3. Data Types

1. 3. Data Types

2. Objectives “.NET is designed around the CTS, or Common Type System. The CTS is what allows assemblies, written in different languages, to work together. To ensure interoperability across languages, Microsoft has also defined the CLS, or Common Language Specification, a subset of the CTS that all languages support. Otherwise, the types in C# are what you would expect from a modern OOPL…” • The Common Type System • Value vs. reference types • Arrays • Namespaces

3. Part 1 • The Common Type System…

4. The Common Type System (CTS) • CTS is based on a hierarchy of classes defined in FCL • all types inherit from Object (all except interface types)

5. The Common Language Specification (CLS) • Not all languages support all CTS types and features • C# supports unsigned integer types, VB.NET does not • C# is case sensitive, VB.NET is not • C# supports pointer types (in unsafe mode), VB.NET does not • C# supports operator overloading, VB.NET does not • CLS was drafted to promote language interoperability • vast majority of classes within FCL are CLS-compliant

6. Mapping C# to CTS • Language keywords map to common CTS classes:

7. Example • An example of using types in C# • declare before you use (compiler enforced) • initialize before you use (compiler enforced) public class App { public static void Main() { int width, height; width = 2; height = 4; int area = width * height; int x; int y = x * 2; ... } } declarations decl + initializer error, x not set

8. Type conversion • Some automatic type conversions available • from smaller to larger types • Otherwise you need a cast or an explicit conversion… • typecast syntax is type name inside parentheses • conversion based on System.Convert class int i = 5; double d = 3.2; string s = "496"; d = i; i = (int) d; i = System.Convert.ToInt32(s); implicit conversion typecast required conversion required

9. Part 2 • Value vs. reference types…

10. "calico" Value vs. reference types • C# separates data types into two categories • Value types: • variable represents a value ("bits") • Reference types: • variable represents a reference to a heap-based object • actual data resides in the object int i; i = 10; 10 string s; s = "calico";

11. How do you know which types are which? • Memorization! • Though it's pretty obvious based on past experience • primitive types like bool, int and double are values • remainder are reference types int i; string s; Customer c1, c2; i = 23; s = "a message"; c1 = null; c2 = new Customer(…);

12. Boxing and Unboxing • When necessary, C# will auto-convert value <==> object • value ==> object is called "boxing" • object ==> value is called "unboxing" int i, j; object obj; string s; i = 32; obj = i; // boxed copy! i = 19; j = (int) obj; // unboxed! s = j.ToString(); // boxed! s = 99.ToString(); // boxed!

13. User-defined reference types • Classes! • for example, Customer class we worked with earlier… public class Customer { public string Name; // fields public int ID; public Customer(string name, int id) // constructor { this.Name = name; this.ID = id; } public override string ToString() // method { return "Customer: " + this.Name; } }

14. Working with reference types… • Creating, assigning, and comparing: Customer c1, c2, c3; string s1, s2; c1 = new Customer("joe hummel", 36259); c2 = new Customer("marybeth lore", 55298); c3 = null; // c3 references no object c3 = c1; // c3 now references same obj as c1 if (c1 == null) ... // do I ref an object? if (c1 == c2) ... // compares references if (c1.Equals(c2)) ... // compares objects if (s1 == s2) ... // exception: == overloaded to // compare string data

15. Defining equality • Classes should override Equals public class Customer { . . . public override bool Equals(object obj) { Customer other; if ((obj == null) || (!(obj is Customer))) return false; // definitely not equal other = (Customer) obj; // typecast to access return this.ID == other.ID; // equal if same id... }

16. GetHashCode • If you override Equals, must also override GetHashCode: • public class Customer • { • . • . • . • public override int GetHashCode() • { • return this.id.GetHashCode(); • }

17. Part 3 • Arrays…

18. Arrays • Arrays are reference types • based on Array class in FCL • must be created using new • 0-based indexing • assigned default values (0 for numeric, null for references, etc.) int[] a; a = new int[5]; a[0] = 17; a[1] = 32; int x = a[0] + a[1] + a[4]; int l = a.Length; create element access number of elements

19. Multi-dimensional arrays • C# supports arrays as a single object OR array of arrays • latter allows you to implement jagged arrays Customer[,] twoD; int[][] jagged2D; // 2D array as single object twoD = new Customer[10, 100]; twoD[0, 0] = new Customer(…); twoD[9, 99] = new Customer(…); // 2D array as array of arrays jagged2D = new int[10][]; jagged2D[0] = new int[10]; jagged2D[1] = new int[20]; jagged2D[9] = new int[100]; jagged2D[0][0] = 1; jagged2D[9][99] = 100;

20. Part 4 • Namespaces…

21. Namespaces • Namespaces are a means for organizing types • a namespace N is a set of names scoped by N • namespaces are often nested namespace Workshop { public class Customer { . . . } public class Product { . . . } }//namespace Workshop.Customer

22. Example • Framework Class Library (FCL) contains 1000's of classes • how to organize? • how to avoid name collisions? • with FCL • within FCL

23. FCL namespaces • FCL's outermost namespace is "System" • FCL technologies nested within System…

24. Namespace != Assembly • Orthogonal concepts: • namespace for organization • assembly for packaging • One namespace could be spread across multiple assemblies • One assembly may contain multiple namesspaces • e.g. mscorlib.dll

25. Summary • CTS is the common type system • same type system for all languages • types implemented by classes in FCL • fundamental difference between value & reference types • CLS is the common language specification • types that are guaranteed to work across languages • Try not to confuse namespaces with assemblies… • namespaces help with organization • assemblies denote implementation / packaging

26. References • Books: • I. Pohl, "C# by Dissection" • S. Lippman, "C# Primer" • J. Mayo, "C# Unleashed"