how to generate qr code in asp.net using c# All Types Are Derived from Object in C#

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All Types Are Derived from Object
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.NET comes to our rescue again. It turns out that every type in the system is derived from Object. Every one value types (struct) and reference types (class) alike, even the built-in types such as Int32. It is easy to see how that would work for a class declaration in C#. If you don t specify a particular base class, you get Object by default. But what about a struct, or enum, or the built-in types; what happens if we try to talk to them through their Object base class
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Boxing and Unboxing Value Types
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Let s give it a try. This code snippet will compile and work quite happily:
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// Int variable int myIntVariable = 1; object myObject = myIntVariable;
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What happens under the covers is that the runtime allocates a new object and puts a copy of the value inside it. This is called boxing, and, as you might expect given that it involves allocating objects and copying values, it is relatively expensive when compared to a straightforward assignment. You can also convert back the other way:
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// Int variable int myIntVariable = 1; object myObject = myIntVariable; int anotherIntVariable = (int)myObject;
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Notice how we use the type name in parentheses to perform the conversion back to an int. In general, this sort of conversion from one type to another is known as a cast, and will work for classes too (although we ll see a more explicit way of doing that later in this chapter). The runtime looks at that box object for us and checks that it contains a value of the correct type. If so, it will copy the value back out of the box and into the new variable.
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What if it isn t of the correct type The runtime will throw an Invalid CastException. You can find out more about exceptions in 6.
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That process is known as unboxing, and is also quite expensive (although not as expensive as boxing, as it doesn t need to allocate the object). Although these performance costs are individually fairly small, if you are processing large numbers of value types in a way that requires them to be repeatedly boxed and unboxed the costs can add up quite rapidly; so you should be aware of boxing and unboxing when you are profiling your application. So the only common base of both Firefighter and Administrator is Object at the moment (remember, everything is ultimately derived from Object). That seems a bit low level, but it is all we have to go on for now, so we ll make do. Example 4-17 shows our first pass at a FireStation.
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class FireStation { List<object> clockedInStaff = new List<object>(); public void ClockIn(object staffMember) { if (!clockedInStaff.Contains(staffMember)) { clockedInStaff.Add(staffMember); } } public void RollCall() { foreach(object staffMember in clockedInStaff) { // Hmmm... What to do } }
}
Our ClockIn method is making use of a list of objects to keep track of who is in the station. To do that it is using the generic collection class List<T> we first saw in 2. Using the List.Contains method, the implementation checks that they weren t already in the station, and adds them if necessary. Everything is fine so far. Then we reach the RollCall method. We re using foreach to iterate over the clocked-in staff, but we don t actually have a method to call to get their names! We want a way of indicating that these disparate object types (firefighters, fire chiefs, and administrators) all support giving out their name. We saw one way of doing that already: we could create a common base class, and move the Name functionality in there. Let s see what happens if we try to do that. Practically speaking, we have two completely different implementations of the Name property. We saw that we can model that situation with an abstract base class from which Firefighter and Administrator both derive, both implementing the method in their own way. Here s our NamedPerson base with an abstract property for the Name:
abstract class NamedPerson { public abstract string Name { get; } }
There s no problem when we implement this on our Administrator:
class Administrator : NamedPerson { public override string Name { get { StringBuilder name = new StringBuilder(); AppendWithSpace(name, Title); AppendWithSpace(name, Forename); AppendWithSpace(name, Surname); return name.ToString(); } } } // ...
Notice how we derived from NamedPerson and added the override modifier to our Name property so that it overrides the abstract method in our base.
That s fine so far. What about our FirefighterBase Let s try doing exactly the same thing:
abstract class FirefighterBase : NamedPerson { public abstract void ExtinguishFire(); public override string Name { get; set; } } // ...
If we compile that, we get an error:
'FirefighterBase.Name.set': cannot override because 'NamedPerson.Name' does not have an overridable set accessor
We run into difficulty because FirefighterBase has both a getter and a setter for the Name property, but our base allows only a getter.
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