April 30, 2008

C# Pre Processor Directives

One of the good features of C# that we rarely use are the Pre Processor directives. Let's see how we can use them for us.

In general practice, we always write some test code in the application. As in JavaScript we use temporary alerts, in C# code behind too (or even in class library project) we use test code like temporary emailing and writing hardcode emails in the application emails. And very often this kind of code goes to production machine that is undesirable as well disastrous. To avoid this we can use the conditional compilation directives. Let's see how to use them

We are going to use three directives #define, #if, #endif. We can define the symbol in a file using the #define directive like

#define TESTCODE (this will define a symbol named TESTCODE)

And then we can write our test code within the conditional compilation #if directive as below

#if TESTCODE (if the TESTCODE symbol is defined using the #define then true, false otherwise)

// Test Code goes here.....

Also this is a very good way to handle multiple environments like in one project we had three environments like DEVELOPER, STAGE, PRODUCTION and we also had different databases. Now it may happen that we do not want the test data in our production database so that we can define these three symbols and then in the function returning the connection string we can conditionally return the appropriate connection string as below

internal string GetConnectionString() {

string conStr = "";

conStr = "development database connection string";
conStr = "Stage database connection string";
conStr = "Production database connection string";
return conStr;


So now only if you have the above line (#define TESTCODE) in your file the above conditional code will compile, not otherwise. So in case you do not want the test code to be moved to the production environment dll then just go to the file and remove the #define TESTCODE.
These conditional compilation symbols can be defined at file level or the project level. To define it at file level we can put the #define SYMBOLNAME in the file itself. To define the project level symbol follow the below steps.
Right click the .csproj (project file) -> click Properties -> go to Build tab -> find the Conditional compilation symbol and write the symbols in the text boxes. Multiple symbols can be separated by comma.
Now in ASP.NET 2.0 website this can be in the web.config file too as below
compiler language="c#;cs;csharp" extension=".cs"
type="Microsoft.CSharp.CSharpCodeProvider, System, Version=,
Culture=neutral, PublicKeyToken=b77a5c561934e089"

USE #warning AND #error IN CODE REVIEW:

Two other important directives are #warning and #error. Let us see how can we use them. These directives can be used while reviewing the code. As when some senior programming guy reviewing the code and if he find some better alternative for a piece of code he/she can put a warning there like

#warning Method implemented incorrectly. Do not use static variables.

So whenever the code is compiled, we will find the above warning in the output window and the Error list window. If we find some bad code that is written wrongly then we can put #error MESSAGE so that the code will not be compiled ant the message will be shown in the output and the Error list window

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Nirav Patel said...

Hi Paresh,

Its very good article. Seems very useful.

Thanks for posting.