13 Things to keep in mind before using DLL in Delphi

Keep in mind the following tips when writing your DLL:

1. Make sure you use the proper calling convention (C or stdcall).

2. Know the correct order of the arguments passed to the function.

3. NEVER resize arrays or concatenate strings using the arguments passed directly to a function.

4. Remember, the parameters you pass are LabVIEW data. Changing array or string sizes may result in a crash by overwriting other data stored in LabVIEW memory. You MAY resize arrays or concatenate strings if you pass a LabVIEW Array Handle or LabVIEW String Handle and are using the Visual C++ compiler or Symantec compiler to compile your DLL.

5. When passing strings to a function, remember to select the correct type of string to pass . C or Pascal or LabVIEW string Handle.

6. Remember, Pascal strings are limited to 255 characters in length.

7. Remember, C strings are NULL terminated. If your DLL function returns numeric data in a binary string format (for example, via GPIB or the serial port), it may return NULL values as part of the data string. In such cases, passing arrays of short (8-bit) integers is most reliable.

8. If you are working with arrays or strings of data, ALWAYS pass a buffer or array that is large enough to hold any results placed in the buffer by the function unless you are passing them as LabVIEW handles, in which case you can resize them using CIN functions under Visual C++ or Symantec compiler.

9. Remember to list DLL functions in the EXPORTS section of the module definition file if you are using _stdcall.

10. Remember to list DLL functions that other applications call in the module definition file EXPORTS section or to include the _declspec (dllexport) keyword in the function declaration.

11. If you use a C++ compiler, remember to export functions with the extern .C.{} statement in your header file in order to prevent name mangling.

12. If you are writing your own DLL, you should not recompile a DLL while the DLL is loaded into memory by another application (for example, your VI). Before recompiling a DLL, make sure that all applications making use of the DLL are unloaded from memory. This ensures that the DLL itself is not loaded into memory. You may fail to rebuild correctly if you forget this and your compiler does not warn you.

13. Test your DLLs with another program to ensure that the function (and the DLL) behave correctly. Testing it with the debugger of your compiler or a simple C program in which you can call a function in a DLL will help you identify whether possible difficulties are inherent to the DLL or LabVIEW related.

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Anatomy of document.ready function in jQuery

All jQuery methods come inside a document.ready() function. This is to prevent any jQuery code from running before the document is finished loading (is ready).

Here are some examples of actions that can fail if functions are run before the document is fully loaded:

1. Trying to hide an element that doesn't exist
2. Trying to get the size of an image that is not loaded

document.ready syntax:

<div id="divTest1"></div>
<script type="text/javascript">
function DocumentReady()
{
        $("#divTest1").text("Hello, world!");   
}

$(document).ready(DocumentReady);
</script>

<div id="divTest2"></div>
<script type="text/javascript">
$(document).ready(function()
{
        $("#divTest2").text("Hello, world!");   
});
</script>

But of course, this syntax wasn't even short enough for the jQuery team, so they decided to create a version (overload) of the jQuery constructor which takes a ready function as a parameter, to make it even shorter:

<div id="divTest3"></div>
<script type="text/javascript">
$(function()
{
        $("#divTest3").text("Hello, world!");   
});
</script>

In the last example, our anonymous function is passed directly to the jQuery constructor, which assigns it to the ready event. As you will see when you test the code, the event is fired as soon as the page is loaded, most of the time so fast that you won't even realize it.

Difference between body onload() function used in javascript and document.ready() function used in jQuery

Document.ready() function is different from body onload() function because of 2 reasons:

1. We can have more than one document.ready() function in a page where we can have only one onload function.

2. Document.ready() function is called as soon as DOM is loaded whereas body.onload() function is called when everything gets loaded on the page that includes DOM, images and all associated resources of the page.

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How to abort an AJAX call using jQuery?

There may be situations where you need to cancel a running AJAX request before it ends. It's usually in cases where the user might perform an action, which sets of an AJAX request, several times within a short time period.

A good example of this is auto-complete functionality for a search box, where you might try to help the user by finding related search terms based on their current input, by making an AJAX request each time they press a key in the search field. In that case, it's very likely that the user types faster than your AJAX request can be performed and therefore you would want to abort any non-finished requests, before starting the next one.

Consider the following example:

<input type="button" name="btnDoRequest" value="Start" onclick="PerformSimpleCalculation();" />

<script type="text/javascript">

function PerformSimpleCalculation()
{
        $.get("/tests/calc.php", function(data, textStatus)
        {
                alert(data);
        });
}
</script>

It requests a PHP script which is doing a very complicated calculation (as you will see from the result), which means that it usually takes ~3 seconds to finish. Now, try the example and push the button several times after each other. The same "calculation" will be performed multiple times and the result will also be displayed multiple times (with a 3 second delay).

Fortunately, a call to the get() method and pretty much any other jQuery AJAX method, returns an object which, among others, contains an abort() method. We can save this reference and then call the abort() method on it if needed. Have a look at this slightly modified example:

<input type="button" name="btnDoRequest" value="Start" onclick="PerformAbortableCalculation();" />

<script type="text/javascript">

var calculationRequest = null;

function PerformAbortableCalculation()
{
        if(calculationRequest != null)
                calculationRequest.abort();
        calculationRequest = $.get("/tests/calc.php", function(data, textStatus)
        {
                alert(data);
        });
}
</script>

We start off by defining a common variable for containing the request reference. In the PerformAbortableCalculation() method, we assign the return value of the get() call to this variable, but before we do so, we check to see if it's null (the method hasn't been used yet) and if not, we call the abort() method on it. If you try this example and click several times, you will see that no matter how many times you click the button, it only executes the callback function once.

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Positioning Property and Z-Index in CSS

The CSS positioning properties allow you to position an element. It can also place an element behind another, and specify what should happen when an element's content is too big.

Elements can be positioned using the top, bottom, left, and right properties. However, these properties will not work unless the position property is set first. They also work differently depending on the positioning method.

There are four different positioning methods:

1. Static Positioning

HTML elements are positioned static by default. A static positioned element is always positioned according to the normal flow of the page. Static positioned elements are not affected by the top, bottom, left, and right properties.

2. Fixed Positioning

An element with fixed position is positioned relative to the browser window. It will not move even if the window is scrolled:

p
{
position:fixed;
top:30px;
right:5px;
}

Fixed positioned elements are removed from the normal flow. The document and other elements behave like the fixed positioned element does not exist.

3. Relative Positioning

A relative positioned element is positioned relative to its normal position.

h2
{
position:relative;
left:-20px;
}

or

h2
{
position:relative;
left:20px;
}

The content of relatively positioned elements can be moved and overlap other elements, but the reserved space for the element is still preserved in the normal flow.

4. Absolute Positioning

An absolute position element is positioned relative to the first parent element that has a position other than static. If no such element is found, the containing block is <html>

h2
{
position:absolute;
left:100px;
top:150px;
}

Absolutely positioned elements are removed from the normal flow. The document and other elements behave like the absolutely positioned element does not exist. Absolutely positioned elements can overlap other elements.

Z-INDEX Property (Overlapping Elements)

The z-index property specifies the stack order of an element. An element with greater stack order is always in front of an element with a lower stack order.

Note: z-index only works on positioned elements (position:absolute, position:relative, or position:fixed).

img
{
position:absolute;
left:0px;
top:0px;
z-index:-1;
}

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Validation Controls in ASP.NET: System.Web.UI.WebControls Class

System.Web.UI.WebControls contains all the validation controls. Here is a brief description of all the validation controls in ASP.NET:

1. RequiredFieldValidator (<asp:RequiredFieldValidator>)

Checks that the validated control contains a value. It cannot be empty.

<asp:RequiredFieldValidator
id="validateTxtName"
runat="server"
display="static"
controlToValidate="txtName"
errorMessage="Name must be entered" >
</asp:RequiredFieldValidator>

2. RegularExpressionValidator (<asp:RegularExpressionValidator>)

Checks the value against a regular expression (pattern). Checks that the value in the control matches a specified regular expression. If the validated control is empty, no validation takes place. The most important property in the RegularExpressionValidator is ValidationExpression.

<asp:RegularExpressionValidator
id="regvH"
runat="server"
display="static"
controlToValidate="txtH"
errorMessage="Hours must be 1-3 digits only"
validationExpression="\d{1,3}">
</asp:RegularExpressionValidator>

3. CompareValidator (<asp:CompareValidator>)

Checks if the value is acceptable compared to a given value or compared to the content of another control. In other words, it checks that the value in the validated control matches the value in another control or a specific value. The data type and comparison operation can be specified. If the validated control is empty, no validation takes place. The most important properties in the CompareValidator are ControlToCompare, Operator, and type.

<asp:CompareValidator
id="comvR"
runat="server"
display="static"
controlToValidate="txtR"
errorMessage="Rate must be numeric"
ValueToCompare="txtA">
</asp:CompareValidator>

4. RangeValidator (<asp:RangeValidator>

Checks if the input control’s value is within a specified range. In other words, it checks that the value in the validated control is within the specified text or numeric range. If the validated control is empty, no validation takes place. The most important properties in the RangeValidator are MaximumValue, MinimumValue, and type.

<asp:RangeValidator
id="ranvDependents"
runat="server"
display="static"
controlToValidate="txtDependents"
errorMessage="Must be from 0 to 10"
type="Integer"
minimumValue=0
maximumValue=10>
</asp:RangeValidator>

5. CustomValidator (<asp:CustomValidator>)

Allows you to develop custom validation. Performs user-defined validation on an input control using a specified function (client-side, server-side, or both). If the validated control is empty, no validation takes place. The most important property in the CustomValidator is ClientValidationFunction.

<asp:CustomValidator
id="cusvDeptNum"
runat="server"
display="static"
controlToValidate="txtDeptNum"
onServerValidate="validateDeptNum"
errorMessage="Must be in multiples of 10" >
</asp:CustomValidator>

6. ValidationSummary (<asp:ValidationSummary>)

Displays a summary of all current validation errors. In other words, reports a summary of all errors. The most important properties in the ValidationSummary are DisplayMode, ShowHeaderText, ShowMessageBox, and ShowSummary.

<asp:ValidationSummary
id="valSummary"
runat="server"
display="static"
headerText="Please correct the following errors"
showSummary= "True" />

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COM Family: COM+ and DCOM, Interop, RPC and TLB

Microsoft COM (Component Object Model) technology enables different software components to communicate. It was introduced by Microsoft in 1993. The family of COM technologies includes COM+, Distributed COM (DCOM) and ActiveX Controls.

COM provides interoperability. COM allows reuse of objects with no knowledge of their internal implementation.COM objects can be used with all .NET languages through .NET COM Interop.

COM interfaces have bindings in several languages, such as C, C++, Visual Basic, Delphi, and several of the scripting languages implemented on the Windows platform. All access to components is done through the methods of the interfaces.

Type Library in COM (.tlb file)

First, you must create a type library for the assembly. A type library is the COM equivalent of the metadata contained within a .NET assembly. Type libraries are generally contained in files with the extension .tlb. A type library contains the necessary information to allow a COM client to determine which classes are located in a particular server, as well as the methods, properties, and events supported by those classes. Each component or dll file must contain type library.

COM Interop

COM Interop is a technology included in the .NET CLR that enables COM objects to interact with .NET objects, and vice versa. COM Interop allows COM developers to access managed objects as easily as they access other COM objects.

The .NET Framework creates a type library and special registry entries when a component (compiled in the form of DLL) is registered. It provides a specialized utility that exports the managed types into a type library and registers the managed component as a traditional COM component. When the type is instantiated through COM, the .NET CLR is the actual COM object that executes and it merely marshals any method calls or property access to the type implementation.

Reference Counting, Pooling and Memory Management in COM:

The COM specifications require a technique called reference counting to ensure that individual objects remain alive as long as there are clients which have acquired access to one or more of its interfaces and, conversely, that the same object is properly disposed of when all code that used the object have finished with it and no longer require it. A COM object is responsible for freeing its own memory once its reference count drops to zero. Pooling for objects can also be set for different components.

RPC (Remote Procedure Call)

Microsoft Remote Procedure Call (RPC) is a powerful technology for creating distributed client/server programs. RPC is an interprocess communication technique that allows client and server software to communicate.

dcomcnfg:

To access settings on a computer running Windows 2000, Windows XP and earlier, click Start > Run, and type "dcomcnfg". (Click NO for any warning screens that appear.) To access DCOM settings on a computer running Windows Vista or later, click Start, type "dcomcnfg", right-click "dcomcnfg.exe" in the list, and click "Run as administrator".

COM is very similar to other component software interface technologies, such as CORBA and Java Beans, although each has its own strengths and weaknesses.

COM Family:

COM+: COM+ is the name of the COM-based services and technologies first released in Windows 2000. It provides  distributed transactions, resource pooling, disconnected applications, event publication and subscription, better memory and processor (thread) management.

DCOM: DCOM isDistributed COM. DCOM uses RPC mechanism for communication. It allows COM components to communicate across network boundaries.It provides load balancing.COM installation and execution is done in its clien machine where DCOM is intalled and run on selected server machine.

DCOM is generally equivalent to the Common Object Request Broker Architecture ( CORBA ) in terms of providing a set of distributed services. But unlike CORBA, which runs on many operating systems, DCOM is currently implemented only for Windows.

Migrate from COM application

Although calling a COM component from managed code is simple, you should not consider this a permanent solution for most applications. Over the long run, you'll want to migrate heavily used components to native .NET code. This will provide you with the full range of .NET capabilities, as well as make your code faster by eliminating the RCW layer.

Decline of COM

Several of the services that COM+ provides have been largely replaced by recent releases of .NET. For example, the System.Transactions namespace in .NET provides the TransactionScope class, which provides transaction management without resorting to COM+. Similarly, queued components can be replaced by Windows Communication Foundation with an MSMQ transport.

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Frameset, Frame and IFrame Elements in HTML

Frame Element

With frames, you can display more than one HTML document in the same browser window. Each HTML document is called a frame, and each frame is independent of the others.

Disadvantages of using frames:

1. Frames are not expected to be supported in future versions of HTML
2. Frames are difficult to use. (Printing the entire page is difficult).
3. The web developer must keep track of more HTML documents

Frameset Element

The frameset element holds one or more frame elements. Each frame element can hold a separate document. The frameset element states HOW MANY columns or rows there will be in the frameset, and HOW MUCH percentage/pixels of space will occupy each of them.

The <frameset> tag defines a frameset. The <frameset> element holds one or more <frame> elements.

Example: The first column is set to 25% of the width of the browser window. The second column is set to 75% of the width of the browser window. The document "frame_a.htm" is put into the first column, and the document "frame_b.htm" is put into the second column:

<frameset cols="25%,75%">
   <frame src="frame_a.htm" />
   <frame src="frame_b.htm" />
</frameset>

IFrame Element

IFrame defines an inline sub window (frame). An iframe is used to display a web page within a web page.

Syntax: <iframe src="URL"></iframe>

The height and width attributes are used to specify the height and width of the iframe.
The attribute values are specified in pixels by default, but they can also be in percent (like "80%").

Example
<iframe src="demo_iframe.htm" width="200" height="200"></iframe>

The frameborder attribute specifies whether or not to display a border around the iframe.
Set the attribute value to "0" to remove the border:

Example
<iframe src="demo_iframe.htm" frameborder="0"></iframe>

Points to note about Frames, Framesets and IFrames

1. The <frameset> tag is supported in all major browsers.
2. If you want to validate a page containing frames, be sure the <!DOCTYPE> is set to either "HTML Frameset DTD" or "XHTML Frameset DTD".
3. If a frame has visible borders, the user can resize it by dragging the border. To prevent a user from doing this, you can add noresize="noresize" to the <frame> tag.
4. Add the <noframes> tag for browsers that do not support frames.
5. Inline frame is just one “box” that is placed anywhere on browser page. In contrast, frame is a bunch of boxes put together to make one site.

 

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