Object Typecasting Data Structure - StudySection

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Object Typecasting

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Introduction

• Object Typecasting is a process of converting one
  data structure into another. The type system is
  categorized into 2 types, primitives and
  references.
• Primitive types are also known as value types,
  which means a variable contains the value itself.
  Most simply type data structure would look like
  this.

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• In this figure, we can see that primitive data
  types are stored inside stack memory and the
  variables point to a memory location. The
  variable value is placed next to the variable
  label in memory.
• On the other hand, reference types hold reference
  or address of the object data, however, that
  address binding with the label of variable is
  inside the stack memory, opposite to this
  variable data is stored inside Heap memory.

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Type Casting Rules
Compile-time checking 1

• Type of D (refers to the General typecasting
  equation) and C must have some relation in
  between them. Either child to parent or parent to
  child or must be of the same type, otherwise, we
  will get a compile-time error.
• Case 1.
• Object o "Hello World"
• StringBuilder sb (StringBuilder)o
• This code will compile successfully. The object
  is the topmost class or parent of all types. A
  string of data can easily hold an Object type.
• After that, object and StringBuilder have a
  parent/child relationship, however, to convert
  from parent to child type, we need to explicitly
  mention the typecasting.
• Case 2.
• string s "Hello World"
• StringBuilder sb (StringBuilder)s
• This code will raise a compile-time error in the
  second line, where we are explicitly converting a
  string to the StringBuilder type. There is no
  relationship between string and StringBuilder
  type.
• Error description
• Compile Time Error Cannot convert type string
  to System.Text.StringBuilder

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Compile-time checking 2

• C must be either the same or derived/child type
  of A, otherwise we will get a compile-time
  error.
• Case 1.
• Object o "Hello World"
• StringBuilder sb (StringBuilder)o
• This code will compile successfully.
• Case 2.
• Object o "Hello World"
• StringBuilder sb (string)o
• The string is not a derived class of
  StringBuilder and not the same at all, so this
  code fails to follow rule Compile-time checking
  2. Consequently, it will raise a compile-time
  error.
• Error description
• Compile Time Error
• Cannot implicitly convert type string to
  System.Text.StringBuilder

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Runtime checking

• Type of D must be either the same or derived
  type of C, otherwise, we will get a runtime
  exception,
• Case 1.
• Object o "Hello World"
• StringBuilder sb (StringBuilder)o
• This code will compile with no errors. However,
  when we try to run it, it will throw a runtime
  exception.
• At runtime o will be of string type, which
  means, when the runtime environment will execute
  a second statement, it has to convert a string
  type variable to a StringBuilder type and we have
  already seen that there is no any relationship
  between them.
• Error Description
• Runtime Exception System.InvalidCastException
• Unable to cast object of type System.String to
  type System.Text.StringBuilder
• Case 2.
• What do you think, will the following code
  snippet compile and run without any issue.
• Object o "Hello World"
• Object o1 (string)o
• At runtime, o will be of type string the same as
  string, resulting in a successful type
  conversion and the program runs smoothly.

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