title: How to enable unit testing on a tightly coupled business\data layers without DI?
body: <p>I'm working on a project that is an older 3 tier design, any new functionality added needs to be unit testable.  </p>
  
  <p>The problem is the business layer / data layers are tightly coupled like in the sample below.  The BL just news up a data layer object... so it's almost impossible to mock up this way.  We don't have any Dependency Injection implemented so constructor injection isn't possible.  So what is the best way to modify the structure so that the data layer can be mocked up without the use of DI?</p>
  
  <pre><code>public class BLLayer()
  {
  
     public GetBLObject(string params)
     {
       using(DLayer dl = new DLayer())
       {  
          DataSet ds = dl.GetData(params);
  
          BL logic here....
  
       }
     }
  }
  </code></pre>