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Chances are that if you ve created even a rudimentary .NET application you ve converted data from one type to another. Perhaps the most common conversion is the ToString() method. Every time you call ToString(), you are taking a given object and converting it to its String representation. In fully managed applications, you can specify conversion functionality by using the TypeConverter class. When using unmanaged code, however, you typically need to take a different approach. The first mechanism for converting data types is the MarshalAs attribute. MarshalAs can be applied to a property or a parameter. Either way, it works essentially the same. You simply create your property, decorate it with the MarshalAs attribute, and then specify the type it should be converted from, as shown here:
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' VB Imports System.Runtime.CompilerServices Imports System.Runtime.InteropServices Public Class MarshalAsDemo <MarshalAs(UnmanagedType.LPStr)> _ Public FirstName As String Public LastName As String <MarshalAs(UnmanagedType.Bool)> _ Public IsCurrentlyWorking As Boolean End Class // C# using System.Runtime.CompilerServices; using System.Runtime.InteropServices; namespace NetForComDemoCS { class MarshalAsDemo { [MarshalAs(UnmanagedType.LPStr)] public String FirstName; public String LastName; [MarshalAs(UnmanagedType.Bool)] public Boolean IsCurrentlyWorking; } }
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One convenient feature of using this attribute is that it has full Microsoft IntelliSense support, so you can find the equivalent data type much more easily.
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Lesson 3: Using Unmanaged Code
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Just as this attribute can be applied to a method or property, it can be applied to a parameter as well. The following code shows the same sample with the LastName property converted to a function that takes a String type named firstName:
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' VB Imports System.Runtime.CompilerServices Imports System.Runtime.InteropServices Public Class MarshalAsDemo <MarshalAs(UnmanagedType.LPStr)> _ Public FirstName As String Public Sub LastName(<MarshalAs(UnmanagedType.LPStr)> _ ByVal lastName As String) End Sub <MarshalAs(UnmanagedType.Bool)> _ Public IsCurrentlyWorking As Boolean End Class // C# using System.Runtime.CompilerServices; using System.Runtime.InteropServices;
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namespace NetForComDemoCS { class MarshalAsDemo { [MarshalAs(UnmanagedType.LPStr)] public String FirstName; public String LastName( [MarshalAs(UnmanagedType.LPStr)] String lastName); [MarshalAs(UnmanagedType.Bool)] public Boolean IsCurrentlyWorking; } }
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Marshaling Structures
Structures are commonly used in many Windows APIs and methods that you will use through P/Invoke. So to understand how the unmanaged structures are marshaled, it s worth a brief discussion of how managed types are handled as well. Performance is a major objective of the CLR and, as a general rule, it s safe to assume that the CLR will optimize performance wherever it can. Types provide the best illustration. When the Type class is created, its developer can lay out the class s members however she sees fit. Typically, you ll see members arranged in an aesthetically pleasing
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Interoperation
and logical manner. Aesthetically pleasing and logical, however, are subjective assessments. What s logical and intuitive to one developer might be quite different to another. Microsoft has published standards for library developers, and most companies have a set of rules that developers are encouraged to conform to. However, compliance to such standards largely remains the responsibility of the developer.
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Suggestions for code library developers
Tools such as Microsoft FX Cop (which can be found at http://www.gotdotnet.com/team/fxcop/) can help ensure compliance to standards. Microsoft has a set of recommended rules for library developers, and they are available at http:// www.gotdotnet.com/team/libraries/.
By default, when a type is created, the CLR will decide how best to arrange the class s members. To manually direct the CLR about how to handle (or not handle) the layout of a type, the System.Runtime.InteropServices.StructLayoutAttribute attribute is provided. Table 13-3 lists the definition of the StructLayoutAttribute from the MSDN documentation.
Table 13-3 StructLayoutAttribute Definition
Name StructLayoutAttribute Constructor StructLayoutAttribute
Scope/Definition
Description
Public/Constructor
Overloaded. Initializes a new instance of the StructLayout Attribute class.
StructLayoutAttribute Public Fields CharSet Public/Field Indicates how string data fields within the class should be marshaled as LPWSTR or LPSTR. (The default is LPSTR.) Controls the alignment of data fields of a class or structure in memory. Indicates the absolute size of the class or structure.
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