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Coding in GUI environment is easy and quicker as compare to traditional, linear programming languages. To place a check mark in a menu item, the checked property is set to true. Each argument acts like a variable in the procedure. Arrays have upper and lower bounds and elements have to lie within those bounds. They are Breakpoint: There are twips in one inch.
Net Black Book , please sign up. Be the first to ask a question about Visual Basic. Net Black Book. Lists with This Book. This book is not yet featured on Listopia. Community Reviews. Showing Rating details. Sort order. Jan 30, Minhajur Rahman Khan rated it liked it. Basic Programming in Visual Basic. View 1 comment. Mar 25, Eric Lyng rated it it was amazing.
After 15 years, this book is still one of my most useful VB. A very well written book for beginners also.
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Visual Basic can locate compile-time bugs if the Visual Basic application is set up correctly. Visual Basic set the several points to users, which can be change as needed. Compile-time bugs are detected by visual basic automatically when the program is compiled using F5.
If the Start With Full Compile option is used, the Compile On Demand can be turned on, and the developer will not find any bugs until the line of code with the bugs is actually executed.
Compile errors occur as a result of incorrectly constructed code such as Next statement without a corresponding For statement or programming mistakes that violate the rules of Basic, such as misplace word, missing separator, or type mismatch. Compile errors included syntax errors.
These include mismatched parentheses or an incorrect number of arguments passed to an intrinsic function. Runtime bugs A Visual Basic runtime error is the exception generated by Visual Basic when it ascertains that the code is about to perform something illegal. An illegal function could be something as simple as trying to determine the size of a file that dose not exist or attempting to multiply two numbers, the result of which exceeds the storage space that could be contained by the data type.
However, using an error handler could prevent this error dialog box from being shown and save the program from crashing. By practices we can also use our knowledge that a particular error identified by its error number will be generated to help guide our logic.
This error can be finally rectified by changing the code suitably. Consider the following example where we declare two variable x and y. Visual Basic Debugging Tools The best way to keep bugs out is to prevent then in the first place. Visual Basic gives a programming several tools, include IDE options and compile directives, to help achieve this goal. Visual Basic offer several IDE option that can help the user write better code. Additional settings and debugging aids can be viewed in a dialog box shown in the fig.
Select Option from Tools Menu 2. Choose the Editor tab in the Option dialog box. Require Variable declaration: This option explicitly inserts the Option Explicit statement in each new form, module, or class created.
A dim, private, public, ReDim or static statement must be used to declare a variable before the value is assigned to it. If an undeclared variable is used, an error occurs at compile time. Auto Syntax Check: The Auto Syntax Check option forces Visual Basic to check the line of code when the user moves the cursor off the line.
Auto Quick Info: VB display all possible parameters while highlighting the one we are currently entering. Debugging Tools are designed to help the user with logic and run-time errors.
Visual Basic provides several buttons in the ToolBar that are helpful for debugging. They are. Defines a line in the code window where Visual Basic suspends execution of application Instant Watch: Lists the current value of an expression while the application is in the break-mode. Presents a dialog box that shows all procedures that have been called but not yet run completely. Visual Basic provides several built-in methods for controlling the execution of the program in real time. It is possible to execute the program line by line or procedure by procedure or a combination of the two.
These basic debugging actions are called Stepping. Because it enables the developer to walk through the program, examining the variables and logic, stepping is the most powerful debugging tool offered by Visual Basic. The various debugging commands are discussed below. Step Into: Execute the next executable line of the code in the application and steps into procedures.
This enables to check every line of code as it is being executed. This option can be accessed from debug menu or by pressing F8. Step Over: Execute the next executable line of the code in the application without stepping into procedure. The procedure the user steps over does execute, but Visual Basic does not walk through the stepped-over procedure line-by-line.
Step Out: Moves program execution back to the calling procedure. This is the functional equivalent of Exit sub or Exit Function. It is simple exits the current procedure without executing any more code in that procedure. Set Next: Enables the user to move program execution to any executable line of code in the current procedure.
While debugging an application, we should clearly understand which of the three modes such as design time, run time or break mode we are in at a given time. It automatically opens at run time. In break mode, the debug window can be used to execute individual line of code, View or change values of variables and properties, and view watch expressions. At run time, it can be used to display data or messages as the programs as the program runs.
At design time, the developer can view the previous output to the Debug window, but cannot execute code. The debug window has two part- Watch window and immediate window. The split bar separates the Debug window into two panes. The upper pane display the Watch Window. The lower pane display the Immediate Window. Watch Window Watch pane displays the current watch expressions, which are expressions whose values are decided by the user as code is executed.
The Watch pane appears automatically if the watch expressions are defined in the project. At times we want to monitor the value of a variable for a certain state- for example to determine weather a flag is set to True or False. Steps to add watch expression 1. Add watch command is chosen from the Debug menu. The Expression is entered in the Expression box. If necessary, the scope of the variables is set and an option button is selected. The OK button is clicked. Immediate Window The immediate pane appears by default the first time the debug window is opened.
From break mode, the code is executed immediately by entering it in this pane. This window is the right place for the. We can enter any valid expression in this window and VB will execute it. If a reference is made to an object outside the scope of the current code execution, Visual Basic will generate an error.
Testing data and procedure in Immediate Pane While debugging an application, Sometimes it may necessary to execute individual procedures, evaluate expressions or assign new values to the variables or properties.
The Immediate Pane can be used to accomplish new values to the variables or properties. The Immediate Pane can be used to accomplish such tasks. Expressions can be evaluated by printing in Immediate Pane. Some advantages of these facility. On executing the above code the value of c is displayed in the Immediate window using the Debug. Using a Breakpoint to selectively Halt Execution At run time, a breakpoint tells VB to halt before executing a specific line of code.
A breakpoint can be set or removed at design time or at break mode. To set or remove a breakpoint, The insertion point is moved to the line of code where the breakpoint is to be set or removed. Writing Error Handlers Visual Basic has specific built-in ways to handle runtime errors, called trappable errors. When such an error occurs, you can direct the execution of your program to an error handler, which is a section of code written specifically to deal with errors.
On Error GoTo Label. The visual basic On error Goto statement is the foundation of handling trappable errors. Execution is transferred to the code starting at label if a trappable error has occurred.
The code following that label is your error handler. It is a object Contains information about run-time errors. When a run-time error occurs, the properties of the Err object are filled with information that uniquely identifies the error and information that can be used to handle it.
To generate a run-time error in your code, use the Raise method. Main Properties of Err object Err. Return error number Err. Return error description e. MsgBox "Des: Besides using a label to start an error handler, you can refer to an error handler by line number in visual basic, using on error goto line statement.
Once you execute this statement, execution continues with the next line of code if the current line generates an error, and the error is disregarded.
Err Function Returns a Variant of subtype Error containing an error number specified by the user. Syntax CVErr error number The required errornumber argument is any valid error number. Remarks Use the CVErr function to create user-defined errors in user-created procedures.
For example, if you create a function that accepts several arguments and normally returns a string, you can have your function evaluate the input arguments to ensure they are within acceptable range. If they are not, it is likely your. In this event, CVErr allows you to return an error number that tells you what action to take.
Note that implicit conversion of an Error is not allowed. For example, you can't directly assign the return value of CVErr to a variable that is not a Variant.
However, you can perform an explicit conversion using CInt, CDbl, and so on of the value returned by CVErr and assign that to a variable of the appropriate data type. Print CalculateDouble " End Function. The first way is working with the special controls that support them. What are Databases? Nearly all business applications need to store large volumes of data, organized in a format that simplifies retrieval. This is accomplished with a database management system DBMS , a mechanism for manipulating tabular with high-level commands.
The database management system hides low-level details, such as how data are store in a database, and frees the programmers to concentrate on managing information, rather than on the specifics of manipulating files or maintaining links among them.
A collection of fields together, such as the Name and Grade fields in our table, make up a record. Each record gets its own row in a table, and each column in that row represents a different field. A collection of records- that is, row of records where each column is a field-become a table. A database is just a collection of one or more tables. In this chapter we discuss about three architectures. Centralized System Architecture Companies that needed real computing power turned to the mainframe computer, which is centralized system architecture.
The salient most feature of a centralized system is that the only movement of marshalling are keystrokes between the client and the host machine.
Marshalling is the process of packing interface elements and sending them across process boundaries. Thus in a centralized system, keystrokes are marshalling from the terminal client to the host. The centralized system architecture is illustrated in fig. Application Data Data. In personal computers made an entry into the business world along with which, came the resources like printers, modems and hard-disk storage. The introduction of the local area network LAN and the use of file server architectures soon following the introduction of personal computer.
File Server Architecture The file server system brought a complete change in implementation of the computer architecture from the mainframe. In this system, the application logic now executed on the client workstation instead of the server. These servers also provided access to computing resources like printer and large hard disk. The complete File Server architecture is illustrated in fig.
The merit of the file server system is the low cost entry point with flexible arrangement. Computer resources can be added or reduced as and when necessary using this system. The drawback of the file server architecture is that all application logic is executed on the client machine. The job of the server is to provide files only to store the data.
Taking into account the de-merits of the centralized and file server system architecture, the client server architecture made its advent. Client Server Model Client server architecture is a process involving a minimum of two independent entities, one is the client and the other is the server. The client makes a request to the server and the server services the requests and the resulting data is sent to the client. In this application, two separate applications operating independently could work together to complete a task.
The client server architecture is well illustrated in fig. Instead, instructions are communicated to an application running on the server, and the server executes those requests and sends a response to the client. Terminal1 Terminal2 Terminal3 Terminal4. Logic1 App. Logic2 App. Logic3 App.
The database engine in turn processes the request and a result set is send to the client. Thus the two independent process work together to accomplish a task and exemplifies the client-server relationship. These applications are referred to as backend and offer support for storage manipulation and retrieval of business persistent data. They use structure query language SQL.
As a standard method for submitting client requests. Now let us move on to discuss the creation of in information system that can change with business needs, needs that mandate tighter budget and higher quality in addition to simply processing transactions generating report.
The logic in two in two-tier model is split between the two physical location namely the server and client. The salient point for a two-tier application is that the business logic must physically reside either on the client or be implemented on the back-end within the DBMS in the form of triggers and stored procedures. Both triggers and procedures are stored as precompiled collections of SQL statements and control-of-flow statements.
Merits of two-tier The merits of this two-tier model are — data access is simplified, allowing very rapid development of applications. The GUI is bound directly to the data source and all the details of data manipulation are handled automatically.
But there is a disadvantage in this, i. Not Scalable: Inability of two-tier approach to grow beyond the physical boundaries of a client and server machine. Business rules cannot be encapsulated and deployed centrally because of which sharing common process and reusing work become difficult. Poor Performance: The graphical interface binding to the data consumes major resources on the client machine, which results in poor performance and dissatisfied clients.
Partitioning an application breaks our code into logical components.
The Service Model employed to design this architecture suggests that these components can be logically grouped into three tier. User services, Business services and Data services. The service and its location are given below.
On developing an application using this model and technique, each component can then be placed on a suitable machine that will provide the best performance depending on our situation and the current business need. The time invested in designing and implementing components is not wasted as they can be shared among applications.
As components can be placed on machines other than the client workstation, load processing can be shifted from a client machine that may be underpowered to a server with extra horsepower. Manageability Encapsulation of applications services into components allows us to break down large, complex applications into more manageable pieces.
They become easier to redeploy on making any amendments to keep pace with business needs. They are: It communicates with the data source through the JET database engine. It provides a framework for using code to create and manipulate components of a remote ODBC database system. This is a programming model that eliminates the need to choose from among DAO and RDO and all other data access methods.
It is designed to provide a common bridge between different database, files systems and e-mail servers. They provide properties and methods that allow to accomplish all the operations to manage such a system, including features for creating databases, defining tables, navigating and querying a database. The DBEngine is the top level database object and corresponding to the Jet database engine.
This object is used to set database engine system parameters and default workspace. The Workspace object is used to support simultaneous transactions and it acts as a container for open database. A default Workspace object Workspace 0 is created when the DAO are referenced in the language at run time.
The default workspace is always available and never be closed or removed from the collection. The TableDef object corresponds to stored table definition. Each TableDef in a collection represents the Definition of a current table in a database or an attached table in the external database. The Recordset object corresponds to a cursored view into a database table or the results of query. A cursored view is that stores rows of data in buffer and points to one row of data at a time called current record.
The cursor may be positioned to any row of data using Move, Seek or find methods. The field object corresponds to a column of data type and set of properties. TableDef, Recordset objects have collection of field objects.
The collection of Field object associated with Recordset cursor describes a single row of data. The index object is a stored index allows to quickly reorder the records in a table. A parameter object represents a parameter associated with a QueryDef object created from a parameter query.
The User object is used to define and enforce database security. The DBEngine object supports a set of user collection. The Users collection contains all stored User objects supports a collection of system groups. Each user in the group inherits the permissions to access the objects that the group can access. A Relation object represents a relationship between fields in the tables or queries.
A Relations collection contains stored Relation objects of Database object. The DBEngine enforces certain update and delete conditions on the data associated with the fields of the Relation object, to maintain referential integrity.
A property object represents a built-in characteristic or user-define characteristic of a data access object. Properties collection contain all the property object for a specific instance of an object. Accessing and Navigating Databases In order to work with data objects, a reference has to be set to the appropriate DAO library. To set reference to DAO type library: Opening Database To open an existing database, the OpenDatabase method of workspace object is used Syntax Opendatabase dbname,[option],[readonly],[connect].
By default, a database that is opened can be shared and modified by any user. To specify that the database is to be opened fro exclusive use, the following statement can be used:. In the above code True value indicates that no users will be able to open the database. In the above statement, the True value specified as the third argument will provide only a read access on the database. A recordset is an object that contains a set of records from the database. There are mainly five types of Recordset object.
Table-Type Recordset: The table type recordset object is a set of records that represents a single table can be used to add, change or delete records. They are fastest type of Recordset. Dynaset-Type Recordset: The dynaset-type recordset object is a set of records that represent a table, or attachment tables, or the results of queries containing fields from one or more tables.
A dynaset enables us to update data from more than one table. Snapshot-Type Recordset: A snapshot cannot be updated and does not reflect changes to data made by the users. Dynamic type Recordset: This recordset type represent a query result set from one or more base tables in which we can add, change or delete records from a row returning query. Further, records that other users add, delete, or edit in the base tables also appear in our recordset.
Forward Only Type recordset: This recordset type is identical to a snapshot except that we can only scroll forward through its records. This improves performance in situations where only need to make a single pass through a result set. Creating a Recordset: The OpenRecordset method is used to open a Recordset and create a recordset variable.
To create a read-only recordset for the table emp, the following code is used. In the above statement, db is the variable that represents the database object. Here dbOpenTable specifies the type of the Recordset to be created.
Navigating a Recordset: After creating a Recordset object, the various methods can be used to navigate through the recordset. The BOF Beginning of file property is true when the user moved to a position before the first record in the recordset. Modifying and Deleting Records To manipulate a recordset, the following methods are used. Edit Method: The user can edit the current record using the edit method.
The Update method is used to save the necessary changes made to the records.
AddNew Method: AddNew method is used to add a blank record in the recordset. Delete Method: This method can be used to delete record in the dynaset-type recordset. The jet engine deletes the current record without any warning when the Delete method used.
Finding Records The Find methods can be used to locate a record in dynaset-type or snapshot-type Recordset. VB supports four Find methods. Once this driver is available, the client machine should be configured with this driver. Double click on ODBC configuration option under control panel.
A list of ODBC data source is displayed as shown in fig. Select Add button to add new ODBC drivers that are currently installed in system are displayed as shown in fig.
Type any Data source name and select database for which you want to create DSN by clicking on Select button. Click on OK button to complete process. Syntax Dim db as Database.
A connection object, which establishes a connection to the database, be it a local file or remote SQL Server. A command object, which executes commands against the database. A Recordset Object, which holds the records retrieved from the database or the records to be updated on the database. The Connection object is used to specify a particular provider and any parameters.
To connect to a data source, you use a Connection object. An ADO transaction marks the beginning and end of series of data operations that are executed across a connection. You can cancel or allow changes. This application lets you open the db1. You can move through the database using arrow buttons you see in fig. To edit a record, just type the new value into the text box and click on update button. To add new record, use add button and insert new values, after it click on update button.
Here are the arguments for this method. Connection String-String containing connection information. UserID- String containing a username to use when establishing the connection Password — String containing a password to use when establishing the connection.
OpenOptions - If set adConnectAsync, the connection will be opened asynchronously. Connection cn. End sub. Source- A valid command object variable name, an SQL statement, a table name, a stored procedure call, or the file name of Recordset. ActiveConnection- A valid Connection object variable name or a string containing ConnectionString parameter. Opens a dynaset-type recordset object, which is like an ODBC keyset cursor.
Open a static type recordset object. Opens a forward-only-type Recordset object, where you can only use MoveNext to move. A value what determines what type of locking the provider should use when opening the record set.
Here are the possible values for the LockType: Pessimistic locking, record by record. Optimistic locking, record by record. Recordset cn. Binding Controls to Record sets To bind a control to ADO Recordset objects, you just set that controls DataSource property to that object, and then set whatever other data properties that control needs to have set.
A single value or an array of values for the fields in the new record. If Fields is an array, values must also be an array with the same number of members. Updating A Records in a Record Set. Here are the arguments for this method: Fields - A single name or an array of names or ordinal positions of the fields in the new record. Values - A single value or an array of values for the fields in the new record. If fields is an array, Values must also be an array with the same number of members.
Method moves to the Previous row in the Recordset. The Jet-engine deletes the current record without any warning when the Delete method used. Input Validation What is Input Validation? Input validation is the process of checking the data entered by the user before that data saved to the database.
This is some times referred to as Client-side validation. Common Input Validation Rules Almost every field in your database required some type of input validation. Before you design form, put together a list of all fields you need on the form and answer the following questions. Is it required field? If space are not allowed, make sure spacebar is disabled.
If there is a limited set of valid inputs for a field, give the user a pick list or set of radio buttons to choose from.
If the field has the low and high limit, tell the user what the limits are. Field level Validation The first level of validation is at the field level. This is the place where you can make sure the user is entering the right character in the field, entering the data into the field in proper format, and entering a valid value based on a list of possible choices.
You declare a string that holds the list of valid keys. The next line loads the string with the valid key fields, and the next line checks to see weather the key pressed is in the string of valid keys.
It does this by converting the numeric value passed by Visual Basic in the keyascii parameter the ASCII value of key Pressed into readable character using function chr. Input Masking It is very common to have fields on your form that required special input formats. VB 6 ships with a bound data control that handles special input and display formatting: The MaskedEdit control works like the standard VB 6 textbox control, with few added properties that make it a powerful tool for your input validation.
OCX select it and click on OK button. The Masked Edit control provides restricted data input as well as formatted data output. This control supplies visual cues about the type of data being entered or displayed. This is what the control looks like as an icon in the Toolbox:. Properties Format: Specifies the format for displaying and printing numbers, dates, times, and text. You can define input masks at both design time and run time.
However, the following are examples of standard input masks that you may want to use at design time. The control can distinguish between numeric and alphabetic characters for validation, but cannot check for valid content, such as the correct month or time of day. Acts like a standard text box. Medium time. Short time. Mask Description character Digit placeholder.
Decimal placeholder. The actual character used is the one specified as the decimal placeholder in your international settings.
This character is treated as a literal for masking purposes. The actual character used is the one specified as the thousands separator in your international settings. Time separator. The actual character used is the one specified as the time separator in your international settings. The actual character used is the one specified as the date separator in your international settings. Valid values for this placeholder are ANSI characters in the following.
A Alphanumeric character placeholder entry required. For example: A Alphanumeric character placeholder entry optional. C Character or space placeholder entry optional.
Letter placeholder. Literal All other symbols are displayed as literals; that is, as themselves. The PromptChar property can only be set to exactly one character. To clear the Text property when you have a mask defined, you first need to set the Mask property to an empty string, and then the Text property to an empty string: Form Level Validation: Form level validation is an essential part of designing a good validation scheme for your form.
Although many input errors can be caught and corrected at the field level, there are several validation steps that can be performed well only at the form level. The form level validation is performed at the time when the user presses Enter, or clicks the OK or Save button. These are validations that are done after the user has entered all the fields, but before any attempt is made to store the values to a table.
Visual Basic 6. These notes are developed for visual basic 6. Flag for inappropriate content. Related titles. Jump to Page. Search inside document.
Notes For Visual Basic 6. Trivedi Introduction to Visual Basic 6. Trivedi This is bar display the commands that are required to build an application. Tool Bar The toolbar in the menu bar provide quick access to the commonly used commands. Fig 1. Trivedi 2. This variable is only available to the code inside the procedure and can be declared using dim statement as given below Dim I as integer Static Variables Static variables are not reinitialized each time visual basic invokes a procedure and thus retains or preserves value even when a procedure ends.
Static in as integer Module Level Variables Module level variables are available to all the procedure in the module. Trivedi String 0 to approximately 2 billion characters Currency ,,,, A procedure can be sub, function or property procedure Sub Procedure A sub procedure can be placed in standard, class and form modules. Execute when load form. It is good programming practice to write common statements in separate procedure and then call them in the event procedure Function Procedures Function are like sub procedures, except they return a value to calling procedure.
The following code entered in the Form Load procedure illustrates the structure. Trivedi A sequence of variables by the same name can be referred using array. The size of can be changed Fixed Size Array Fixed array can be declared by giving a name with the upper limit in the parentheses. Dim newarray The actual elements can be allocate using ReDim statement ReDim newarray n Each time on the executing the ReDim statement, the current data of array is lost and the default value is set.
But if we want to change the size of array without lost any previous elements then use keyword Preserve ReDim Preserve newarray 20 User-Define Data Type Variable of different data types when combined as a single variable to hold several related information is called user-define data type.
Private type student Name as string Age as integer Per as double End type e. Creating variables Dim s1 as student e. Trivedi Dim s 10 as student Accessing members of user-define type Variable name. Syntax DateDiff interval, date1, date2[, firstdayofweek[, firstweekofyear]] The DateDiff function syntax has these named arguments: Trivedi firstweekofyear Optional. Caption The title for the window is store in the caption property.
Trivedi MousePointer This property sets the value that indicate the type of mouse pointer displayed. Moveable By setting it false it is possible to prevent the user from moving window.
StartUpPosition Set the starting position of window. Displaying Forms Show Method The show method is used to display the form object. Hide Using the Unload Statement The unload statement removes the form from the display and releases the memory.
Unload frmcalculator Setting Run time and design time properties Many properties can be set at either design time or runtime. Trivedi Form1.
Controls Classification of controls Visual Basic Controls are broadly classified as standard controls, Active X controls and insert-able objects. Sorting the List To sort the list set sorted property to True. Dropdown Combo style 0 Simple Combo style 1 Dropdown list style 2 Using ScrollBar Control The Scrollbar is a commonly used control, which enables the user to select a value by positioning it at the desired location.
Title Bar Menu Bar Separator bar The menu bar appears appear below the title bar and it may contain one or more menu titles. Trivedi Using Access and Shortcut keys Access keys allow the user to open a menu by pressing the Alt key with a letter key. Trivedi presses any mouse button and MouseUp occurs when the user releases any mouse button.
The third and fourth arguments X and Y are the coordinates of the mouse location at the mouse button was clicked e. CurrentY - X, Y End If End Sub Dragging and dropping Dragging is a process of clicking the mouse button in a control and moving the mouse while holding down the mouse button.
Trivedi Here in e. Trivedi The number of times a twip is used by all visual basic movements, sizing and graphical drawing statements is limited to one. The position, length, color and style of the line control can be positioned to customize the look of an application Shape Control A shape control is a Visual element that contains several predefined shapes.
The following statement draw a point on the form Form1.
Show End Sub Debugging in Visual Basic Debugging is a process by which error are identified and resolved in source code. Trivedi Compile-time bugs are those that occur when we attempt to create the program executable file EXE or run the project. Choose the Editor tab in the Option dialog box Require Variable declaration: They are Breakpoint: Trivedi Visual Basic provides several built-in methods for controlling the execution of the program in real time.
Trivedi users to modify data or test functions during development.