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Overload the wrapper methods and provide a reduced number of
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required parameters by inserting defaults for the extern method call
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Use enum or const to provide constant values for the API as part of the
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For all P/Invoke methods that support GetLastError(), be sure to
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assign the SetLastError named attribute to true This allows the reporting of errors via SystemComponentModelWin32Exception
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Wrap resources, such as handles, into classes that derive from SystemRuntimeInteropServicesSafeHandle or that support IDisposable Function pointers in unmanaged code map to delegate instances in
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of relying on pointers The last bullet implies C# s support for pointers, described in the next section
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On occasion, developers will want to be able to access and work with memory, and with pointers to memory locations, directly This is necessary for certain operating system interaction as well as with certain types of time-critical algorithms To support this, C# requires use of the unsafe code construct
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Unsafe Code One of C# s great features is that it is strongly typed and supports type checking throughout the runtime execution What makes this feature especially great is that it is possible to circumvent this support and manipulate memory and addresses directly You would do this when working with things such as memory-mapped devices, or if you wanted
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to implement time-critical algorithms The key is to designate a portion of the code as unsafe Unsafe code is an explicit code block and compilation option, as shown in Listing 2011 The unsafe modifier has no effect on the generated CIL code itself It is only a directive to the compiler to permit pointer and address manipulation within the unsafe block Furthermore, unsafe does not imply unmanaged
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Listing 2011: Designating a Method for Unsafe Code
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class Program { unsafe static int Main(string[] args) { // } }
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You can use unsafe as a modifier to the type or to specific members within the type In addition, C# allows unsafe as a statement that flags a code block to allow unsafe code (see Listing 2012)
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Listing 2012: Designating a Code Block for Unsafe Code
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class Program { static int Main(string[] args) { unsafe { // } } }
Code within the unsafe block can include unsafe constructs such as pointers
NOTE
It is important to note that it is necessary to explicitly indicate to the compiler that unsafe code is supported
20: Platform Interoperability and Unsafe Code
From the command line, this requires the /unsafe switch For example, to compile the preceding code, you need to use the command shown in Output 201
OUTPUT 201:
cscexe /unsafe Programcs
You need to use the /unsafe switch because unsafe code opens up the possibility of buffer overflows and similar possibilities that expose the potential for security holes The /unsafe switch includes the ability to directly manipulate memory and execute instructions that are unmanaged Requiring /unsafe, therefore, makes the choice of potential exposure explicit
Pointer Declaration Now that you have marked a code block as unsafe, it is time to look at how to write unsafe code First, unsafe code allows the declaration of a pointer Consider the following example
byte* pData;
Assuming pData is not null, its value points to a location that contains one or more sequential bytes; the value of pData represents the memory address of the bytes The type specified before the * is the referent type, or the type located where the value of the pointer refers In this example, pData is the pointer and byte is the referent type, as shown in Figure 201 Because pointers (which are just byte values) are not subject to garbage collection, C# does not allow referent types other than unmanaged types,
byte* pData
0x0338EE9C 0x18 0x42
0x0338EE98
0x0338EE9C
byte[] data
Figure 201: Pointers Contain the Address of the Data
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which are types that are not reference types, are not generics, and do not contain reference types Therefore, the following is not valid:
string* pMessage
Neither is this:
ServiceStatus* pStatus
where ServiceStatus is defined as shown in Listing 2013; the problem again is that ServiceStatus includes a string field
Listing 2013: Invalid Referent Type Example
struct ServiceStatus { int State; string Description; }