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1 1 JSL Uncommon C# Authoring Consulting Crafting Designing Mentoring Training Jon Jagger JSL phone +44 (0) mobile +44 (0) C#

2 2 JSL Definite Assignment 12.3 Definite Assignment A variable shall be definitely assigned at each location where its value is obtained. The occurrence of a variable in an expression is considered to obtain the value of the variable, except when: The variable is the left operand of a simple assignment. The variable is passed as an output parameter. The variable is a struct type variable and occurs as the left operand of a member access. static void Main() int value; unsafe int * ptr = &value; Console.WriteLine(value);

3 3 JSL 2 Conformance The text in this International Standard that specifies requirements is considered normative. Normative text is further broken down into required and conditional categories. Conditionally normative text specifies a feature and its requirements where the feature is optional. Conformance 7 General Description all of clause 27 with the exception of the beginning is conditionally normative; 27 Unsafe code An implementation that does not support unsafe code is required to diagnose any usage of the unsafe keyword. The remainder of this clause, including all of its subclauses, is conditionally normative.

4 4 JSL 27.2 Pointer Types In an unsafe context several constructs are available for operating on pointers: Spot the bug The unary * operator can be used to perform pointer indirection (27.5.1) The -> operator can be used to access a member of a struct through a pointer (27.5.2) The [] operator can be used to index a pointer (27.5.3) The ==,!=, <, >, <=, and => operators can be used to compare pointers (27.5.7) The stackalloc operator can be used to allocate memory from the call stack (27.7) The fixed statement can be used to temporarily fix a variable so its address can be obtained (27.6)

5 5 JSL Protected access for instance members Protected access public public class class A protected protected int int x; x; static static void void F(A F(A a, a, B b) b) a.x a.x = 1; 1; // // Ok Ok b.x b.x = 1; 1; // // Ok Ok public public class class B : A static static void void F(A F(A a, a, B b) b) a.x a.x = 1; 1; // // Error Error b.x b.x = 1; 1; // // Ok Ok

6 6 JSL namespace pitfall Namespace lookup is inside outside Widget not in Company.Other.Framework.Tests Widget not in Company.Other.Framework Widget not in Company.Other Widget IS in Company namespace namespace Company.Widget.Framework public public class class Widget Widget using using Company.Widget.Framework; namespace namespace Company.Other.Framework.Tests [TestFixture] [TestFixture] public public class class FubarTests FubarTests [Test] [Test] public public void void SomeTest() SomeTest() Widget Widget w = new new Widget(); Widget();......

7 7 JSL Guidelines avoid namespace-class class name clashes prefer a namespace prefix style namespace namespace Company.WidgetFramework public public class class Widget Widget Solution using using Company.WidgetFramework; namespace namespace Tests.Company.OtherFramework [TestFixture] [TestFixture] public public class class FubarTests FubarTests [Test] [Test] public public void void SomeTest() SomeTest() Widget Widget w = new new Widget(); Widget();......

8 8 JSL Default top-level class access is internal visible only inside the assembly Default access using using NUnit.Framework; namespace Company.WidgetLibTests [TestFixture] class class WidgetTests [Test] public void void SomeTest() //... //... Question: Does the test pass or fail?

9 9 JSL Make sure test classes are public Otherwise NUnit won't see them! Default pitfall using using NUnit.Framework; namespace Company.WidgetLibTests [TestFixture] public class class WidgetTests [Test] public void void SomeTest() //... //... Question: What else should you do?

10 10 JSL Check if [TestFixture]'d[ classes are internal Testing the tests [TestFixture] [TestFixture] public public class class InternalFixtureTests InternalFixtureTests [Test] [Test] public public void void AccidentalNonPublicTestFixture() AccidentalNonPublicTestFixture() Type Type tfa tfa = = typeof(testfixtureattribute); typeof(testfixtureattribute); Assembly Assembly self self = = this.gettype().module.assembly; this.gettype().module.assembly; foreach foreach (Type (Type type type in in self.gettypes()) self.gettypes()) object[] object[] attributes attributes = = type.getcustomattributes(tfa, type.getcustomattributes(tfa, false); false); if if (attributes (attributes!=!= null null && && attributes.length attributes.length > > 0 0 && && type.isnotpublic) type.isnotpublic) Assert.Fail(type.ToString() Assert.Fail(type.ToString() + + " " is is not not public!"); public!");

11 11 JSL What's this? Generic ambiguity F(G<A, B>(7)); A call to F with two arguments: G < A B > (7) viz redundant parentheses A call to F with one argument: G < A,B > (7) Two type parameters: A,B One regular argument: 7

12 12 JSL Operators and punctuators right-shift: > > >>== tokenization right-shift-assignment: > >= delegate void void D(); D(); class class C static void void F<T>() static D d = F<F<int>>; static void void Main() bool bool b = F<F<int>>==d; Compile-time error

13 13 JSL String literals When two or more string literals that are equivalent according to the string equality operator, appear in the same assembly, these string literals refer to the same string instance. String Interning Unfortunate example of over-specification Implementation!= specification Serves no useful purpose for C# users String reference equality does not mean strings are necessarily in the same assembly

14 14 JSL Hex Escape Sequence Character Literals A hex-escape escape-sequence sequence character literal has 1, 2, 3, or 4 hex-digits hexadecimal-escape-sequence: \x hex-digit hex-digit? hex-digit? hex-digit? s1 s1 = "\x7scotland"; // // BEL Scotland s2 s2 = "\x7england"; // // ~ ngland Advice: use unicode-escapes escapes

15 15 JSL are inadvisable in some places using using Integer = int; int; // // compile-time error error using using Integer = System.Int32; // // ok ok keywords Type Type ti ti = Type.GetType("int"); // // runtime failure Type Type ti ti = Type.GetType("System.Int32"); // // ok ok Type Type ti ti = typeof(int); // // better extern unsafe static void void Process(int length, S * array); // // ok ok extern unsafe static void void Process(Int32 length, S * array); // // better

16 16 JSL Properties are not variables public public struct struct Point Point private private int int x, x, y; y; public public int int X get get set set public public struct struct Rectangle Rectangle private private Point Point topleft; topleft; public public Point Point TopLeft TopLeft get get set set Properties class class App App static static void void Main() Main() Rectangle Rectangle r = new new Rectangle(); Rectangle(); r.topleft.x r.topleft.x = 42; 42; // // compile-time compile-time error error

17 17 JSL Calling a method on a readonly struct? The method is called on a copy of the struct! readonly + struct struct struct Mile Mile public public void void Add(Mile Add(Mile rhs) rhs) value value += += rhs.value; rhs.value; private private int int value; value; class class Trap Trap public public void void Ouch(Mile Ouch(Mile more) more) distance.add(more); private private readonly readonly Mile Mile distance; distance;

18 18 JSL Solution Make struct immutable rely on assignment readonly struct struct struct Mile Mile public public Mile(int Mile(int value) value) this.value this.value = value; value; public public static static Mile Mile operator+ operator+ Add(Mile Add(Mile lhs, lhs, Mile Mile rhs) rhs) return return new new Mile(lhs.value Mile(lhs.value + rhs.value); rhs.value); private private readonly readonly int int value; value; class class Trap Trap public public void void Ooops(Mile Ooops(Mile more) more) distance distance += += more; more; // // compile-time compile-time error error private private readonly readonly Mile Mile distance; distance;

19 19 JSL Checked pitfall public public class class Eg Eg public public void void Ok(int Ok(int x) x) x *= *= 2; 2; public public class class Eg Eg public public void void AlsoOk(int AlsoOk(int x) x) checked checked x *= *= 2; 2; public public class class Eg Eg public public void void NotOk(int NotOk(int x) x) checked(x checked(x *= *= 2); 2); public public class class Eg Eg public public void void OkAgain(int OkAgain(int x) x) x = checked(x checked(x * 2); 2);

20 20 JSL Checking noses The checked and unchecked statements The checked statement causes all expressions in the block to be evaluated in a checked context, and the unchecked statement causes all expressions in the block to be evaluated in an unchecked context. The checked and unchecked statements are precisely equivalent to the checked and unchecked operators ( ) except that they operate on blocks instead of expressions. checked checked int int value; value; value value = unchecked(checked(f() * G()) G()) + 42); 42);

21 21 JSL Constant expressions Must be evaluated at compile time And, by default, are checked public class class Eg Eg public virtual void void Method() const const int int factor = 0; 0; int int count; if if (factor!=!= 0) 0) count count *= *= 5 / factor;......

22 22 JSL Invariant meaning Invariant meaning in blocks For each occurrence of a given identifier as a simple-name in an expression or declarator, every other occurrence of the same identifier as a simple-name in an expression or declarator within the immediately enclosing block or switch-block shall refer to the same entity. class class Example void void F() F() if if (true) int int v = 42; 42; int int v = 1; 1;

23 23 JSL In retrospect a poor choice Default == unsealed public /*unsealed*/ class class Vulnerable ~Vulnerable() public /*unsealed*/ class class Vulnerable protected override void void Finalize() public class class Attacker : Vulnerable ~Attacker() for(;;);

24 24 JSL Guidelines Don't rely on defaults Make type and member access explicit Classes: static, sealed, abstract, or /*unsealed*/ Solutions public /*unsealed*/ class class DoesntCompile protected sealed override ~Vulnerable() public sealed class class Best Best ~Best()......

25 25 JSL virtual first implementation virtual: C#!= C++ public abstract class class Middle public virtual void void Foo() Foo() public sealed class class Bottom : Middle public virtual void void Foo() Foo() >Warning: Bottom.Foo() hides inherited member Middle.Foo()

26 26 JSL override another implementation public abstract class class Middle public virtual void void Foo() Foo() Solution public sealed class class Bottom : Middle public override void void Foo() Foo()......

27 27 JSL ref/out overloading too easy to forget the ref/out Overload pitfall - 1 public sealed class class Dodgy Dodgy public void void Foo(Wibble value) public void void Foo(ref Wibble value) public void void Bar(Wibble value) public void void Bar(out Wibble value)......

28 28 JSL Better conversion pitfall Overload pitfall - 2 public sealed class class A public static implicit operator B(A B(A from) from) public sealed class class B public sealed class class App App static void void Method(A a, a, B b) b) static void void Method(B b, b, A a) a) static void void Main() A a = new new A(); A(); Method(a, null); // // ambiguous

29 29 JSL Overload pitfall Method invocations The set of candidate methods for the method invocation is constructed. For each method F associated with the method group M: The set of candidate methods is reduced to contain only methods from the most derived types: public class class Base Base public virtual void void Method(int value) public class class Derived : Base Base public void void Method(double value) public override void void Method(int value) public class class Demo Demo public static void void Main() Derived d = new new Derived(); d.method(42);

30 30 JSL Guidelines Don't mix overloading and overriding Don't overload solely on ref/out Reference conversions inheritance Overloading does not happen in CIL Overload

31 31 JSL A class is implicitly convertible to an interface Only if it actually realizes the interface Conversion Oddity interface interface IWibble IWibble class class Alpha Alpha : IWibble IWibble class class Beta Beta public public static static implicit implicit operator operator Alpha(Beta Alpha(Beta from) from) return return new new Beta(); Beta(); class class App App static static void void Main() Main() Beta Beta b = new new Beta(); Beta(); IWibble IWibble iw iw = (IWibble)b; (IWibble)b; // // cast-required cast-required

32 32 JSL Reference type equality operators Every class type C implicitly provides the following predefined reference type equality operators: Hidden boxing? bool operator ==(C x, C y); bool operator!=(c x, C y); there are special rules for determining when a reference type equality operator is applicable. struct S S s1,s2; if if (s1 == == s2)...

33 33 JSL Addition operator string operator +(string x, string y); string operator +(string x, object y); string operator +(object x, string y); Hidden boxing message = "Answer==" + 42; message = operator+("answer==", (object)42); message = "Answer==" + 42.ToString();

34 34 JSL How to tell if a struct instance is boxed? Boxing Trick interface IBoxable bool bool? IsBoxed(); unsafe struct Eg Eg : IBoxable public Eg(int value) fixed fixed (Eg (Eg * ptr ptr = &this) this.address = ptr; ptr; public bool bool? IsBoxed() if if (address == == null) null) return null; null; else else fixed(eg * ptr ptr = &this) return ptr ptr!=!= address; private readonly Eg Eg * address;

35 35 JSL The lock statement A lock statement of the form lock lock (x) (x) is precisely equivalent to: object obj obj = x; x; System.Threading.Enter(obj); // // comment: weak weak spot spot here... try try finally System.Threading.Exit(obj); Q: What happens if a Thread.Abort occurs at the comment? A: The call to System.Threading.Exit is bypassed!

36 36 JSL Event Thread Unsafely The C# 1.0 Standard contained this example Ooops public delegate void void EventHandler(object sender, EventArgs e); e); public class class Button : Control public event event EventHandler Click; protected void void OnClick(EventArgs e) e) if if (Click!=!= null) null) Click(this, e); e);

37 37 JSL Event Thread safely The C# 2.0 Standard the example is now Much better public delegate void void EventHandler(object sender, EventArgs e); e); public class class Button : Control public event event EventHandler Click; protected void void OnClick(EventArgs e) e) EventHandler toraise = Click; if if (toraise!=!= null) null) toraise(this, e); e);

38 38 JSL The C# 1.0 Standard said this Field like events In order to be thread safe, the addition and removal operations are done while holding the lock on the containing object for an instance event, or the type object for a static event. Event Locking public delegate void void D(); D(); class class X public event event D Ev; Ev; class class X private D Ev; Ev; public event event D Ev Ev add add lock(this) Ev Ev += += value; remove lock(this) Ev Ev -= -= value;

39 39 JSL The C# 2.0 Standard says this Field like events The addition and removal operations on all instance events of a class shall be done while holding the lock on an object uniquely associated with the containing object. Event Locking public delegate void void D(); D(); class class X public event event D Ev; Ev; class class X private readonly object key key = new new object(); private D Ev; Ev; public event event D Ev Ev add add lock( key) Ev Ev += += value; remove lock( key) Ev Ev -= -= value;?

40 40 JSL Events inside structs? Event Locking? Field like events The addition and removal operations on all instance events of a class shall be done while holding the lock on an object uniquely associated with the containing object. public delegate void void D(); D(); struct S public event event D Ev; Ev; struct S private D Ev; Ev; public event event D Ev Ev add add Ev Ev += += value; remove Ev Ev -= -= value;

41 41 JSL Captured Variables delegate void void F(); F(); class class CapturedPitfall static void void Main() F[] F[] array array = new new F[5]; F[5]; for for (int (int at at = 0; 0; at at!=!= 5; 5; at++) at++) array[at] = delegate Console.Write(at); ; ; for for (int (int cat cat = 0; 0; cat cat!=!= 5; 5; cat++) array[cat](); 55555

42 42 JSL Captured Variables delegate void void F(); F(); class class CapturedPerLoop static void void Main() F[] F[] array array = new new F[5]; F[5]; for for (int (int at at = 0; 0; at at!=!= 5; 5; at++) at++) int int value value = at; at; array[at] = delegate Console.Write(value); ; ; for for (int (int at at = 0; 0; at at!=!= 5; 5; at++) at++) array[at](); 01234

43 43 JSL Captured Variables Local variable shared between threads! using using System.Threading; class class Demo Demo static void void Main() int int i = 0; 0; ThreadStart t1 t1 = delegate for for (;;) (;;) Thread.Sleep(102); i++; i++; ; ; ThreadStart t2 t2 = delegate for(;;) Thread.Sleep(500); System.Console.Write("0 ", ", i); i); ; ; new new Thread(t1).Start(); new new Thread(t2).Start();

44 44 JSL Not all exceptions are managed Exceptions class class Vulnerable static void void Main() try try // // catch catch (Exception error) // // clean-up code. code. could could result in in // // security hole hole if if not not run. run. // // Ooops Ooops

45 45 JSL C# 1.0 General catch clause Exceptions class class Vulnerable static void void Main() try try // // catch catch (Exception error) CleanUp(); catch catch CleanUp();

46 46 JSL CLR 2.0 Unmanaged exception RuntimeWrappedException Exceptions class class Vulnerable static void void Main() try try // // catch catch (Exception error) CleanUp(); catch catch // // new new compiler warning // // now now unreachable...

47 47 JSL Spot the bug Garbage Collection using using System.Threading; class class App App static static void void Main() Main() bool bool firstinstance; firstinstance; Mutex Mutex key key = new new Mutex(@"Global\App", out out firstinstance); firstinstance); if if (firstinstance) (firstinstance) // // we're we're the the only only instance instance running running // // else else // // another another instance instance detected detected // //......

48 48 JSL One solution Delayed Collection using using System.Threading; class class App App static static void void Main() Main() bool bool firstinstance; firstinstance; using using (new (new out out firstinstance)) firstinstance)) if if (firstinstance) (firstinstance) // // we're we're the the only only instance instance running running // // else else // // another another instance instance detected detected // //......

49 49 JSL That's all Folks! Any Questions? Authoring Consulting Crafting Designing Mentoring Training Jon Jagger JSL phone +44 (0) mobile +44 (0) C#

50 50 JSL What's this? 1.D 1 an int literal. the member access operator D the field/property called D or 1.D? 1. a double literal* D a double type suffix confirming 1. as a double

51 51 JSL portions of this presentation are from the forthcoming book Annotated C# Standard by Jon Jagger, Nigel Perry, Peter Sestoft published by Morgan Kaufmann copyright 200? Elsevier Inc Plug

52 52 JSL Absence, defaults, forbidden, compulsory Access Modifiers public interface IX IX void void M(); M(); public class class X : IX IX public static operator static X() X() ~X() ~X() void void IX.M() public not allowed public required Static constructor not callable Finalizer not callable Explicit Impl. not callable*

53 53 JSL Uninitialized variables Invocation expressions For an invocation expression expr of the form: primary-expression(arg 1, arg 2,, arg N ) For each argument arg i, the definite assignment state of v after arg i is determined by the normal expression rules, ignoring any ref or out modifiers. public class class Eg Eg static void void Method(out int int x, x, int int y) y) Console.WriteLine(y); x = 42; 42; static void void Main() int int x; x; Method(out x, x, x); x);