create and print barcode c# Value ByValTStr in .NET

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Value ByValTStr
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Currency I1, I2, I4, I8 IDispatch Interface
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IUnknown LPArray
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LPWStr R4, R8 SafeArray Struct SysInt, SysUInt U1, U2, U4, U8 VariantBool
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30:
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PInvoke and COM Interop
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The following code shows how you can use the MarshalAs attribute to pass a Unicode string to the Windows API function that changes the current directory. This example is for illustration purposes only; you can achieve the same result by using the CharSet.Unicode setting in the DllImport attribute.
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<DllImport( kernel32", EntryPoint:="SetCurrentDirectoryW )> _ Function SetCurrentDirectory( _ <MarshalAs(UnmanagedType.LPWStr)> ByVal lpPathName As String) As Integer End Function
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The following structure containing two string fields flagged with the MarshalAs attribute. When this structure is passed to an external procedure, the first field is trans lated to a null-terminated ANSI string, whereas the second field becomes a 256-characters fixed-length string.
Structure MyStructure <MarshalAs(UnmanagedType.LPStr)> Public f1 As String <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=256)> Public f2 As String End Structure
Here s another example that uses the SizeConst optional argument:
Structure Employee Marshal the Name string as an ANSI fixed-length string of 100 chars. (We must account for the extra null character.) <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=101)> _ Dim Name As String End Structure
The StructLayout Attribute
Visual Basic .NET defines a couple of attributes that let you control how the elements of a Structure block are arranged in memory and how the runtime should marshal them when they re passed to a function in an external DLL. Fields of a structure are arranged in memory in the order in which they appear in source code, even though the compiler is free to insert padding bytes to arrange members so that 16-bit values are aligned with word boundaries, 32-bit values are aligned with doubleword boundaries, and so on. This arrangement known as an unpacked layout delivers the best performance because Intel processors work faster with aligned data. Visual Basic .NET lets you finely control where each member of the structure or the class is located by means of the StructLayout attribute. The allowed values for this attribute are StructLayout.Auto (the compiler can reorder elements for best performance for example, by grouping value types together), StructLayout.Sequential (elements are laid out and properly aligned sequentially in memory), and StructLayout.Explicit.
<StructLayout(LayoutKind.Explicit)> _ Structure ARGBColor End Structure
Part VII:
Advanced Topics
By default, Visual Basic .NET uses the StructLayout.Auto setting for classes and StructLayout.Sequential for structures. The StructLayout attribute supports three additional fields: CharSet, Pack, and Size. CharSet defines how string members in the structure are marshaled when the structure is passed to a DLL and can be Unicode, Ansi, or Auto. This argument, which defaults to Auto, has the same meaning as the DllImport argument with the same name. The Pack field defines the packing size for the structure, and can be 1, 2, 4, 8 (the default), 16, 32, 64, 128, or the special value 0 that uses the default packing size for the current platform. A structure whose LayoutKind is Sequential always aligns elements to this number of bytes; if you omit the StructLayout attribute, elements align to their nat ural boundary (2-byte words for Short elements, 4-byte double words for Integer and Single, addresses that are multiples of 8 for Long and Double, and so forth). The Size field determines the total length of the structure when passed to unmanaged code; you can use this argument to increase the length of a structure. (You can reach the same goal by appending dummy, unused fields to the structure.)
<StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Unicode, Pack:=4)> _ Structure ARGBColor End Structure
By default, the Visual Basic compiler is free to arrange class members in a way that optimizes memory usage and performance. This corresponds to the StructLayout.Auto setting. When passing an object as opposed to a structure to unmanaged code, you must apply the StructLayout attribute with a different setting, typically StructLay out.Explicit. Classes flagged in this way are known as formatted classes. PInvoke supports classes as arguments only if they are formatted classes, with only a few exceptions such as the String and StringBuilder types; conversely, COM Interop doesn t enforce any limitation in passing a reference type. In practice, you will rarely pass a reference to an object other than String or StringBuilder as an argument to an external DLL via PInvoke. If you do, however, remember that the DLL receives a pointer to the data in the object. From the perspective of the called routine, an object reference passed with ByVal is similar to a structure passed with ByRef because in both cases the routine receives an address. If you pass an object reference to a ByRef argument, the routine receives the address of a pointer that points to the object s data.
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