Visual Basic Questions
What are the three main differences between flexgrid control and dbgrid(Data bound Grid) control
The Microsoft FlexGrid (MSFlexGrid) control displays and operates on tabular data. It allows complete flexibility to sort, merge, and format tables containing strings and pictures. When bound to a Data control, MSFlexGrid displays read-only data.Adaptation to existing Visual Basic code for the data-bound grid (DBGrid).
dbgrid is A spreadsheet-like bound control that displays a series of rows and columns representing records and fields from a Recordset object.
The data grids are bound controls; that is, they require a data source that actually connects to a database and retrieves their data. And it seems that the root of the problem with DBGrid is that there's no data source that can be readily included along with the DBGrid control.
In Visual Basic, the solution is simply to include the Data Control on the same form as DBGrid. But the Data Control is an intrinsic control; it's unavailable to anything outside of the Visual Basic environment itself. and VB 6.0 has a new set of data controls (DataGrid, DataList, DataCombo, MSHFlexGrid, MSFlexGrid) that once again are bound controls. Unlike DBGrid, though, they support OLE DB, and therefore rely on the an ADO Data Source (and in particular the ActiveX Data Objects Data Control, or ADO DC) for data access. Unlike the Data Control, the ADO
DC is a custom control (that is, an .OCX) that can be added to any project. In short, if you add ADO DC to your project along with the
DataGrid control.
ActiveX and Types of ActiveX Components in VB ?
Standard EXE
ActiveX EXE
ActiveX DLL
ActiveX document
ActiveX Control
What is difference between inprocess and out of process ?
An in-process component is implemented as a DLL, and runs in the same process space as its client app, enabling the most efficient communication between client and component.Each client app that uses the component starts a new instance of it.
An out of process component is implemented as an EXE, and unlike a dll, runs in its own process space. As a result, exe's are slower then dll's
because communications between client and component must be marshalled across process boundaries. A single instance of an out of process component can service many clients.
Advantage of ActiveX Dll over Active Exe ?
ACTIVEX DLL:
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An in-process component, or ActiveX DLL, runs in another application’s process. In-process components are used by applications or other in-process components. this allows you to wrap up common functionality (like an ActiveX Exe).
ACTIVEX EXE:
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An out-of-process component, or ActiveX EXE, runs in its own address space. The client is usually an application running in another process.The code running in an ActiveX Exe is running in a separate process space. You would usually use this in N-Tier programming.
An ActiveX EXE runs out of process while an ActiveX DLL runs in the same process space as VB app. Also, and ActiveX EXE can be run independent of your application if desired.
Explain single thread and multithread thread apartments
All components created with Visual Basic use the apartment model, whether they’re single-threaded or multithreaded. A single-threaded component has only one apartment, which contains all the objects the component provides.
This means that a single-threaded DLL created with Visual Basic is safe to use with a multithreaded client. However, there’s a performance trade-off for this safety. Calls from all client threads except one are marshaled, just as if they were out-of-process calls.
What is a Component?
If you compile an ActiveX dll, it becomes a component.
If you compile an ActiveX Control, it becomes both a component and a control. Component is a general term used to describe code that's grouped by functionality. More specifically, a component in COM terms is a compiled collection of properties/methods and events.
Typically a component is loaded into your project via the References whereas an ActiveX Control is loaded into your project via "components".
What is meant by "Early Binding" and "Late Binding"? Which is better?
Early binding and late binding refer to the method used to bind an interface's properties and methods to an object reference (variable). Early binding uses type library information at design time to reference procedures, while late binding handles this at run time. Late binding
handles this by interrogating the reference before each call to insure that it supports a particular method. Since every call to a late bound
object actually requires two calls
("Do you do this?" followed by "Okay, do it then"), late binding is much less efficient than early binding. Except where early binding is not supported (ASP, scripting, etc.), late binding should only be used in very special cases.
It is a common misconception that any code using the CreateObject function instead of Set = New is using late binding. This is not the case. The type declaration of the object variable determines whether
it is late or early bound, as in the following:
Dim A As Foo
Dim B As Foo
Dim C As Object
Dim D As Object
Set A = New Foo 'Early Bound
Set B = CreateObject("FooLib.Foo") 'Early Bound
Set C = CreateObject("FooLib.Foo") 'Late Bound
Set D = New Foo 'Late Bound
What are the Advantages of disconnected recordsets?
A disconnected Recordset, as its name implies, is a Recordset that lacks a connection.
seen that a Recordset that does not have a database connection can be very useful as a tool in your programming. It can save you time and effort and make your code more scalable.
In order to create a disconnected Recordset two Recordset properties must be set appropriately.
It is a requirement that the CursorLocation property is set to adUseClient and the LockType property is set to adLockBatchOptimistic. Note that the CursorType will default to adUseStatic if we don't explicitly state that it should be set to adUseClient.) i.e
rst.LockType = adLockBatchOptimistic
rst.CursorLocation = adUseClient
However, we've recently discovered that these steps aren't necessary. VB automatically assigns batch optimistic locking to newly created,
connectionless recordsets. And, of course, without a connection, a recordset can't have any other cursor but a client-side one. To create one of these structures, then, the only thing you need do is create
the object variable instance. After that, you can simply begin adding fields to the construct.
To add fields, you use the Fields collection's Append method. This method requires two parameters , the field name and the field data type. So, to create a connectionless recordset with two fields,you'd use code similar to:
Dim rst As ADODB.Recordset
Set rst = New ADODB.Recordset
rst.Fields.Append "CustID", adVarChar
rst.Fields.Append "CustName", adVarChar
Additional, optional Append method parameters include DefinedSize and Attrib. The DefinedSize argument takes the size of the field. Fill the Attrib parameter with constants that define additional field characteristics, such as whether it will allow null values or is updatable.
Since, in our technique, we want the fields to mirror the structure of the original recordset, we'll simply use existing values for these parameters.
Disconnected Recordsets, first available with ADO 2.0, are the most commonly used mechanism to retrieve a Recordset and open a connection for only the necessary amount of time, thus increasing scalability. They are call disconnected because the connection to the database is closed.
The collections, properties, and methods of a disconnected Recordset are still available even though the connection is closed. This frees up server resources, given that the number of open connections is limited and database locking is a non-issue.
What are Benefits of wrapping database calls into MTS transactions?
If database calls are made within the context of a transaction, aborting the transaction will undo and changes that occur within that transaction.
This removes the possibility of stranded, or partial data. Transaction that uses the Microsoft® Transaction Server (MTS) environment. MSMQ implicitly uses the current MTS transaction if one is available.
BENIFTIS OF USING MTS :
Database Pooling, Transactional operations, Deployment, Security, Remote Execution This allows MTS to reuse database connections. Database connections are put to ?sleep? As opposed to being created and destroyed and are activated upon request.
How to register a component?
Compiling the component, running REGSVR32 MyDLL.dll
Controls which do not have events ?
Shape and line controls are useful for drawing graphical elements on
the surface of a form. These controls don't support any events; they
are strictly for decorative purposes.
EXTRA INFO::
The image, shape and line controls are considered to be lightweight controls; that is, they support only a subset of the properties, methods, and events found in the picture box. Because of this, they typically require less system resources and load faster than the picture box control.
What are the Control Categories
a)Intrinsic controls:
such as the command button and frame controls. These controls are contained inside the Visual Basic .exe file. Intrinsic controls are always included in the toolbox
b)ActiveX controls:
which exist as separate files with a .ocx file name extension. These include controls that are available in all editions of Visual Basic (DataCombo, DataList controls, and so on) and those that are available only in the Professional and Enterprise editions (such as Listview, Toolbar, Animation, and Tabbed Dialog). Many third-party ActiveX controls are also available.
c)Insertable Objects:
such as a Microsoft Excel Worksheet object containing a list of all your company's employees, or a Microsoft Project Calendar object containing the scheduling information for a project. Since these can
be added to the toolbox, they can be considered controls.Some of these objects also support Automation (formerly called OLE Automation),which allows you to program another application's objects from within a Visual Basic application.
DIFF between Image and Picture box controls?
The sizing behavior of the image control differs from that of the picture box. It has a Stretch property while the picture box has an AutoSize property. Setting the AutoSize property to True causes a picture box to resize to the dimensions of the picture; setting it to False causes the picture to be cropped (only a portion of the picture is visible). When set to False (the default) , the Stretch property of the image control causes it to resize to the dimensions of the picture.
Setting the Stretch property to True causes the picture to resize to the size of the image control, which may cause the picture to appear
distorted.
Default property of datacontrol ?...
connect property......
Define the scope of Public, Private, Friend procedures?
The set of public variables, methods, properties, and events described in a class module define the interface for an object. The interface consists of the object members that are available to a programmer who's using the object from code.
You can create private variables, methods, properties, and events that are used by other procedures within the class module but are not part of the object's public interface. Additionally, constants user-defined types, and Declare statements within a class module must always be private.
The Friend keyword makes a procedure private to the project: The procedure is available to any code running within the project, but it is not available to a referencing project.
Describe Database Connection pooling relative to MTS ?
This allows MTS to reuse database connections. Database connections are put to ?sleep? as opposed to being created and destroyed and are activated upon request.
Object pooling is an important design concept required for high-performance applications. A performance optimization based on using collections of preallocated resources, such as objects or database connections. Pooling results in more efficient resource allocation.
Difference between a function and a subroutine ?
A function accepts any number of parameters (possibly zero), does something with them, and returns a value. A subroutine is performs an action, but doesn't return a value.
There are two differences between a function and a subroutine: A)How they are invoked. B)How they are accessed.
A function call has the following syntax ::
function(arg1, arg2, ...)
where: function Is the name of the function. arg1, arg2, ... Are the arguments.
A subroutine call has the following syntax
::subroutine (arg1, arg2, ... {outputfield|'format'})
where: subroutine -->Is the name of the subroutine. arg1, arg2, ... Are the arguments. {outputfield|'format'} Is the name of the output field or its format.
In addition, on some platforms, the functions are available immediately; whereas, the subroutines are available in a special subroutine library that you must access.
Difference between Linked Object and Embedded Object?
Embedding objects -
When you embed an object, a copy of the object is inserted into the destination document. There's no link to the original file. When you change information in the source document, no changes will be reflected in the destination document. The actual data for the object is stored within the destination file. To make changes to the embedded object, double click it and it will launch the original application the source file was in.
Linking objects -
Information is updated when you modify the original source file when you use a linked object. This dynamic updating is very handy for things such as the aforementioned monthly report. You can open up the Excel spreadsheet that is referenced within your Word document.Make changes to the spreadsheet, close Excel, and when you open your Word document... viola! The changes are already there. If that object is linked to ten other Word files, the changes are already in those ten files, too! actually linking or embedding an object is fast and easy.
Difference between listbox and combo box?
A LISTBOX CONTROL displays a list of items from which the user can select one or more. If the number of items exceeds the number that can be displayed, a scroll bar is automatically added to the ListBox control. A COMBOX CONTROL combines the features of a text box and a list box. This control allows the user to select an item either by typing text into the combo box, or by selecting it from the list.
DIFF::Generally, a combo box is appropriate when there is a list of suggested choices, and a list box is appropriate when you want to limit input to what is on the list. A combo box contains an edit field, so choices not on the list can be typed in this field.
Difference between Dynaset and Snapshot?
All Recordset objects are constructed using records (rows) and fields (columns). There are five types of Recordset objects:
Table-type Recordset ::
representation in code of a base table that you can use to add, change, or delete records from a single database table (Microsoft Jet workspaces only).
Dynaset-type Recordset ::
the result of a query that can have updatable records. A dynaset-type Recordset object is a dynamic set of records that you can use to add, change, or delete records from an underlying database table or tables. A dynaset-type Recordset object can contain fields from one or more tables in a database. This type corresponds to an ODBC keyset cursor.
Snapshot-type Recordset ::
a static copy of a set of records that you can use to find data or generate reports. A snapshot-type Recordset object can contain fields from one or more tables in a database but can't be updated. This type corresponds to an ODBC static cursor.
Forward-only-type Recordset::
identical to a snapshot except that no cursor is provided. You can only scroll forward through records. This improves performance in situations where you only need to make a single pass through a result set. This type corresponds to an ODBC forward-only cursor.
Dynamic-type Recordset ::
a query result set from one or more base tables in which you can add, change, or delete records from a row-returning query. Further, records other users add, delete, or edit in the base tables also appear in your Recordset. This type corresponds to an ODBC dynamic cursor (ODBCDirect workspaces only).
Difference Listindex and Tab index?
LIST INDEX::
Returns or sets theindex of the currently selected item in the control. Not available at design time.Default LIST INDEX IS -1 for ComboBox, DirListBox, and DriveListBox controls
TAB INDEX::
Returns or sets thetab order of most objects within their parent form. Visual Basic automatically renumbers the TabIndex of other controls to reflect insertions and deletions. You can make changes atdesign time using theProperties window or atrun time in code.The TabIndex property isn't affected by the ZOrder method.
Difference modal and moduless window?
MODAL forms are forms which require user input before any other actions can be taken place. In other words, a modal form has exclusive focus in that application until it is dismissed. When showing a modal form, the
controls outside this modal form will not take user interaction until the form is closed. The internal MsgBox and InputBox forms are examples of modal forms. To show a form modally, use the syntax:
MyForm.SHOW.vbModal ' a predeclared constant for 1
MODELESS forms are those which are shown but do not require immediate user input. MDI child forms are always modeless. To show a form modeless, use the syntax:: MyForm.SHOW