net qr code reader open source Reading Text Input in VS .NET

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9 Reading Text Input
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given a reference, all you re told is how to get hold of something. The following code would create a new array and then set pressedKeys with the telephone number of that array:
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pressedKeys = new Keys[4];
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The only way that you can get hold of this array is by using pressedKeys to locate it. If the pressedKeys variable is destroyed or overwritten, the object might as well not exist, as you have no way of getting in touch with it. This is the same as if you met someone wonderful at a party and failed to get their telephone number. If you can t get to an object, it might as well not be there.
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Say Hello to the Garbage Collector
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Within C#, there s a special mechanism for getting rid of objects that might as well not be there. The garbage collector process has the job of looking for objects that do not have anything referring to them and removing them from memory. Memory allocation is an important part of programming, and you need to be careful not to make too much work for the garbage collector. An inexperienced programmer could write the following C# code:
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Keys[] pressedKeys ; pressedKeys = new Keys[100]; pressedKeys = new Keys[200];
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This is very stupid code. It s completely legal and would compile and run, but it s still very stupid because the 100-element array that was created in the second statement is immediately made useless in the third, where pressedKeys is made to refer to another, larger array. This is like building a brand-new office and then destroying the only copy of the telephone number that could be used to contact it. The garbage collector would eventually notice that the array object had no references referring to it and would destroy it, but until this happened, a large chunk of memory would be unusable. The Great Programmer makes sure that when she writes a program, it does not repeatedly create and destroy objects in this way; as a result, her software runs quickly and uses only the minimum amount of memory.
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Using References and Values
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You can see that reference variables are quite different from ordinary variables that simply hold values. It s important that you understand the difference. A reference variable holds the telephone number of an instance of an object. A value variable holds a particular value, for example:
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int myAge; myAge = 21;
Part II
Images, Sound, and Text
These statements declare an int variable called myAge and set it to the rather optimistic (in my case) value of 21. You can think of myAge as a piece of paper with space to write a single integer value on it. When a value is assigned to the variable, it s equivalent to writing a new number on the paper. If I assign the value in myAge to another int variable, the value on the paper is copied across:
int myAge; myAge = 21; int tempAge; tempAge = myAge;
You now have a new int variable called tempAge. This has the value 21 written on it because that s the value that was copied from the myAge variable. In other words, when you work with value types, you re copying values from one piece of paper to another. Changing the value written on one piece of paper does not change the value on another:
tempAge++; // this will not change the value in myAge
If the value in tempAge is increased by 1, it now holds the value 22, but myAge still has the value 21. However, consider what happens when the program performs assignments using references:
Keys[ ] pressedKeys ; pressedKeys = new Keys[100]; Keys[ ] oldKeys; oldKeys = pressedKeys;
The oldKeys variable is a reference that can refer to an array of Keys. If I set oldKeys equal to pressedKeys, it means that it refers to the same object as pressedKeys does. In other words, it contains the same office phone number. Whether you use oldKeys.Length or pressedKeys.Length, you get the same Mrs. Length on the end of the line. So you can see that the following statements both set the element at the start of the same array:
oldKeys[0] = Keys.X; pressedKeys[0] = Keys.Y;
First, the element is set to X; then it is set to Y. At the end of these two statements, both oldKeys[0] and pressedKeys[0] contain Y. An object managed by reference doesn t have a name; rather, it s identified only in terms of the things that are referring to it. You should never say the array called oldKeys ; you can say only the array that oldKeys is currently referring to. During the lifetime of the oldKeys reference, it could be made to refer to many different arrays.
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