create and print barcode c# PInvoke and COM Interop in Visual Studio .NET

Creation Quick Response Code in Visual Studio .NET PInvoke and COM Interop

PInvoke and COM Interop
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Sub TestFindWindow() This works only on English and US versions of Windows. Dim hWnd As Integer = FindWindow(Nothing, Untitled - Notepad ) If hWnd <> 0 Then MoveWindow(hWnd, 0, 0, 600, 300, 1) Else MessageBox.Show( Window not found", Error ) End If End Sub
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As this snippet demonstrates, you can use Nothing to pass a null string. In earlier ver sions of Visual Basic, you would have passed the vbNullString constant in these cir cumstances. (This constant is still supported but is superfluous under Visual Basic .NET.) Remember that a null string is always different from an empty string that is, a string that contains no characters when calling a function in a DLL. A Declare statement can include optional arguments; if you omit an optional argument when calling the function, its default value is pushed on the stack anyway. If the external function receives an incorrect number of arguments, your application is likely to crash.
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See Also You can learn more about the Declare keyword and see many advanced examples
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of how to use it in Appendix of Programming Microsoft Visual Basic 6, available on the compan ion CD. All examples are for Visual Basic 6; you need to adapt them to Visual Basic .NET.
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The DllImport Attribute
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Although the Declare keyword is still supported, you should abandon it in favor of the DllImport attribute. This attribute is preferable because it offers additional options and also because it works in virtually all .NET languages, including C#, so it makes your code more readable to other developers and more easily ported to other languages. The DllImport attribute is defined in the System.Runtime.InteropServices namespace and can be applied only to static methods namely, methods defined in a module or class methods flagged with the Shared keyword. The method marked with this attribute is used only as a blueprint for deriving the type of arguments and the type of the return value (if there is one); a compilation error occurs if the method contains exe cutable code. In the simplest case, the DllImport attribute specifies only the name of the DLL that contains the external function:
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Class WindowsFunctions <DllImport( user32 )> _ Public Shared Function FindWindow(ByVal lpClassName As String, _ ByVal lpWindowName As String) As Integer No code here End Function
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Part VII:
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Advanced Topics
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<DllImport( user32 )> _ Shared Function MoveWindow(ByVal hWnd As Integer, ByVal x As Integer, _ ByVal y As Integer, ByVal nWidth As Integer, ByVal nHeight As Integer, _ ByVal bRepaint As Integer) As Integer No code here End Function End Class
As you see, the .dll extension in the DLL name can be omitted. If the path is also omit ted, the DLL must be in a system folder or another directory listed in the PATH envi ronment variable. Once you have declared an external function, you can invoke it as if it were defined in managed code:
Dim hWnd As Integer = WindowsFunctions.FindWindow(Nothing, Untitled - Notepad ) If hWnd <> 0 Then WindowsFunctions.MoveWindow(hWnd, 0, 0, 600, 300, 1) Else MessageBox.Show( Window not found", Error ) End If
The DllImport attribute supports several optional arguments, which let you precisely define how the external procedure should be called and how it returns a value to the caller.
<DllImport( filename.dll", CharSet:=charsetoption, ExactSpelling:=bool, _ EntryPoint:="procname", CallingConvention:=calloption, _ SetLastError:=bool, BestFitMapping:=bool, ThrowOnUnmappableChar:=bool>
The CharSet argument tells how strings are passed to the external routine; it can be CharSet.Ansi (the default), CharSet.Unicode, or CharSet.Auto. This argument has the same meaning as the Ansi, Unicode, or Auto qualifier in the Declare statement. In addi tion, if you specify the Ansi setting and a function with that name isn t found in the DLL, Visual Basic appends the A character to the function name and tries again. If you specify the Unicode setting, Visual Basic appends the W character to the function name before searching for it; if this first search fails, Visual Basic searches for the name you ve pro vided. Notice the subtle difference between the Ansi and Unicode settings the latter works as described because .NET uses Unicode strings, so a function that takes Unicode strings is preferable because it is more efficient than one that doesn t. The ExactSpelling argument is a Boolean value that determines whether the method name must match exactly the name in the DLL; if True (the default setting), the CharSet setting has no effect on the function name being searched. The EntryPoint argument specifies the actual function name in the DLL and is therefore equivalent to the Alias clause in a Declare statement. In practice, you use this argument if the entry point name is an invalid or a reserved name in Visual Basic (such as Friend), if it duplicates a name already defined in the application, or if it s an ordinal entry point (such as #123).
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