Preprocessor directives across different files in C#

Question

I know that I can use preprocessor directives in C# to enable/disable compilation of some part of code.

If I define a directive in the same file, it wor

Answer 1

Instead of adding conditional compilation to your files, and adding blocks of code which use Linq, and others, which don't. I'd moved all data-access logic (i.e. code which have two implementations - with Linq and without) to separate libraries.

E.g. create interfaces, which your main application will use. And create two implementations of those interfaces - one which use Linq, and another which don't use:

In main project:

public interface IUserRepository
{
    IEnumerable GetUsersByCompanyName(string companyName);
}

In Persistence.Linq.dll :

using System.Linq; 

public class UserRepository : IUserRepository
{
    public IEnumerable GetUsersByCompanyName(string companyName)
    // use Linq here
}

In Persistence.SomethingOther.dll:

public class UserRepository : IUserRepository
{
    public IEnumerable GetUsersByCompanyName(string companyName)
    // do not use Linq here
}

Now you can inject any implementation of IUserRepository into your main application classes.