2018: Modifying an Immutable with the Index Operator in Unsafe Code in Visual C#

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Listing 2018: Modifying an Immutable with the Index Operator in Unsafe Code
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string text; text = "S5280ft"; ConsoleWrite("{0} = ", text); Unsafe // Requires /unsafe switch { fixed (char* pText = text) { pText[1] = 'm'; pText[2] = 'i'; pText[3] = 'l'; pText[4] = 'e'; pText[5] = ' '; pText[6] = ' '; } } ConsoleWriteLine(text);
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The results of Listing 2018 appear in Output 203
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OUTPUT 203:
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S5280ft = Smile
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Summary
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Modifications such as those in Listing 2017 and Listing 2018 lead to unexpected behavior For example, if you reassigned text to "S5280ft" following the ConsoleWriteLine() statement and then redisplayed text, the output would still be Smile because the address of two equal string literals is optimized to one string literal referenced by both variables In spite of the apparent assignment
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text = "S5280ft";
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after the unsafe code in Listing 2017, the internals of the string assignment are an address assignment of the modified "S5280ft" location, so text is never set to the intended value
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Accessing the Member of a Referent Type Dereferencing a pointer makes it possible for code to access the members of the referent type However, this is possible without the indirection operator (&) As Listing 2019 shows, it is possible to directly access a referent type s members using the -> operator (that is, a->b is shorthand for (*a)b)
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Listing 2019: Directly Accessing a Referent Type s Members
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unsafe { Angle angle = new Angle(30, 18, 0); Angle* pAngle = ∠ SystemConsoleWriteLine("{0} {1}' {2}\"", pAngle->Hours, pAngle->Minutes, pAngle->Seconds); }
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The results of Listing 2019 appear in Output 204
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OUTPUT 204:
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30 18' 0
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SUMMARY
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This chapter s introduction outlined the low-level access to the underlying operating system that C# exposes To summarize this, consider the Main()
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20: Platform Interoperability and Unsafe Code
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function listing for determining whether execution is with a virtual computer (see Listing 2020)
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Listing 2020: Designating a Block for Unsafe Code
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using SystemRuntimeInteropServices; class Program { unsafe static int Main(string[] { // Assign redpill byte[] redpill = { 0x0f, 0x01, 0x0d, 0x00, 0x00, 0x00, 0x00, 0xc3};
args)
// asm SIDT instruction // placeholder for an address // asm return instruction
unsafe { fixed (byte* matrix = new byte[6], redpillPtr = redpill) { // Move the address of matrix immediately // following the SIDT instruction of memory *(uint*)&redpillPtr[3] = (uint)&matrix[0]; using (VirtualMemoryPtr codeBytesPtr = new VirtualMemoryPtr(redpillLength)) { MarshalCopy( redpill, 0, codeBytesPtr, redpillLength); MethodInvoker method = (MethodInvoker)MarshalGetDelegateForFunctionPointer( codeBytesPtr, typeof(MethodInvoker)); method(); } if (matrix[5] > 0xd0) { ConsoleWriteLine("Inside Matrix!\n"); return 1; } else { ConsoleWriteLine("Not in Matrix\n"); return 0; }
Summary
} // fixed } // unsafe } }
The results of Listing 2020 appear in Output 205
OUTPUT 205:
Inside Matrix!
In this case, you use a delegate to trigger execution of the assembler code The delegate is declared as follows:
delegate void MethodInvoker();
This book has demonstrated the power, flexibility, consistency, and fantastic structure of C# This chapter demonstrated the ability, in spite of such high-level programming capabilities, to perform very low-level operations as well Before I end the book, the next chapter briefly describes the underlying execution platform and shifts the focus from the C# language to the broader platform in which C# programs execute
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The Common Language Infrastructure
that C# programmers encounter beyond the syntax is the context under which a C# program executes This chapter discusses the underpinnings of how C# handles memory allocation and deallocation, type checking, interoperability with other languages, crossplatform execution, and support for programming metadata In other words, this chapter investigates the Common Language Infrastructure (CLI) on which C# relies both at compile time and during execution It covers the execution engine that governs a C# program at runtime and how C# fits into a broader set of languages that are governed by the same execution
NE OF THE FIRST ITEMS
Metadata Application Domains Assemblies Manifests Modules Base Class Library Common Language Specification Common Type System Common Intermediate Language
Components
What Is the CLI
Common Language Infrastructure
Garbage Collection Type Safety Code Access Security Platform Portability Performance
CLI Implementations
Runtime
C# Compilation
21: The Common Language Infrastructure
engine Because of C# s close ties with this infrastructure, most of the features that come with the infrastructure are made available to C#
Defining the Common Language Infrastructure (CLI)
Instead of generating instructions that a processor can interpret directly, the C# compiler generates instructions in an intermediate language, the Common Intermediate Language (CIL) A second compilation step occurs, generally at execution time, converting the CIL to machine code that the processor can understand Conversion to machine code is still not sufficient for code execution, however It is also necessary for a C# program to execute under the context of an agent The agent responsible for managing the execution of a C# program is the Virtual Execution System (VES), generally more casually referred to as the runtime (Note that the runtime in this context does not refer to a time, such as execution time; rather, the runtime the Virtual Execution System is an agent responsible for managing the execution of a C# program) The runtime is responsible for loading and running programs and providing additional services (security, garbage collection, and so on) to the program as it executes The specification for the CIL and the runtime is contained within an international standard known as the Common Language Infrastructure (CLI) This is a key specification for understanding the context in which a C# program executes and how it can seamlessly interact with other programs and libraries, even when they are written in alternate languages Note that the CLI does not prescribe the implementation for the standard, but rather identifies the requirements for how a CLI platform should behave once it conforms to the standard This provides CLI implementers with the flexibility to innovate where necessary, while still providing enough structure that programs created by one platform can execute on a different CLI implementation, and even on a different operating system