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What Is a Reference Type
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Reference types store the address of their data, also known as a pointer, on the stack. The actual data that address refers to is stored in an area of memory called the heap. The runtime manages the memory used by the heap through a process called garbage collection. Garbage collection recovers memory periodically as needed by disposing of items that are no longer referenced.
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BEST PRACTICES Garbage collection
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Garbage collection occurs only when needed or when triggered by a call to GC.Collect. Automatic garbage collection is optimized for applications where most instances are short-lived, except for those allocated at the beginning of the application. Following that design pattern will result in the best performance.
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Comparing the Behavior of Reference and Value Types
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Because reference types represent the address of data rather than the data itself, assigning one reference variable to another doesn t copy the data. Instead, assigning a reference variable to another instance merely creates a second copy of the reference, which refers to the same memory location on the heap as the original variable.
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1
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Framework Fundamentals
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Consider the following simple structure declaration:
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' VB Structure Numbers Public val As Integer Public Sub New(ByVal _val As Integer) val = _val End Sub Public Overloads Overrides Function ToString() As String Return val.ToString End Function End Structure // C# struct Numbers { public int val; public Numbers(int _val) { val = _val; } public override string ToString() { return val.ToString(); } }
Now consider the following code, which creates an instance of the Numbers structure, copies that structure to a second instance, modifies both values, and displays the results:
' VB Dim n1 As Numbers = New Numbers(0) Dim n2 As Numbers = n1 n1.val += 1 n2.val += 2 Console.WriteLine("n1 = {0}, n2 = {1}", n1, n2) // C# Numbers n1 = new Numbers(0); Numbers n2 = n1; n1.val += 1; n2.val += 2; Console.WriteLine("n1 = {0}, n2 = {1}", n1, n2);
This code would display n1 = 1, n2 = 2 because a structure is a value type, and copying a value type results in two distinct values. However, if you change the Numbers type declaration from a structure to a class, the same application would display n1 = 3, n2 = 3 . Changing Numbers from a structure to a class causes it to be a reference type rather than a value type. When you modify a reference type, you modify all copies of that reference type.
Lesson 2: Using Common Reference Types
Built-in Reference Types
There are about 2500 built-in reference types in the .NET Framework. Everything not derived from System.ValueType is a reference type, including these 2500 or so built-in reference types. Table 1-3 lists the most commonly used types, from which many other reference types are derived.
Table 1-3
Common Reference Types
Type System.Object
Use for The Object type is the most general type in the Framework. You can convert any type to System.Object, and you can rely on any type having ToString, GetType, and Equals members inherited from this type. Text data. Dynamic text data. Arrays of data. This is the base class for all arrays. Array declarations use language-specific array syntax. Buffer for file, device, and network I/O. This is an abstract base class; task-specific classes are derived from Stream. Handling system and application-defined exceptions. Task-specific exceptions inherit from this type.
System.String System.Text.StringBuilder System.Array System.IO.Stream
System.Exception
Strings and String Builders
Types are more than just containers for data, they also provide the means to manipulate that data through their members. System.String provides a set of members for working with text. For example, the following code does a quick search and replace:
' VB Dim s As String = "this is some text to search" s = s.Replace("search", "replace") Console.WriteLine(s) // C# string s = "this is some text to search"; s = s.Replace("search", "replace"); Console.WriteLine(s);
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