Compilation refers to the processing of source code files (.c,.cc, or.cpp) and the creation of an 'object' file. This step doesn't create anything the user can actually run. Instead, the compiler merely produces the machine language instructions that correspond to the source code file that was compiled. In this article Syntax. Expression expression expression!= expression. The binary equality operators compare their operands for strict equality or inequality. The equality operators, equal to () and not equal to (!=), have lower precedence than the relational operators, but they behave similarly. A class that derives from CodeDomProvider can typically provide methods for generating and compiling code for the language the provider supports. Using a CodeDOM code provider to generate source code. To generate source code in a particular language, you need a CodeDOM graph that represents the structure of the source code to generate. Visual Studio includes a command-line C and C compiler. You can use it to create everything from basic console apps to Universal Windows Platform apps, Desktop apps, device drivers, and.NET components. In this walkthrough, you create a basic, 'Hello, World'-style C program by using a text editor, and then compile it on the command line.

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The System.CodeDom.Compiler namespace provides interfaces for generating source code from CodeDOM object graphs and for managing compilation with supported compilers. A code provider can produce source code in a particular programming language according to a CodeDOM graph. A class that derives from CodeDomProvider can typically provide methods for generating and compiling code for the language the provider supports.

Using a CodeDOM code provider to generate source code

To generate source code in a particular language, you need a CodeDOM graph that represents the structure of the source code to generate.

The following example demonstrate how to create an instance of a CSharpCodeProvider:

The graph for code generation is typically contained in a CodeCompileUnit. To generate code for a CodeCompileUnit that contains a CodeDOM graph, call the GenerateCodeFromCompileUnit method of the code provider. This method has a parameter for a TextWriter that it uses to generate the source code, so it is sometimes necessary to first create a TextWriter that can be written to. The following example demonstrates generating code from a CodeCompileUnit and writing the generated source code to a file named HelloWorld.cs.

Using a CodeDOM code provider to compile assemblies

Invoking compilation

To compile an assembly using a CodeDom provider, you must have either source code to compile in a language for which you have a compiler, or a CodeDOM graph that source code to compile can be generated from.

If you are compiling from a CodeDOM graph, pass the CodeCompileUnit containing the graph to the CompileAssemblyFromDom method of the code provider. If you have a source code file in a language that the compiler understands, pass the name of the file containing the source code to the CompileAssemblyFromFile method of the CodeDom provider. You can also pass a string containing source code in a language that the compiler understands to the CompileAssemblyFromSource method of the CodeDom provider.

Configuring compilation parameters

All of the standard compilation-invoking methods of a CodeDom provider have a parameter of type CompilerParameters that indicates the options to use for compilation.

You can specify a file name for the output assembly in the OutputAssembly property of the CompilerParameters. Otherwise, a default output file name will be used.

By default, a new CompilerParameters is initialized with its GenerateExecutable property set to false. If you are compiling an executable program, you must set the GenerateExecutable property to true. When the GenerateExecutable is set to false, the compiler will generate a class library.

If you are compiling an executable from a CodeDOM graph, a CodeEntryPointMethod must be defined in the graph. If there are multiple code entry points, it may be necessary to set the MainClass property of the CompilerParameters to the name of the class that defines the entry point to use.

To include debug information in a generated executable, set the IncludeDebugInformation property to true.

If your project references any assemblies, you must specify the assembly names as items in a StringCollection as the ReferencedAssemblies property of the CompilerParameters you use when invoking compilation.

You can compile an assembly that is written to memory rather than disk by setting the GenerateInMemory property to true. When an assembly is generated in memory, your code can obtain a reference to the generated assembly from the CompiledAssembly property of a CompilerResults. If an assembly is written to disk, you can obtain the path to the generated assembly from the PathToAssembly property of a CompilerResults.

To specify a custom command-line arguments string to use when invoking the compilation process, set the string in the CompilerOptions property.

If a Win32 security token is required to invoke the compiler process, specify the token in the UserToken property.

To link a Win32 resource file into the compiled assembly, specify the name of the Win32 resource file in the Win32Resource property.

To specify a warning level at which to halt compilation, set the WarningLevel property to an integer that represents the warning level at which to halt compilation. You can also configure the compiler to halt compilation if warnings are encountered by setting the TreatWarningsAsErrors property to true.

The following code example demonstrates compiling a source file using a CodeDom provider derived from the CodeDomProvider class.

Languages with Initial Support

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The .NET Framework provides code compilers and code generators for the following languages: C#, Visual Basic, C++, and JScript. CodeDOM support can be extended to other languages by implementing language-specific code generators and code compilers.

See also

When programmers talk about creating programs, they often say, 'it compilesfine' or, when asked if the program works, 'let's compile it and see'. Thiscolloquial usage might later be a source of confusion for new programmers.Compiling isn't quite the same as creating an executable file! Instead,creating an executable is a multistage process divided into two components:compilation and linking. In reality, even if a program 'compilesfine' it might not actually work because of errors during the linking phase. The total process of going from source code files to an executable might better be referred to as a build.

Compilation

Compilation refers to the processing of source code files (.c, .cc, or.cpp) and the creation of an 'object' file. This step doesn't create anythingthe user can actually run. Instead, the compiler merely produces the machinelanguage instructions that correspond to the source code file that wascompiled. For instance, if you compile (but don't link) three separate files,you will have three object files created as output, each with the name<filename>.o or <filename>.obj (the extension will dependon your compiler). Each of these files contains a translation of your sourcecode file into a machine language file -- but you can't run them yet! You needto turn them into executables your operating system can use. That's where thelinker comes in.

Linking

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Linking refers to the creation of a single executable file from multipleobject files. In this step, it is common that the linker will complain aboutundefined functions (commonly, main itself). During compilation, if thecompiler could not find the definition for a particular function, it would justassume that the function was defined in another file. If this isn't the case,there's no way the compiler would know -- it doesn't look at the contents ofmore than one file at a time. The linker, on the other hand, may look atmultiple files and try to find references for the functions that weren'tmentioned.

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You might ask why there are separate compilation and linking steps. First,it's probably easier to implement things that way. The compiler does itsthing, and the linker does its thing -- by keeping the functions separate, thecomplexity of the program is reduced. Another (more obvious) advantage is thatthis allows the creation of large programs without having to redo thecompilation step every time a file is changed. Instead, using so called'conditional compilation', it is necessary to compile only those source filesthat have changed; for the rest, the object files are sufficient input for thelinker. Finally, this makes it simple to implement libraries of pre-compiledcode: just create object files and link them just like any other object file.(The fact that each file is compiled separately from information contained inother files, incidentally, is called the 'separate compilation model'.)
To get the full benefits of condition compilation, it's probably easier to geta program to help you than to try and remember which files you've changed sinceyou last compiled. (You could, of course, just recompile every file that has atimestamp greater than the timestamp of the corresponding object file.) Ifyou're working with an integrated development environment (IDE) it may alreadytake care of this for you. If you're using command line tools, there's a niftyutility called make thatcomes with most *nix distributions. Along with conditional compilation, it hasseveral other nice features for programming, such as allowing differentcompilations of your program -- for instance, if you have a version producingverbose output for debugging.

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Knowing the difference between the compilation phase and the link phase canmake it easier to hunt for bugs. Compiler errors are usually syntactic innature -- a missing semicolon, an extra parenthesis. Linking errors usuallyhave to do with missing or multiple definitions. If you get an error that afunction or variable is defined multiple times from the linker, that's a goodindication that the error is that two of your source code files have the samefunction or variable.