gistfile1.cs

public class MyClass
{
  public IDb MyDB {get;set;}

  public MyClass(ISys A) {}
  public DoSomething() 
  {
    if (MyDB != null)
    ..
    else
    ..
  }

}

public class MyClass
{
  private bool UsingDB = false;

  public MyClass(ISys A, IDb B) { UsingDB = true; }
  public MyClass(ISys A) {}
  public DoSomething() 
  {
    if (UsingFB)
    ..
    else
    ..
  }

}

public class MyClass
{
  public bool UsingDB = false;

  public MyClass(ISys A, IDb B) {  }

  public DoSomething() 
  {
    if (UsingFB)
    ..
    else
    ..
  }

}