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Object Typecasting is a process of converting one data structure into another. The type system is categorized into 2 types, primitives and references.
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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.
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.
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
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’
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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