PART I in C#.NET

Encoder QR in C#.NET PART I

As the comments indicate, the variable x is declared at the start of Main( ) s scope and is accessible to all subsequent code within Main( ) Within the if block, y is declared Since a block defines a scope, y is visible only to other code within its block This is why outside of its block, the line y = 100; is commented out If you remove the leading comment symbol, a compile-time error will occur because y is not visible outside of its block Within the if block, x can be used because code within a block (that is, a nested scope) has access to variables declared by an enclosing scope Within a block, variables can be declared at any point, but are valid only after they are declared Thus, if you define a variable at the start of a method, it is available to all of the code within that method Conversely, if you declare a variable at the end of a block, it is effectively useless, because no code will have access to it If a variable declaration includes an initializer, then that variable will be reinitialized each time the block in which it is declared is entered For example, consider this program:

// Demonstrate lifetime of a variable using System; class VarInitDemo { static void Main() { int x; for(x = 0; x < 3; x++) { int y = -1; // y is initialized each time block is entered ConsoleWriteLine("y is: " + y); // this always prints -1

Part I:

y = 100; ConsoleWriteLine("y is now: " + y); } } }

The output generated by this program is shown here:

y y y y y y is: -1 is now: 100 is: -1 is now: 100 is: -1 is now: 100

As you can see, y is always reinitialized to 1 each time the inner for loop is entered Even though it is subsequently assigned the value 100, this value is lost There is one quirk to C# s scope rules that may surprise you: Although blocks can be nested, no variable declared within an inner scope can have the same name as a variable declared by an enclosing scope For example, the following program, which tries to declare two separate variables with the same name, will not compile

/* This program attempts to declare a variable in an inner scope with the same name as one defined in an outer scope *** This program will not compile *** */ using System; class NestVar { static void Main() { int count; for(count = 0; count < 10; count = count+1) { ConsoleWriteLine("This is count: " + count); int count; // illegal!!! for(count = 0; count < 2; count++) ConsoleWriteLine("This program is in error!"); } } }

If you come from a C/C++ background, then you know that there is no restriction on the names you give variables declared in an inner scope Thus, in C/C++ the declaration of count within the block of the outer for loop is completely valid However, in C/C++, such a declaration hides the outer variable The designers of C# felt that this type of name hiding could easily lead to programming errors and disallowed it

3:

In programming, it is common to assign one type of variable to another For example, you might want to assign an int value to a float variable, as shown here:

When compatible types are mixed in an assignment, the value of the right side is automatically converted to the type of the left side Thus, in the preceding fragment, the value in i is converted into a float and then assigned to f However, because of C# s strict type-checking, not all types are compatible, and thus, not all type conversions are implicitly allowed For example, bool and int are not compatible Fortunately, it is still possible to obtain a conversion between incompatible types by using a cast A cast performs an explicit type conversion Both automatic type conversion and casting are examined here