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    1. Variables are temporary memory locations that have a name (called an identifier), a data type, and a scope. A constant also has a name, data type, and scope, but it also must have a value assigned to it. The value stored in a variable can be changed during the execution of the project; the values stored in constants cannot change.
    2. The data type determines what type of values may be assigned to a variable or constant. The most common data types are string, int, decimal, single, and bool.
    3. Identifiers for variables and constants must follow the C# naming rules and should follow good naming standards, called conventions. An identifier should be meaningful and have the data type appended at the end. Variable names should begin with a lowercase character and be mixed upper- and lowercase while constants are all uppercase.
    4. Intrinsic constants, such as Color.Red and Color.Blue, are predefined and built into the .NET Framework. Named constants are programmer-defined constants and are declared using the const statement. The location of the const statement determines the scope of the constant.
    5. The location of the declaration statement determines the scope of the vari- able. Use the data type without the private or public keyword to declare local variables inside a method; use the private statement to declare class-level variables at the top of the program, outside of any method.
    6. The scope of a variable may be namespace, class level, local, or block level. Block level and local variables are available only within the method in which they are declared; class-level variables are accessible in all methods within a class; namespace variables are available in all methods of all classes in a namespace, which is usually the entire project.
    7. The lifetime of local and block-level variables is one execution of the method in which they are declared. The lifetime of class-level variables is the length of time that the class is loaded.
    8. Identifiers should include the data type of the variable or constant.
    9. Use the Parse methods to convert text values to numeric before perform- ing any calculations.
    10. Calculations may be performed using the values of numeric variables, con- stants, and the properties of controls. The result of a calculation may be assigned to a numeric variable or to the property of a control.
    11. A calculation operation with more than one operator follows the order of precedence in determining the result of the calculation. Parentheses alter the order of operations.
    12. To explicitly convert between numeric data types, use casting or the Con- vert class. Some conversions can be performed implicitly.
    13. The decimal.Round method rounds a decimal value to the specified num- ber of decimal positions.
    14. The ToString method can be used to specify the appearance of values for display. By using formatting codes, you can specify dollar signs, commas, percent signs, and the number of decimal digits to display. The method rounds values to fit the format.
    15. try/catch/finally statements provide a method for checking for user errors such as blank or nonnumeric data or an entry that might result in a calculation error.
    16. A run-time error is called an exception; catching and taking care of excep- tions is called error trapping and error handling.
    17. You can trap for different types of errors by specifying the exception type on the catch statement, and you can have multiple catch statements to catch more than one type of exception. Each exception is an instance of the Exception class; you can refer to the properties of the Exception object for further information.
    18. A message box is a window for displaying information to the user.
    19. The Show method of the MessageBox class is overloaded, which means that the method may be called with different argument lists, called signatures.
    20. You can calculate a sum by adding each transaction to a class-level vari- able. In a similar fashion, you can calculate a count by adding to a class- level variable.







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