HOWTO: Get Started Programming With the Windows API (LONG)

ID: Q190000

The information in this article applies to:

Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, version 6.0
Microsoft Visual Basic Professional and Enterprise Editions for Windows, version 5.0
Microsoft Visual Basic Standard, Professional, and Enterprise Editions, 32-bit only, for Windows, version 4.0
Microsoft Visual Basic for Applications version 5.0

SUMMARY

This article demonstrates how to get started calling functions in the Windows Application Programming Interface (Windows API) in your Visual Basic projects. This article assumes you are familiar with using Visual Basic but have never used it to call the functions in the Windows API.

MORE INFORMATION

Although Visual Basic has an extensive list of statements, methods, events, and procedures to help you create a Windows program quickly and easily, there are limitations within the development environment that may prevent you from implementing your ideas. One way to go beyond these limitations is to call the Windows API from within your Visual Basic project. At the heart of the Windows operating system are the functions that make up the Windows API. The majority of these functions are accessible from Visual Basic programs.

The following topics are covered in this article:

What is the Windows API? - A brief overview of the Windows Application Programming Interface.

Why Use the Windows API? - Explains the advantages of calling Windows API functions in addition to using the functions, methods, and procedures provided by Visual Basic.

How to call a Windows API function - The details of calling a function from a Visual Basic program.

Sample Project - Shows how to create your first Visual Basic project that calls two functions in the Windows API.

Tips - Some tips and tricks to make Windows API programming easier.

Where to go from here? - additional resources and articles on the topics of using the Windows API with Visual Basic.

What is the Windows API?

If you look at the Windows System directory, typically \Windows\System under Window 95/98 and \Winnt\System32 under Windows NT, you will find a number of Dynamic Link Library (.DLL) files. These files contain functions that are used to run the operating system and to ensure a consistent user interface and operating environment. These files make up the Windows API.

The purpose of the Windows API is to allow you to develop programs consistent with the Windows operating system and user interface. Instead of individuals writing the code to create components of the Windows operating system, such as forms, command buttons, and menus, you can call the appropriate functions in the Windows API and let the operating system create those components.

The purpose of each function in the Windows API is documented in the Platform Software Development Kit (SDK). The Platform SDK is shipped with Microsoft Visual C++ and as part of a subscription to the Microsoft Developer Network (MSDN.)

You can also download the Platform SDK from the following internet site:

Microsoft Platform SDK at http://msdn.microsoft.com/developer/sdk/platform.asp

As with any other Windows program, Visual Basic uses the functions in the Windows API. When you perform a task in Visual Basic, such as adding a command button to a form, Visual Basic calls the appropriate functions to perform the task. The features of Visual Basic are wrappers to these functions.

Why Use the Windows API?

The reason why you would use the Windows API is to perform a task beyond what is available from Visual Basic. For example, you cannot determine or set the cursor blinking rate through any statement, property, procedure, or method in Visual Basic. You can get the blink rate by calling the GetCaretBlinkTime function, which returns the cursor blink rate in milliseconds. If you want to set the cursor to blink at another rate under specified conditions, you can call the SetCaretBlinkTime function, passing the specified time in milliseconds.

You can get the same functionality as almost any of the features of Visual Basic by calling the appropriate Windows API functions. However, calling these functions directly is typically more complicated and can cause unpredictable behavior if not done correctly, resulting in system errors. For the best performance and predictable behavior, you should use the intrinsic Visual Basic functionality.

How to Call a Windows API Function

There are two steps required to call a Windows API function:

Declare the function in a module or form.

Call the function in your program as you would any other function.

The topic "Declaring a DLL Procedure" in "Part 4 Accessing DLLs and the Windows API" of the "Visual Basic Component Tools Guide" explains how to declare a function in a module. The topic also shows how to use the API Text Viewer to find the appropriate declaration and copy and paste the declaration to your project.

For example, to declare the GetCaretBlinkTime function in a module, copy the following text to the code window of your module:


Declare Function GetCaretBlinkTime Lib "user32" Alias _
                 "GetCaretBlinkTime" () As Long

If you want to use the same function only in a specific form, add the keyword "Private" to the beginning of the function declaration. The GetCaretBlinkTime function declaration is written in the code window of a form as follows:


Private Declare Function GetCaretBlinkTime Lib "user32" Alias _
                         "GetCaretBlinkTime" () As Long

This function returns a Long value that is the cursor blink rate in milliseconds. The Lib keyword precedes the name of the file that contains the function; in this case, User32.DLL. The Alias keyword is used to define the specific function in the DLL file. This clause is optional when the function is being declared with the same name as in the DLL.

You will often see the Private notation used in Microsoft Knowledge Base articles because not using a module streamlines the implementation of the sample code. If you need to use a Windows API function in more than one source file in a project, it should be declared in a module.

For more information about the Declare statement, see "Declare Statement" in the Language Reference in Books Online.

Once you have declared the function, you can call it as you would any Visual Basic function and a pass any appropriate parameters. The following example shows how to call the GetCaretBlinkTime function:


Dim lngCaretBlinkTime as Long   ' Blink Rate variable
   lngCaretBlinkTime =  GetCaretBlinkTime

For functions that require parameters, you can pass parameters by reference or by value.

A parameter passed by reference passes the 32-bit address of the location where the value is stored. By default, Visual Basic passes parameters by reference. You can also explicitly pass parameters by reference using the ByRef keyword.

The values of parameters passed by reference can be changed in the called procedure. For example, the InvertRect function inverts a specified rectangle by inverting the value of each pixel. The function requires the handle to the device context containing the rectangle and the address of a user defined type (UDT) variable with the logical coordinates of the rectangle. The address of the user defined variable is passed by reference.

When you call this function, the logical coordinates of the rectangle are placed in the user defined variable.

To use this function, add the following declarations to a module:


   Public Type RECT
      Left As Long
      Top As Long
      Right As Long
      Bottom As Long
   End Type

   Declare Function InvertRect Lib "user32" Alias "InvertRect" _
   (ByVal hdc As Long, ByRef lpRect As RECT) As Long

The first declaration specifies a user defined type required for a parameter of the InvertRect function while the second declaration is for the function.

Passing a parameter by value sends the actual value of the parameter to the function. You pass a parameter by value if the parameter is not changed by the called procedure. To pass a parameter by value, add the ByVal keyword to each parameter you want to pass to the function.

NOTE: When you pass a string variable to an API call, you actually pass the memory address of the string, so you should always pass string parameters as ByVal. If you pass a string parameter by reference, you pass the memory address containing the memory address of the string, which causes the API function receiving the parameter to behave incorrectly and may cause a memory violation error.

The SetCaretBlinkTime function requires the rate in milliseconds parameter to be passed by value to the function. Declare this function by adding the following statement to a module:


   Declare Function SetCaretBlinkTime Lib "user32" Alias _
   "SetCaretBlinkTime" (ByVal wMSeconds As Long) As Long

To use this function to set the cursor blink rate to 250 milliseconds, add the following to your project:


Dim lBlinkRate as Long   ' Blink rate variable
Dim lResult as Long      ' Result Variable
lBlinkRate = 250
lResult = SetCaretBlinkTime (lBlinkRate)
If lResult = 0 Then
   ' Error code
End If

Sample Project

This section shows you how to create a sample project that controls the cursor blink rate by calling two functions in the Windows API. The GetCaretBlinkRate function returns the current blink rate in milliseconds while the SetCaretBlinkRate function sets the blink rate. Both of these functions will be declared in a module. You can also declare these functions in the form by prefacing each function declaration with the Private keyword.

NOTE: The SetCaretBlinkRate function sets the cursor blink rate for the operating system. If you exit the project before setting the blink rate back to the default, the cursor blink rate will change for all programs.

Step-by-Step Example

Start a new Standard EXE project in Visual Basic. Form1 is created by default.

Add a Module to the project by completing the following steps:

From the Project menu, click Add module. The Add Module dialog box appears.

From the New tab, choose Module and click OK. A new module, Module1, is added to your project.

Copy the following code to the Code window of Module1:


Option Explicit

' Returns the current cursor blink rate
Declare Function GetCaretBlinkTime Lib "user32" () As Long

' Sets the cursor blink rate
Declare Function SetCaretBlinkTime Lib "user32" _
                (ByVal wMSeconds As Long) As Long

' Returns the error code if either GetCaretBlinkTime or
' SetCaretBlinkTime functions fails
Public Declare Function GetLastError Lib "kernel32" () As Long

Add a HScrollBar, a TextBox, and a CommandButton to Form1.

Copy the following code to the Code window of Form1:


Option Explicit
Dim lDefaultTime As Long
Dim lResult As Long
Dim lErrorCode As Long

Private Sub Form_Load()
   lResult = GetCaretBlinkTime

   If lResult = 0 Then
      Call DisplayError(0)
   Else
      lDefaultTime = lResult
      HScroll1.Min = 10
      HScroll1.Max = 1000
      HScroll1.Value = lDefaultTime
      Text1.Text = CStr(lDefaultTime)
      Command1.Caption = "Return to Default"
   End If
End Sub

Private Sub Command1_Click()
   lResult = SetCaretBlinkTime(lDefaultTime)
   
   ' If the function fails then display a message box with the error code
   If lResult = 0 Then
      Call DisplayError(1)
   Else
      ' Display the new blink rate.
      HScroll1.Value = lDefaultTime
      Text1.Text = CStr(GetCaretBlinkTime)
      HScroll1.SetFocus
   End If
End Sub

Private Sub HScroll1_Change()
   lResult = SetCaretBlinkTime(HScroll1.Value)

   If lResult = 0 Then
      Call DisplayError(1)
   Else
      lResult = GetCaretBlinkTime
      If GetCaretBlinkTime = 0 Then
         Call DisplayError(0)
      Else
         Text1.Text = CStr(lResult)
      End If
   End If
End Sub

Private Sub HScroll1_Scroll()
   Text1.Text = CStr(HScroll1.Value)
End Sub

Private Sub Text1_KeyPress(KeyAscii As Integer)
   If KeyAscii = vbKeyReturn Then
      Dim iTextValue As Integer
      iTextValue = CInt(Text1.Text)

      If iTextValue > 1000 Or iTextValue < 10 Then
         MsgBox "Enter a number between 10 and 1000."
         Text1.Text = CStr(HScroll1.Value)
         Exit Sub
      Else
         HScroll1.Value = iTextValue
      End If
   End If
End Sub

Private Sub Form_Terminate()
   lResult = SetCaretBlinkTime(lDefaultTime)

   If lResult = 0 Then
      lErrorCode = GetLastError
      MsgBox ("SetCaretBlinkTime failed. Error" & CStr(lErrorCode))
   End If
End Sub

Private Sub DisplayError(iFail As Integer)
   Dim sErrorMsg As String
   lErrorCode = GetLastError

   Select Case iFail
      Case 0   ' GetCaretBlinkRate Function Failed
         sErrorMsg = "GetCaretBlinkRate Failed. Error Code "
         sErrorMsg = sErrorMsg & CStr(lErrorCode)
      Case 1
         sErrorMsg = "SetCaretBlinkRate Failed. Error Code "
         sErrorMsg = sErrorMsg & CStr(lErrorCode)
      Case Else
         sErrorMsg = "Unknown Error"
   End Select

   MsgBox (sErrorMsg)
End Sub

On the Run menu, click Start or press the F5 key to start the program. Move the scroll bar to change the cursor blink rate. You can also enter a number between 10 and 1000 in the TextBox to change the blink rate. Click the "Return to Default" button to set the blink rate back to the default setting. To exit the application, you should click the Close button at the far right of the title bar instead of the stop button in the Visual Basic environment. This will restore your original cursor blink rate.

Tips

Save your work often: because you are calling functions in the Windows API, you are bypassing the syntax checking feature of Visual Basic. If the function fails, then an application error can occur and lock up your system. You can set Visual Basic to always save your work prior to running the project by enabling the Prompt to Save Changes option in the Environment Tab of the Options dialog box. You can get to this dialog box by clicking Options from the Tools menu.

Specify the variable type explicitly rather than using the Any variable type; a number of functions in the API Text Viewer can accept any type of variable as a parameter. These functions have parameters declared as Any. You can change the parameter to as specified variable type in order to simplify debugging your code.

Verify that you are passing the correct variable types, constants, and values required by the function by checking the function documentation in the Platform SDK. Although the SDK was written for a C programmer and therefore uses C data types, you can pass the appropriate data types by using the following Visual Basic data types:

Function Data Type Parameter	Appropriate Visual Basic Data Type
int, INT	ByVal Long
UNINT	ByVal Long
BOOL	ByVal Long
WORD	ByVal Integer
DWORD	ByVal Long

WPARAM	ByVal Long
LPARAM, LRESULT	ByVal Long
COLORREF	ByVal Long
ATOM	ByVal Integer
HANDLE	ByVal Long

BYTE	ByVal Byte
char	ByVal Byte

LPINT, int *	ByRef Long
LPUINT, UINT *	ByRef Long
LPBOOL, BOOL *	ByRef Long
LPBYTE, BYTE *	ByRef Byte
LPWORD, WORD *	ByRef Integer
LPDWORD, DWORD *	ByRef Long
LPHANDLE, HANDLE *	ByRef Long

Use the Add-In Manager to add the API Text Viewer as an add-in to the IDE. To use the Add-In Manger, see Using Wizards and Add-Ins in "Chapter 4-Managing Projects" of the "Visual Basic Programmer's Guide."

When using the API Text Viewer, convert the API text file to an Access database for faster searches.

Always pass strings to a DLL using the ByVal keyword.

REFERENCES

The following are additional sources for information about using the functions in the Windows API:

VB5DLL.DOC - located in the TOOLS\DOCS directory of the Visual Basic product CD. These are notes for developing DLLs for use with Microsoft Visual Basic. Use this information to know what a DLL expects from Visual Basic.

"Visual Basic 5.0 Programmer's Guide to the Win32 API" by Dan Appleman-A "must" book for Visual Basic programmers who want to use the Windows API. The book contains information written for the Visual Basic programmer on using the Windows API.

Hardcore Visual Basic by Bruce McKinney-contains good examples on how to extend the capabilities of Visual Basic through the Windows API.

Microsoft Developer Network Subscription - contains the header files used to compile the DLLs used by the operating system. You can use this information to verify the correct function, type, and constant declarations.

Mastering Visual Basic 5-Chapter 5: Using Dynamic Link Libraries: contains a good tutorial on using the Windows API.

For additional information on looking for the correct parameters, values, and constants required by the Windows API, please see the following article in the Microsoft Knowledge Base:

Q187674 HOWTO: Find Undocumented Constants Used by Windows API Functions

The following articles in the Microsoft Knowledge Base show you how to use functions in the Windows API to accomplish specific tasks:

Q161161 HOWTO: Search a ListBox Control Quickly

Q175512 HOWTO: Get a Short Filename from a Long Filename

Q151799 HOWTO: Use the Animated Copy Functions in Windows 95

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