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Although the most straightforward way to call unmanaged DLLs is to add a reference to them in Visual Studio, you can also use the System.Runtime.InteropServices namespace and the DllImport attribute to import an unmanaged DLL. Then, you can declare a managed prototype that you will use to access the Win32 function. The following Console application shows how to call the unmanaged MessageBox function in User32.dll:
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' VB Imports System.Runtime.InteropServices Module Module1 <DllImport("user32.dll")> _ Public Function MessageBox(ByVal hWnd As Integer, _ ByVal txt As String, ByVal caption As String, _ ByVal Typ As Integer) As IntPtr End Function Sub Main() MessageBox(New IntPtr(0), "Hello, world!", "My box", 0) End Sub End Module // C# using System; using System.Runtime.InteropServices; namespace com_example { class Program { [DllImport("user32.dll")] private static extern int MessageBox(IntPtr hWnd, String text, String caption, uint type);
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Lesson 1: Using COM Components from the .NET Framework
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static void Main(string[] args) { MessageBox( new IntPtr(0), "Hello, world!", "My box", 0); } } }
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The .NET Framework uses interop marshaling to pass data between the .NET Framework and COM. Marshaling is the process of formatting parameters for use by COM, and it is required because COM types and classes in the .NET Framework can use slightly different formats. For example, if you pass a .NET string instance to a COM object, COM interop automatically converts it to the COM type BStr. Although the default settings work for most scenarios, you can use the following properties with the DllImport attribute to control aspects of the external function call:
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Enables or disables best-fit mapping when converting Unicode characters (used by the .NET Framework) to American National Standards Institute (ANSI) characters (used by COM). Setting this property to true enables Interop Marshaling to provide a similar character when no exact match exists. For example, it will convert the Unicode copyright character to c for unmanaged methods that accept ANSI characters. If no similar character exists, it is usually converted to the default ANSI character (which causes an exception to be thrown if ThrowOnUnmappableChar is true).
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Specifies the calling convention to use in passing method arguments. The default, WinAPI, is typically sufficient.
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CallingConvention CharSet
Controls how string parameters are marshaled. The default is CharSet .Ansi, which is sufficient unless you know the target function requires a different character set. this only if you want your managed prototype to have a different name from the COM function.
EntryPoint Specifies the name of the function to be called. You need to specify
ExactSpelling
Controls whether an entry point should be modified to correspond to the character set. Typically, the default setting is sufficient.
Controls whether unmanaged methods that have return values are translated directly or whether HRESULT or retval return values are converted to exceptions automatically. The default is true, which directly translates the return values. Set this to false only if you would rather perform exception handling in your application than examine the return value.
PreserveSig
13
Interoperating with COM
Enables the caller to use the Marshal.GetLastWin32Error API function to determine whether an error occurred while executing the method. In Visual Basic, the default is true (which adds some overhead); in C# and C++, the default is false.
SetLastError ThrowOnUnmappableChar Set this to true to throw an exception if marshaling encounters an unmappable Unicode character that is converted to an ANSI character.
For example, if you want to call the MessageBox function using the method name ShowBox, you need to specify the function name using the EntryPoint property, as the following example demonstrates:
' VB Imports System.Runtime.InteropServices Module Module1 <DllImport("user32.dll", EntryPoint := "MessageBox")> _ Public Function ShowBox(ByVal hWnd As Integer, _ ByVal txt As String, ByVal caption As String, _ ByVal Typ As Integer) As IntPtr End Function Sub Main() ShowBox(New IntPtr(0), "Hello, world!", "My box", 0) End Sub End Module // C# using System; using System.Runtime.InteropServices; namespace com_example { class Program { [DllImport("user32.dll", EntryPoint="MessageBox")] private static extern int ShowBox(IntPtr hWnd, String text, String caption, uint type); static void Main(string[] args) { ShowBox(new IntPtr(0), "Hello, world!", "My box", 0); } } }
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