vb.net generate barcode image Primitive, Reference, and Value Types in C#

Generator PDF-417 2d barcode in C# Primitive, Reference, and Value Types

5 Primitive, Reference, and Value Types
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operands to Object first) because one of the operands types could overload the == operator, giving it semantics other than identity . As you can see, the .NET Framework has a very confusing story when it comes to object equality and identity . By the way, System.ValueType (the base class of all value types) does override Object s Equals method and is correctly implemented to perform a value equality check (not an identity check) . Internally, ValueType s Equals is implemented this way: 1. If the obj argument is null, return false . 2. If the this and obj arguments refer to objects of different types, return false . 3. For each instance field defined by the type, compare the value in the this object with the value in the obj object by calling the field s Equals method . If any fields are not equal, return false . 4. Return true . Object s Equals method is not called by ValueType s Equals method . Internally, ValueType s Equals method uses reflection (covered in 23, Assembly Loading and Reflection ) to accomplish step #3 above . Since the CLR s reflection mechanism is slow, when defining your own value type, you should override Equals and provide your own implementation to improve the performance of value equality comparisons that use instances of your type . Of course, in your own implementation, do not call base.Equals . When defining your own type, if you decide to override Equals, you must ensure that it adheres to the four properties of equality:
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Equals must be reflexive; that is, x.Equals(x) must return true . Equals must be symmetric; that is, x.Equals(y) must return the same value as y.Equals(x) . Equals must be transitive; that is, if x.Equals(y) returns true and y.Equals(z)
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returns true, then x.Equals(z) must also return true .
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Equals must be consistent . Provided that there are no changes in the two values being
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compared, Equals should consistently return true or false . If your implementation of Equals fails to adhere to all of these rules, your application will behave in strange and unpredictable ways . When overriding the Equals method, there are a few more things that you ll probably want to do:
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Have the type implement the System.IEquatable<T> interface s Equals method This generic interface allows you to define a type-safe Equals method . Usually, you ll implement the Equals method that takes an Object parameter to internally call the type-safe Equals method .
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Part II Designing Types
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Overload the == and !=operator methods Usually, you ll implement these operator methods to internally call the type-safe Equals method .
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Furthermore, if you think that instances of your type will be compared for the purposes of sorting, you ll want your type to also implement System.IComparable s CompareTo method and System.IComparable<T> s type-safe CompareTo method . If you implement these methods, you ll also want to overload the various comparison operator methods (<, <=, >, >=) and implement these methods internally to call the type-safe CompareTo method .
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Object Hash Codes
The designers of the FCL decided that it would be incredibly useful if any instance of any object could be placed into a hash table collection . To this end, System.Object provides a virtual GetHashCode method so that an Int32 hash code can be obtained for any and all objects . If you define a type and override the Equals method, you should also override the GetHashCode method . In fact, Microsoft s C# compiler emits a warning if you define a type that overrides Equals without also overriding GetHashCode . For example, compiling the following type yields this warning: warning CS0659: 'Program' overrides Object.Equals(object o) but does not override Object.GetHashCode() .
public sealed class Program { public override Boolean Equals(Object obj) { ... } }
The reason why a type that defines Equals must also define GetHashCode is that the implementation of the System.Collections.Hashtable type, the System.Collections. Generic.Dictionary type, and some other collections require that any two objects that are equal must have the same hash code value . So if you override Equals, you should override GetHashCode to ensure that the algorithm you use for calculating equality corresponds to the algorithm you use for calculating the object s hash code . Basically, when you add a key/value pair to a collection, a hash code for the key object is obtained first . This hash code indicates which bucket the key/value pair should be stored in . When the collection needs to look up a key, it gets the hash code for the specified key object . This code identifies the bucket that is now searched sequentially, looking for a stored key object that is equal to the specified key object . Using this algorithm of storing and looking up keys means that if you change a key object that is in a collection, the collection will no longer be able to find the object . If you intend to change a key object in a hash table, you should remove the original object/value pair, modify the key object, and then add the new key object/value pair back into the hash table .
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