In C++, I can define a macro such as:

#define PRODUCT(x, y, z) x * y * z

and then use it in code:

int a = PRODUCT(3, 2, 1);

C# doesn't allow you to do this. Why?

There are a few reasons why. The first is one of readability.

One of our main design goals for C# is to keep the code very readable. Having the ability to write macros gives the programmer the ability to create their own language - one that doesn't necessarily bear any relation to what the code underneath. To understand what the code does, the user must not only understand how the language works, but he must also understand all of the #define macros that are in effect at that point in time. That makes code much harder to read.

In C#, you can use methods instead of macros, and in most cases, the JIT will inline them, giving you the same performance aspect.

There's also a somewhat more subtle issue. Macros are done textually, which means if I write:

int y = PRODUCT (1 + 2, 3 + 4, 5 + 6)

I would expect to get something that gives me 3 * 7 *11 = 231, but in fact, the expansion as I've defined it gives:

int y = 1 + 2 * 3 + 4 * 5 + 6;

which gives me 33. I can get around that by a judicious application of parenthesis, but its very easy to write a macro that works in some situations and not in others.

Although C# doesn't strictly speaking have a pre-processor, it does have conditional compilation symbols which can be used to affect compilation. These can be defined within code or with parameters to the compiler. The "pre-processing" directives in C# (named solely for consistency with C/C++, despite there being no separate pre-processing step) are (text taken from the ECMA specification):

#define and #undef

Used to define and undefine conditional compilation symbols

#if, #elif, #else and #endif

Used to conditionally skip sections of source code

#line

Used to control line numbers emitted for errors and warnings.

#error and #warning

Used to issue errors and warnings.

#region and #endregion

Used to explicitly mark sections of source code.

See section 9.5 of the ECMA specification for more information on the above. Conditional compilation can also be achieved using the Conditional attribute on a method, so that calls to the method will only be compiled when the appropriate symbol is defined. See section 24.4.2 of the ECMA specifcation for more information on this.

Author: Eric Gunnerson]