for_jim.cs

public interface IWhateverDataTableProvider
{
  DataTable GetData();
}

public class WhateverDataTableProvider : IWhateverDataTableProvider
{
  public DataTable GetData()
  {
    // put all that hard-coded, hard to test stuff here...
    // copy-paste...
    return dataTable;
  }
}

public interface IWhateverDataTableProviderFactory
{
  IWhateverDataTableProvider CreateProvider();
}

public class WhateverDataTableProviderFactory : IWhateverDataTableProviderFactory
{
  private static IWhateverDataTableProviderFactory instance;
  
  public IWhateverDataTableProviderFactory Instance
  {
    get
    {
      if (instance == null)
        instance = new WhateverDataTableProviderFactory();
      
      return instance;
    }
    set { instance = value; }
  }
  
  public IWhateverDataTableProvider CreateProvider()
  {
    return new WhateverDataTableProvider();
  }
}

runtime.cs

public interface IC
{
  void DoSomething();
}

public class C : IC
{
  public void DoSomething()
  {
    // ha!
  }
}

public interface ICFactory
{
  IC Create();
}

public class CFactory : ICFactory
{
  private static ICFactory instance;
  
  public static ICFactory Instance
  {
    get 
    { 
      // If no instance has previously been defined, then
      // create a new factory.
      if (instance == null)
        instance = new CFactory();
      
      return instance;
    }
    // Putting a set accessor here allows us to put in a
    // stub at unit test time.
    set { instance = value; }
  }
  
  public IC Create()
  {
    return new C();
  }
}

test.cs

public class MockCFactory : ICFactory
{
  private readonly Mock<ICFactory> mock;
  
  public MockCFactory()
  {
    // Create your mock and setups here.
    mock = new Mock<ICFactory>();
    mock.Setup(x => x.Create()).Returns(new C());
  }
  
  public IC Create()
  {
    return mock.Object.Create();
  }
}

// Then, somewhere in a test...
[SetUp]
public void BeforeEachTest()
{
  // For purposes of these tests, use our mock factory instead of the real one.
  CFactory.Instance = new MockCFactory();
}

[TearDown]
public void AfterEachTest()
{
  // Reset the factory back to its default behavior.
  CFactory.Instance = null;
}