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int myVariable; // some other code here myVariable = 15; // assign 15 to myVariable
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You can also change the value of a variable later in the program. That is why they re called variables; their values can vary, and that s what makes them useful.
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int myVariable; // some other code here myVariable = 15; // assign 15 to myVariable // some other code here myVariable = 12; // now it is 12
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Technically, a variable is a named storage location (that is, stored in memory) with a type. After the final line of code in the previous example, the value 12 is stored in the named location myVariable. Example 3-1 illustrates the use of variables. To test this program, open Visual Studio .NET and create a console application, just as you did with Hello World in 1. Type in the code shown in bold in Example 3-1.
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using using using using System; System.Collections.Generic; System.Linq; System.Text;
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namespace Example_3_1_ _ _ _Using_variables { class Values { static void Main( )
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Variables and Assignment
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{ int myInt = 7; System.Console.WriteLine("Initialized, myInt: {0}", myInt); myInt = 5; System.Console.WriteLine("After assignment, myInt: {0}", myInt); } } }
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Press Ctrl-F5, or select Debug Start Without Debugging to build and run this application. As we mentioned in 1, if you press F5, the console window will disappear almost immediately; using Ctrl-F5 allows the window to stick around so that you can read it. The output looks like this:
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Initialized, myInt: 7 After assignment, myInt: 5
Example 3-1 initializes the variable myInt to the value 7, displays that value, reassigns the variable with the value 5, and displays it again.
De nite Assignment
C# requires definite assignment; you have to initialize a variable, or assign a value to it, before you can use it that is, before you can output it or manipulate it in any way. To test this rule, change the line that initializes myInt in Example 3-1 to:
int myInt;
Save the revised program shown in Example 3-2.
using using using using System; System.Collections.Generic; System.Linq; System.Text;
namespace Example_3_2_ _ _ _Definite_Assignment { class Values { static void Main( ) { int myInt; System.Console.WriteLine("Initialized, myInt: {0}", myInt); myInt = 5; System.Console.WriteLine("After assignment, myInt: {0}", myInt); } } }
| 3: C# Language Fundamentals
When you try to compile Example 3-2, the C# compiler will open the Error List window in the IDE, and will display the following error message:
Use of unassigned local variable 'myInt'
You can t use an uninitialized variable in C#; doing so violates the rule of definite assignment. In this case, using the variable myInt means passing it to WriteLine( ). So, does this mean you must initialize every variable No, but if you don t initialize your variable, you must assign a value to it before you attempt to use it. Example 3-3 illustrates a corrected program.
using using using using System; System.Collections.Generic; System.Linq; System.Text;
namespace Example_3_3_ _ _ _Definite_assignment { class Values { static void Main( ) { int myInt; //other code here... myInt = 7; // assign to it System.Console.WriteLine("Assigned, myInt: {0}", myInt); myInt = 5; System.Console.WriteLine("Reassigned, myInt: {0}", myInt); } } }
You can even assign the same value to multiple variables, like this:
int a, b, c, d; a = b = c = d = 5;
In this case, a, b, c, and d would all have the value 5. This works because the C# compiler performs the rightmost assignment first; that is, d = 5. That assignment itself returns a value, the value 5. The compiler then assigns that returned value to c. That second assignment also returns a value, and so on, until all the variables have been assigned.
Implicitly Typed Variables
There s one additional type of variable we can discuss, now that you understand assignment: the implicitly typed variable. The C# compiler can determine the type of a variable by analyzing the type of the value that you assign to it. For example, look at these assignment statements:
var firstVariable = 6; var secondVariable = 3.5; var thirdVariable = "I'm a string!";
The compiler assigns firstVariable as type int, secondVariable as type double, and thirdVariable as type string. You don t have to explicitly assign the type. Be very clear, though: these variables are typed, and if you later try to assign an object of the wrong type to them you will generate a compiler error. And once the implicit type is set, it cannot be changed, not even explicitly. If you try to do something like this, you ll get an error:
firstVariable = secondVariable;
Just as if you had explicitly declared firstVariable to be of type int, once it is implicitly typed, you cannot assign the value of a variable of type double to it, because you would lose part of the value, as you ll see in 4. You may be wondering: if the compiler can determine the types of the variables for you, why not just use var all the time, instead of bothering with explicitly declaring types A key reason is that implicit types make your code harder to read, and thus harder to maintain. This is more than enough reason to avoid using var except where needed. A second reason is that the compiler may guess incorrectly, and thus introduce small but nasty errors into your code. If you were writing a math program, and you used var with your assignments like this:
var a = 12; var b = 7;
the compiler will decide that both a and b should be of type int. But if you were thinking they should be of type double, and later try something like this:
a = 7.4; b = 5.5;
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