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Table 13-3 StructLayoutAttribute Definition
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Name StructLayoutAttribute Public Properties TypeId
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When implemented in a derived class, gets a unique identifier for this attribute. (Inherited from the Attribute class.) Gets the LayoutKind value that specifies how the class or structure is arranged.
Value
Public/Property
StructLayoutAttribute Public Methods Equals GetCustomAttribute Public/Method Public/Method Overloaded. (Inherited from the Attribute class.) Overloaded. Retrieves a custom attribute of a specified type applied to an assembly, module, type member, or method parameter. (Inherited from the Attribute class.) Overloaded. Retrieves an array of the custom attributes applied to an assembly, module, type member, or method parameter. (Inherited from the Attribute class.) Returns the hash code for this instance. (Inherited from the Attribute class.)
GetCustomAttributes
Public/Method
GetHashCode
Public/Method
13
Interoperation
Table 13-3
StructLayoutAttribute Definition
Name GetType
Scope/Definition Public/Method
Description Gets the type of the current instance. (Inherited from the Object class.) When overridden in a derived class, indicates whether the value of this instance is the default value for the derived class. (Inherited from the Attribute class.) Overloaded. Determines whether any custom attributes of a specified type are applied to an assembly, module, type member, or method parameter. (Inherited from the Attribute class.) When overridden in a derived class, returns a value that indicates whether this instance equals a specified object. (Inherited from the Attribute class.) Determines whether the specified object instances are the same instance. (Inherited from Object class.) Returns a string that represents the current object. (Inherited from the Object class.)
IsDefaultAttribute
Public/Method
IsDefined
Public/Method
Match
Public/Method
ReferenceEquals
Public/Method
ToString
Public/Method
Arguably the most important aspect of using the StructLayoutAttribute attribute is the constructor, which takes one of the following three values:
LayoutKind.Auto
Causes the developer to relinquish all control over the layout
to the CLR
LayoutKind.Sequential Causes the CLR to preserve the layout specified by the
developer
Lesson 3: Using Unmanaged Code
LayoutKind.Explicit Causes the CLR to use the layout explicitly specified by the
developer by using memory offsets
NOTE Make sure the correct layout is used for structures
By default, the CLR will use LayoutKind.Auto for Reference types and use LayoutKind.Sequential for value types. Because this section of the book is about structures, LayoutKind.Sequential is used by default.
For the sake of illustration, we are going to use the OSVersionInfo structure, which is used to provide information about the OperatingSystemVersion.
Layout.Sequential
The first method to use is the Layout.Sequential option. This method indicates that structure values will appear exactly as they should in the called library:
' VB Imports System.Runtime.CompilerServices Imports System.Runtime.InteropServices <StructLayout(LayoutKind.Sequential)> _ Public Class OSVersionInfo Public dwOSVersionInfoSize As Int32 Public dwMajorVersion As Int32 Public dwMinorVersion As Int32 Public dwBuildNumber As Int32 Public dwPlatformId As Int32 <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=128)> _ Public szCSDVersion As String End Class // C# using System.Runtime.CompilerServices; using System.Runtime.InteropServices; namespace NetForComDemoCS { [StructLayout(LayoutKind.Sequential)] class OSVersionInfo { public Int32 dwOSVersionInfoSize; public Int32 dwMajorVersion; public Int32 dwMinorVersion; public Int32 dwBuildNumber; public Int32 dwPlatformId; [MarshalAs(UnmanagedType.ByValTStr, SizeConst=128)] public String szCSDVersion; } }
13
Interoperation
Layout.Explicit
The other option is to use the Explicit mode. With this method, two things must be done:
The LayoutKind enumeration needs to be set to Explicit. The offset in bytes must be specified for each field.
Other than these differences, using either approach is virtually indistinguishable from using Sequential layouts.
' VB Imports System.Runtime.CompilerServices Imports System.Runtime.InteropServices <StructLayout(LayoutKind.Explicit)> _ Public Class OSVersionInfo <FieldOffset(0)> Public dwOSVersionInfoSize As Int32 <FieldOffset(4)> Public dwMajorVersion As Int32 <FieldOffset(8)> Public dwMinorVersion As Int32 <FieldOffset(12)> Public dwBuildNumber As Int32 <FieldOffset(16)> Public dwPlatformId As Int32 <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=128)> _ <FieldOffset(20)> Public szCSDVersion As String End Class // C# using System.Runtime.CompilerServices; using System.Runtime.InteropServices; namespace NetForComDemoCS { [StructLayout(LayoutKind.Explicit)] class OSVersionInfo { [FieldOffset(0)] public Int32 dwOSVersionInfoSize; [FieldOffset(4)] public Int32 dwMajorVersion; [FieldOffset(8)] public Int32 dwMinorVersion; [FieldOffset(12)] public Int32 dwBuildNumber; [FieldOffset(16)] public Int32 dwPlatformId; [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)] [FieldOffset(20)] public String szCSDVersion; } }
Lesson 3: Using Unmanaged Code
Using a Callback with Unmanaged Code
Callback functions are an extremely important tool in any developer s library. Callbacks are used throughout the .NET Framework and Framework Class Library extensively, and most nontrivial applications will employ them in some fashion or another. Just as callbacks are important in a totally managed environment, they are also important in an unmanaged environment. Traditionally, callbacks were implemented with pointers. This approach afforded a tremendous amount of power to the programmer, but it also had some inherent shortcomings, such as lack of type safety. To address this issue, the .NET Framework provides Delegate objects, which can be used to manage callbacks in a type-safe fashion.
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