1. CHARLES UNIVERSITY IN PRAGUE http://d3s.mff.cuni.cz/~jezek faculty of mathematics and physics C# Language &.NET Platform 4 th Lecture Pavel Ježek pavel.jezek@d3s.mff.cuni.cz Some of the slides are based on University of Linz.NET presentations. © University of Linz, Institute for System Software, 2004 published under the Microsoft Curriculum License (http://www.msdnaa.net/curriculum/license_curriculum.aspx)

2. CLI Type System All types Reference types (allocated on managed heap) PointersValue types (allocated in-place [with exceptions] ) Classes (e.g. strings) Interfaces ArraysDelegates Simple types (Int32, Int64, Double, Boolean, Char, …) Nullables Enumerations Structures User defined structures

3. Visibility Visibility modifiers: publicAccess is not restricted. protectedAccess is limited to the containing class or types derived from the containing class. internalAccess is limited to the current assembly. protected internalAccess is limited to the current assembly or types derived from the containing class. privateAccess is limited to the containing type. Default visibility in: enumpublic classprivate interfacepublic structprivate

4. CLI Type System All types Reference types (allocated on managed heap) PointersValue types (allocated in-place [with exceptions] ) Classes (e.g. strings) Interfaces ArraysDelegates Simple types (Int32, Int64, Double, Boolean, Char, …) Nullables Enumerations Structures User defined structures

5. Parameters value parameters (input parameters) void Inc(int x) {x = x + 1;} void f() { int val = 3; Inc(val); // val == 3 } -"call by value" -formal parameter is a copy of the actual parameter -actual parameter is an expression -"call by value" -formal parameter is a copy of the actual parameter -actual parameter is an expression -"call by reference" -formal parameter is an alias for the actual parameter (address of actual parameter is passed) -actual parameter must be a variable -"call by reference" -formal parameter is an alias for the actual parameter (address of actual parameter is passed) -actual parameter must be a variable ref parameters (transient parameters) void Inc(ref int x) { x = x + 1; } void f() { int val = 3; Inc(ref val); // val == 4 }

6. Parameters value parameters (input parameters) void Inc(int x) {x = x + 1;} void f() { int val = 3; Inc(val); // val == 3 } -"call by value" -formal parameter is a copy of the actual parameter -actual parameter is an expression -"call by value" -formal parameter is a copy of the actual parameter -actual parameter is an expression -similar to ref parameters but no value is passed by the caller. -must not be used in the method before it got a value. -similar to ref parameters but no value is passed by the caller. -must not be used in the method before it got a value. out parameters (output parameters) void Read (out int first, out int next) { first = Console.Read(); next = Console.Read(); } void f() { int first, next; Read(out first, out next); } -"call by reference" -formal parameter is an alias for the actual parameter (address of actual parameter is passed) -actual parameter must be a variable -"call by reference" -formal parameter is an alias for the actual parameter (address of actual parameter is passed) -actual parameter must be a variable ref parameters (transient parameters) void Inc(ref int x) { x = x + 1; } void f() { int val = 3; Inc(ref val); // val == 4 }

7. CLI Type System All types Reference types (allocated on managed heap) PointersValue types (allocated in-place [with exceptions] ) Classes (e.g. strings) Interfaces ArraysDelegates Simple types (Int32, Int64, Double, Boolean, Char, …) Nullables Enumerations Structures User defined structures

8. Declaration of Local Variables void foo(int a) { int b; if (...) { int b; // error: b is already declared in the outer block int c; int d;... } else { int a; // error: a is already declared in the outer block (parameter) int d; // ok: no conflict with d in the if block } for (int i = 0;...) {...} for (int i = 0;...) {...} // ok: no conflict with i from the previous loop int c; // error: c is already declared in a nested block } int e = 1, f; if (e == 1) { f = 2; } e = f; // error: f is not initialized in every possible execution path

9. CLI Type System All types Reference types (allocated on managed heap) PointersValue types (allocated in-place [with exceptions] ) Classes (e.g. strings) Interfaces ArraysDelegates Simple types (Int32, Int64, Double, Boolean, Char, …) Nullables Enumerations Structures User defined structures

10. How Would You Implement List.Clear() ? OptionComplexity with Respect to Number of Elements in List (n) AO(1) BO(n) CSomething else.

11. How Would You Implement List.Clear() ? OptionComplexity with Respect to Number of Elements in List (n) AO(1) BO(n) – set all elements to null ! Mind the GC! CSomething else.

12. What about String.Substring?

13. CLI Type System All types Reference types (allocated on managed heap) PointersValue types (allocated in-place [with exceptions] ) Classes (e.g. strings) Interfaces ArraysDelegates Simple types (Int32, Int64, Double, Boolean, Char, …) Nullables Enumerations Structures User defined structures

14. CLI Type Inheritance System.Object (C# keyword: object ) user-defined classes (C# keyword: class ) delegates (C# keyword: delegate ) pointers (C#: Type * ) System.Delegate System.MulticastDelegate System.ValueType System.Enum System.Array arrays (C#: Type[] or Type[,] ) System.String (C# keyword: string ) interfaces (C# keyword: interface ) user-defined structures (C# keyword: struct ) enumerations (C# keyword: enum ) System.Int32 (C# keyword: int ) System.Int64 (C# keyword: long ) System.Double (C# keyword: double ) System.Boolean (C# keyword: bool ) … simple types System.Nullable (C#: Type? )

15. CLI Type Inheritance System.Object (C# keyword: object ) user-defined classes (C# keyword: class ) delegates (C# keyword: delegate ) pointers (C#: Type * ) System.Delegate System.MulticastDelegate System.ValueType System.Enum System.Array arrays (C#: Type[] or Type[,] ) System.String (C# keyword: string ) interfaces (C# keyword: interface ) user-defined structures (C# keyword: struct ) enumerations (C# keyword: enum ) System.Int32 (C# keyword: int ) System.Int64 (C# keyword: long ) System.Double (C# keyword: double ) System.Boolean (C# keyword: bool ) … simple types System.Nullable (C#: Type? ) Interfaces are not inherited from System.Object, but System.Object members are callable/accessible via any interface type expression.

16. Visibility Visibility modifiers: publicAccess is not restricted. protectedAccess is limited to the containing class or types derived from the containing class. internalAccess is limited to the current assembly. protected internalAccess is limited to the current assembly or types derived from the containing class. privateAccess is limited to the containing type. Default visibility in: enumpublic classprivate interfacepublic structprivate

17. Example: Inheritance of private members

18. CLI Type Inheritance System.Object (C# keyword: object ) user-defined classes (C# keyword: class ) delegates (C# keyword: delegate ) pointers (C#: Type * ) System.Delegate System.MulticastDelegate System.ValueType System.Enum System.Array arrays (C#: Type[] or Type[,] ) System.String (C# keyword: string ) interfaces (C# keyword: interface ) user-defined structures (C# keyword: struct ) enumerations (C# keyword: enum ) System.Int32 (C# keyword: int ) System.Int64 (C# keyword: long ) System.Double (C# keyword: double ) System.Boolean (C# keyword: bool ) … simple types System.Nullable (C#: Type? )

19. CLI Type Inheritance (Sealed Types) System.Object (C# keyword: object ) user-defined classes (C# keyword: class ) delegates (C# keyword: delegate ) pointers (C#: Type * ) System.Delegate System.MulticastDelegate System.ValueType System.Enum System.Array arrays (C#: Type[] or Type[,] ) System.String (C# keyword: string ) interfaces (C# keyword: interface ) user-defined structures (C# keyword: struct ) enumerations (C# keyword: enum ) System.Int32 (C# keyword: int ) System.Int64 (C# keyword: long ) System.Double (C# keyword: double ) System.Boolean (C# keyword: bool ) … simple types System.Nullable (C#: Type? ) sealed Optionally sealed

20. Ref. Type DOES NOT Imply Instances System.Object (C# keyword: object ) user-defined classes (C# keyword: class ) delegates (C# keyword: delegate ) pointers (C#: Type * ) System.Delegate System.MulticastDelegate System.ValueType System.Enum System.Array arrays (C#: Type[] or Type[,] ) System.String (C# keyword: string ) interfaces (C# keyword: interface ) user-defined structures (C# keyword: struct ) enumerations (C# keyword: enum ) System.Int32 (C# keyword: int ) System.Int64 (C# keyword: long ) System.Double (C# keyword: double ) System.Boolean (C# keyword: bool ) … simple types System.Nullable (C#: Type? )

21. Hiding Members can be declared as new in a subclass. They hide inherited members with the same name and signature. class A { public int x; public void F() {...} public virtual void G() {...} } class B : A { public new int x; public new void F() {...} public new void G() {...} } B b = new B(); b.x =...;// accesses B.x b.F();... b.G();// calls B.F and B.G ((A)b).x =...;// accesses A.x! ((A)b).F();... ((A)b).G(); // calls A.F and A.G!

22. What is the output the following program? class A { public string className = “A”; } class B : A { private string className = “B”; } class Program { static void Main(string[] args) { Console.WriteLine(new B().className); } OptionResult AIt will not compile – error in class B. BIt will not compile – error in class Program. CA DB EIt will generate a runtime error.

23. What is the output the following program? class A { public string className = “A”; } class B : A { private string className = “B”; } class Program { static void Main(string[] args) { Console.WriteLine(new B().className); } OptionResult AIt will not compile – error in class B. BIt will not compile – error in class Program. CA DB EIt will generate a runtime error. a compiler warning: use new keyword new

24. What is the output the following program? class A { public string className = “A”; } class B : A { private string className = “B”; } class Program { static void Main(string[] args) { Console.WriteLine(new B().className); } OptionResult AIt will not compile – error in class B. BIt will not compile – error in class Program. CA & a compiler warning: use new keyword. DB EIt will generate a runtime error.

25. What is the output the following program? class A { public string className = “A”; } class B : A { private string className = “B”; public void f() { Console.WriteLine(className); } class Program { static void Main(string[] args) { Console.Write(new B().className); new B().f(); } OptionResult AA BA B CB A DB

26. What is the output the following program? class A { public string className = “A”; } class B : A { private string className = “B”; public void f() { Console.WriteLine(className); } class Program { static void Main(string[] args) { Console.Write(new B().className); new B().f(); } OptionResult AA BA B & a compiler warning: use new keyword. CB A DB