1.C# preprocessor directives

Defining symbols

You use the following two preprocessor directives to define or undefine symbols for conditional compilation， these is mostly combine will the conditional compilation:

• #define: Define a symbol.
• #undef: Undefine a symbol.

Conditional compilation

You use four preprocessor directives to control conditional compilation:

• #if: Opens a conditional compilation, where code is compiled only if the specified symbol is defined.
• #elif: Closes the preceding conditional compilation and opens a new conditional compilation based on if the specified symbol is defined.
• #else: Closes the preceding conditional compilation and opens a new conditional compilation if the previous specified symbol isn't defined.
• #endif: Closes the preceding conditional compilation.

You can use the operators == (equality)]and != (inequality)]to test for the bool values trueor false. truemeans the symbol is defined. The statement #if DEBUG  has the same meaning as #if (DEBUG == true). You can use the [&&`   operators to evaluate whether multiple symbols have been defined. You can also group symbols and operators with parentheses.

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1#define MYTEST
2using System;
3public class MyClass
4{
5    static void Main()
6    {
7#if (DEBUG && !MYTEST)
8        Console.WriteLine("DEBUG is defined");
9#elif (!DEBUG && MYTEST)
10        Console.WriteLine("MYTEST is defined");
11#elif (DEBUG && MYTEST)
12        Console.WriteLine("DEBUG and MYTEST are defined");  
13#else
14        Console.WriteLine("DEBUG and MYTEST are not defined");
15#endif
16    }
17}

Defining regions

You can define regions of code that can be collapsed in an outline using the following two preprocessor directives: #region #endregion

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1#region MyClass definition
2public class MyClass
3{
4    static void Main()
5    {
6    }
7}
8#endregion

Error and warning information

You instruct the compiler to generate user-defined compiler errors and warnings, and control line information using the following directives:

• #error: Generate a compiler error with a specified message.
• #warning: Generate a compiler warning, with a specific message.
• #line: Change the line number printed with compiler messages.

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1#error Deprecated code in this method.
2
3#warning Deprecated code in this method.

Pragmas

This has been used in shaders to define the function name of vertex shader and fragment shader

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1#pragma pragma-name pragma-arguments

2.Unity Platform #define directives

2.1 Platform

2.2 Unity versions

UNITY_X, UNITY_X_Y and UNITY_X_Y_Z. for any unity release version

2.3 Others

Note: Remember to validate the code before compiling if it’s not included in the current editor context. Or it will cause errors when building the application.

It is also possible to add to the built-in selection of #define directives by supplying your own. Open the Other Settings panel of the Player settings and navigate to the Scripting Define Symbols  text box.

Reference

https://docs.unity3d.com/2019.1/Documentation/Manual/PlatformDependentCompilation.html

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/preprocessor-directives