Question

I know how to make pluggable things in c#. Define an Interface, Activator.CreateInstance(<class>), etc. Or I can have a code path that explicitly creates an instance of the plugged class. Many ways.

But what if the service I want to make pluggable is static (I know I could refactor so that it’s not but that’s not the point of the question)

Concrete example. I have a class that provides disk I/O abstraction (Read File, List Directory,….). Now I want different implementations of this abstraction that serves up files from , say, a real FS, a database.

Based on Olivier Jacot-Descombes reply, I will have a FileSystem class (that is real) like this

public static class FileSystem
{
    static IFSImplemenation s_imple;
    static FileSystem()
    {
        if(<some system setting>)
            // converted to singleton instead of static
            s_imple = new OldFileSystem() 
        else
            s_imple = new DbFileSystem();
    }

    public static byte[] ReadFile(string path)
    {
        return s_imple.ReadFile(path);
    }

    ...
}

To reiterate – I have a large body of code that I dont want to change so it was important to keep the calling signature the same – this solution achieves that

Answer 1

Use your static class as facade for non-static implementations

public static class DB
{
    private static IDbInterface _implementation;

    public static void SetImplementation(IDbInterface implementation)
    {
        _implementation = implementation;
    }

    public static Customer GetCustomerByID(int custId)
    {
        return _implementation.GetCustomerByID(custId);
    }

    ...
}