { Setup(); }

protected virtual void Setup() { }

}

class B : A {

private int i;

public B() { i = 5; }

protected override void Setup() {

Console.WriteLine(i);

} } ...

new B()

A) Displays 0

B) Displays 5

C) Runtime error

D) Other

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A) Displays 0

Base constructors are called first.

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class A {

public A() { Setup(); }

protected virtual void Setup() { }

}

class B : A {

private int i = 5;

public B() { }

protected override void Setup() {


} } ...

new B()

A) Displays 0

B) Displays 5

C) Runtime error

D) Other

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B) Displays 5

Inline initializations are done before calling base

constructors.

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Partial classes

partial class A {

public void DoSomething() {

Console.Write("BEGIN ");

PartialMethod();

Console.Write("END");

}

partial void PartialMethod();

}}

new A().DoSomething();

A) Compilation error

B) Displays BEGIN and then

runtime error

C) Displays BEGIN END

D) Other

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Partial classes

C) Displays BEGIN END

Partial methods are not required to be implemented, in that

case it’s as if they were empty.

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Type Constructors

class A {

static A() { throw new Exception("Error"); } }

class B : A {

static B() { Console.WriteLine("STATIC CTOR B"); } }

...

new StaticB();

A) Displays STATIC CTOR B

B) Displays STATIC CTOR B and then runtime error

C) Runtime error, nothing is displayed

D) Other

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Type Constructors

B) Displays STATIC CTOR B and then runtime error

Type ctors are called before type is used so “new B()” makes B type

ctor to be called.

Derived constructors call base ctor (explicitly or implicitly). So when B

ctor calls A ctor, first A type ctor is executed.

When a type ctor throws an exception that type remains unusable.

A type ctor is not called if not used.

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Param execution order

public class A {

public void Do(int x, int y) {}

public static int Display(int n) { Console.WriteLine(n);

return n; }}…

var a = new A();

a.Do(y: A.Display(2), x: A.Display(1));

A) Displays 1 2

B) Displays 2 1


D) Other

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Param execution order

B) Displays 2 1

Parameters are evaluated from left to right, although the call is

always done by position when IL is emitted.

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Var params

public static void Do(int a, params string[] strs) {

Console.WriteLine(strs.Length); }...

Do(10); Do(10, "hello"); Do(10, "hello", "there");

Do(10, new []{"hello", "there"});

Do(10, (string)null);

Do(10, null);

A) Compilation error

B) Runtime error

C) Displays 0 1 2 2 1 1

D) Other

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Var params

B) Runtime error

Displays 0 1 2 2 1 1 and then

NullReferenceException in: Do(10, null)

Variable params can be null.

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Generics inference 1

public List<T> InitList<T>(T initValue) {

return new List<T> { initValue };

} ...

IList<string> list1 = new List<string> {"hello"};

IList<string> list2 = new List<> { "hello" };

IList<string> list3 = InitList("hello");

A) Compilation error only in list1

B) Compilation error only in list2

C) Compilation error only in list3

D) Other

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Inference only works in generic methods.

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public List<T> CreateList<T>() {

return new List<T>();

}…

IList<string> list1 = new List<string>();

IList<string> list2 = new List<>();

IList<string> list3 = CreateList();

A) Compilation error only in list1


C) Compilation error only in list3

D) Other

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D) Other

Compilation error in list2 and list3

Inference only works in generic methods, but only for input params.

Example to choose method:

IList<string> list3 = CreateList<string>();

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Dynamic

void Sum(int a, int b) { Console.WriteLine("{0} and {1} is {2}", a, b, a+b); }

void SumDyn(dynamic a, dynamic b) { Console.WriteLine("{0} and {1} is {2}", a, b,

a+b); }…

dynamic a1 = 10, b1 = 20; Sum(a1, b1);

dynamic a2 = 10.2, b2 = 20.3; Sum(a2, b2);

int a3 = 11, b3 = 22; SumDyn(a3, b3);

double a4 = 11.2, b4 = 22.5; SumDyn(a4, b4);

A) Compilation or runtime error in a1,b1

B) Compilation or runtime error in a2,b2

C) Compilation or runtime error in a3,b3

D) Compilation or runtime error in a4,b4

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Dynamic

B) Compilation or runtime error in a2,b2

RuntimeBinderException

The best overloaded methods has invalid arguments.

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Explicit interfaces

void PrintBool1(IConvertible obj) {

Console.WriteLine(obj.ToBoolean(null)); }

void PrintBool2(int obj) {

Console.WriteLine(obj.ToBoolean(null)); }...

int a = -3; PrintBool1(a); PrintBool2(a);

A) Displays True True

B) Displays False False

C) Runtime error

D) Other

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Explicit interfaces

D) Other

Compilation error in PrintBool2: int does not contain ToBoolean

IConvertible is implemented in basic numerical types as EIMI (Explicit

Interface Method Implementation)

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LINQ 1

int[] list = {0, 1, 2};

var res = list.Where(n => n%2 == 0).

Select(n => n*2).

Any(n => n >= 8);

list[0] = 4;

Console.WriteLine(res);

A) Displays True

B) Displays False

C) Runtime error

D) Other

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LINQ 1

B) Displays False

LINQ queries can be local or interpreted.

Local queries are lazy evaluated except:

- Conversions: ToList, ToArray, etc.

- Elements: First, FirstOrDefault, Last, etc.

- Aggregations: Sum, Min, Max

- Quantifiers: Any, All, Contains, etc.

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LINQ 2

var list = new int[] {};

Console.WriteLine( list.All(x => x > 10) );

Console.WriteLine( ! list.Any(x => x > 10) );

A) False False

B) False True

C) True False

D) True True

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LINQ 2

D) True True

If you don't have any test in you app, all your tests pass because you

don’t have any failing test ;-)

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Delegates 1

Predicate<string> containsHello = s => s.Contains("hello");

Console.WriteLine(containsHello is Predicate<string>);

Console.WriteLine(containsHello is object);

Console.WriteLine(containsHello is MulticastDelegate);

Console.WriteLine(containsHello is Delegate);

Console.WriteLine(containsHello(“hello world”);

Console.WriteLine(containsHello.Invoke(“hello world”);

A) Displays all True

B) Displays at least one False

C) Runtime error

D) Compilation error

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Delegates 1

A) Displays all True

Delegates are “syntactic sugar” to create a

MulticastDelegate class.

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Delegates 2

class Vehicle, class Terrestrial : Vehicle ,

class Car : Terrestrial…

Predicate<Terrestrial> predT = t => t.numWheels() == 2;

Predicate<Car> predCar = predT;

Predicate<Vehicle> predVehicle = predT;

A) predCar and predVehicle are OK

B) Compilation error in predCar and predVehicle

C) Compilation error only in predVehicle

D) Compilation error only in predCar

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Delegates 2

C) Compilation error only in predVehicle

See next puzzle...

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Delegates 3

class Vehicle, class Terrestrial : Vehicle ,

class Car : Terrestrial…

Func<Terrestrial> fnT = () => new Terrestrial();

Func<Car> fnCar = fnT;

Func<Vehicle> fnVechicle = fnT;

A) fnCar and fnVehicle are OK

B) Compilation error in fnCar and fnVehicle

C) Compilation error only in fnVehicle

D) Compilation error only in fnCar

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Delegates 3

D) Compilation error only in fnCar

Delegates can be covariant in generic type params if they’re only in

output positions and contravariant if they’re only in input positions.

Covariant (out) - a base class can be used

Contravariant (in) - a derived class can be used

public delegate bool Predicate<in T>(T obj)

public delegate TResult Func<out TResult>()

public delegate TResult Func<in T, out TResult>(T arg)

public delegate void Action<in T>(T obj)

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Delegates 4

Predicate<string> pred = null;

Predicate<string> predH = s => s.StartsWith("h");

pred += predH;

pred += s => s.StartsWith("a");

pred += predH;

Console.WriteLine(pred("hello"));

A) Displays True

B) Displays False


D) Runtime error

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Delegates 4

A) Displays True

Delegates can have multiple functions, all are executed in order and

the result of the last function is returned.

Can be duplicated functions, they’re all still executed.

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Delegates 5

Action<string> action = null;

action += s => { Console.WriteLine("first"); };

action += s => { throw new Exception("error"); };

action += s => { Console.WriteLine("last"); };

action("test");

A) Displays “first” and “last”

B) Displays “first” and throws exception

C) Nothing displayed and exception thrown

D) Other

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Delegates 5

B) Displays “first” and throws exception

When an exception occurs in a delegate’s function the execution is

stopped, no more functions are executed.

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Lambdas and closure

Func<int, int> fn = null;

for (int i = 0; i < 2; i++) {

fn += n => n + i;

}

Console.WriteLine(fn(5));

A) Displays 18

B) Displays 6

C) Displays 5

D) Other

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Lambdas and closure

D) Other

Displays 7

External variables are captured and used when the delegate

is executed.

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Interface variance

class Vehicle {... } class Car : Vehicle {...}

private static void Do1(IList<Vehicle> list) { … }

private static void Do2(IEnumerable<Vehicle> list) { … }…

var cars = new List<Car> { new Car(), new Car()};

Do1(cars);

Do2(cars);

A) Both are OK

B) Compilation error calling Do1

C) Compilation error calling Do2

D) Compilation error calling Do1 and Do2

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Interface variance

B) Compilation error calling Do1

Interfaces can be variant. An enumerable of cars is an

enumerable of vehicles but a list of cars is not a list of

vehicles.

Covariant: IEnumerable<out T>, IEnumerator<out T>,

IQueryable<out T>, IGrouping<out TKey, out TElement>, etc.

Contravariant: IComparer<in T>, IEqualityComparer<in T>,

IComparable<in T>, etc.

http://msdn.microsoft.com/en-us/library/9eekhta0(VS.100).aspx

http://msdn.microsoft.com/en-us/library/78dfe2yb(VS.100).aspx

http://msdn.microsoft.com/en-us/library/bb351562(VS.100).aspx

http://msdn.microsoft.com/en-us/library/bb344977(VS.100).aspx

http://msdn.microsoft.com/en-us/library/8ehhxeaf(VS.100).aspx

http://msdn.microsoft.com/en-us/library/ms132151(VS.100).aspx

http://msdn.microsoft.com/en-us/library/4d7sx9hd(VS.100).aspx

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Ref parameters

class A { } class B : A {}

class Uti {

public static void DoRefA(ref A a) {}

public static void DoRefB(ref B b) { }}

...

var b = new B(); Uti.DoRefA(ref b); Uti.DoRefB(ref b);

A) Both are OK

B) Compilation error in both

C) Compilation error in Uti.DoRefA call

D) Compilation error in Uti.DoRefB call

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Ref parameters

C) Compilation error in Uti.DoRefA call

Ref and out params must match exactly the type.

B1 b1 = new B1();

Uti.ProblemRefA(ref b1); // doesn’t compile, that’s good

...

class A { } class B1 : A {} class B2 : A {}

public static void ProblemRefA(ref A a) {

a = new B2(); // oops

}

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Exception 1

try { }

finally {

Console.WriteLine("start");

throw new Exception("error");

Console.WriteLine("end");

}

A) Displays “start” and “end”, no exceptions thrown

B) Displays “start”, no exceptions thrown

C) Displays “start” and throws exception

D) Other

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Exception 1

C) Displays “start” and throws exception

Exceptions can be thrown in a finally block.

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Exception 2

public void A() { try { B(); }

finally { Console.WriteLine("A finally"); } }

public void B() {

try { throw new Exception("error"); }

catch (Exception) { Console.WriteLine("B catch"); }

finally { Console.WriteLine("B finally"); } }

A) Displays “B catch”, “A finally”, “B finally”

B) Displays “B catch”, “B finally”, “A finally”

C) Displays “A finally”, “B catch”, “B finally”

D) Other

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Exception 2

B) Displays “B catch”, “B finally”, “A finally”

Chained finally blocks are called in reverse order.

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Struct default constructor

struct MyStruct {

MyStruct(int field) { _field = field; }

int _field;

public int Field { get { return _field; } }

}

MyStruct my = new MyStruct();

Console.WriteLine(my.Field);

A) Displays 0

B) Compilation error: Cannot access constructor


D) Other

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Struct default constructor

A) Displays 0

Value types always have a default constructor.

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Redefining struct default constructor

struct MyStruct {

MyStruct() { _field = 5; }

int _field;

public int Field { get { return _field; } }

}

MyStruct my = new MyStruct();

Console.WriteLine(my.Field);

A) Displays 0


C) Displays 5

D) Other

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Redefining struct default constructor


Structs cannot contain explicit parameterless constructors

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Value type boxing

struct Point : IPoint {

public int X { get; set; }

public void ChangeX(int x) { X = x; }

public override string ToString()

{ return string.Format("X: {0}", X); }

}

interface IPoint

{

void ChangeX(int x);

}

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Value type boxing 1

Point p = new Point {X = 1};

object o = p;

((Point) o).ChangeX(2);

Point po = (Point) o;

Console.WriteLine(p);

Console.WriteLine(po);

A) Displays X: 1 X: 1

B) Displays X: 1 X: 2

C) Displays X: 2 X: 1

D) Displays X: 2 X: 2






}

interface IPoint

{


}

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Value type boxing 1

A) Displays X: 1 X: 1

Point p = new Point {X = 1};

object o = p;

((Point) o).ChangeX(2);

Point po = (Point) o;


Console.WriteLine(po);

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Value type boxing 2

Point p = new Point { X = 1 };

IPoint i = p;

object o = i;

i.ChangeX(2);


Console.WriteLine(o);


A) Displays X: 1 X: 1 X: 1

B) Displays X: 1 X: 2 X: 1

C) Displays X: 2 X: 1 X: 1

D) Displays X: 2 X: 2 X: 1






}

interface IPoint

{


}

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Value type boxing 2

D) Displays X: 2 X: 2 X: 1

Point p = new Point { X = 1 };

IPoint i = p;

object o = i;

i.ChangeX(2);


Console.WriteLine(o);


Boxing and Unboxing are only done when

necessary. An interface is the only way to modify

a boxed value type.

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Numerical castings

double dbl = 1.2; decimal dec = 1.2m; long lon = 1;

double lonToDbl = lon;

double decToDbl = dec;

decimal dblToDec = dbl;

A) 3 compilation errors: All conversions must be explicit

B) lonToDbl and decToDbl are OK, dblToDec must be explicit

C) All conversions are OK

D) Other

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Numerical castings

D) Other

lonToDbl is OK, decToDbl and dblToDec must be explicit.

C# already allows an implicit conversion from long to double, which can lose up to

twelve bits of precision.

The easy answer is: there cannot be an implicit conversion from double to decimal

because of the range discrepancy.

The conversion from decimal to double would lose far more precision; going from 96

bit precision to 52 bit precision seems like too large a drop to make this implicit.

http://ericlippert.com/2013/07/18/why-not-allow-doubledecimal-implicit-conversions/

http://ericlippert.com/2013/07/18/why-not-allow-doubledecimal-implicit-conversions/

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Garbage collector

private void SetTimer() {

TimerCallback callback = state => {

Console.WriteLine("called"); };

var t = new Timer(callback, null, 0, 1000); }…

SetTimer();

var m = GC.GetTotalMemory(true);

Thread.Sleep(5000);

A) Displays “called” 0 times

B) Displays “called” 1 time

C) Displays “called” more than once

D) Other

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Garbage collector

B) Displays “called” 1 time

Second param to Timer constructor is to start immediately.

When garbage collector is triggered, timer is collected so no more

events are triggered.

(Param true in GC.GetTotalMemory means to collect garbage before getting

total memory value)

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Opposite terms

Can a method be both internal and extern ?

Can a method be both dynamic and static ?

A) False False

B) False True

C) True False

D) True True

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Opposite terms

D) True True

[DllImport("msvcrt.dll")]

internal static extern int puts(string c);

static dynamic Hello() { return “hello”; }

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Leo Antoli @lantoliPuzzles C#

¡¡¡Si te ha gustado no olvides rellenar la encuesta!!!Thanks

Y

AX B

Puzles C#

Technology

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