Passing a generic <TObject> class to a form

Question

I can't seem to find out the answer to this through searching, so here goes....

I know that I can pass Class objects generically to other classes by utilising this type of code:

public class ClsGeneric<TObject> where TObject : class
{
    public TObject GenericType { get; set; }
}

Then constructing in this way:

ClsGeneric<MyType> someName = new ClsGeneric<MyType>()

However, I have an application that requires me to open a Form and somehow pass in the generic type for use in that form. I am trying to be able to re-use this form for many different Class types.

Does anyone know if that's possible and if so how?

I've experimented a bit with the Form constructor, but to no avail.

Many thanks in advance, Dave

UPDATED: A Clarification on what the outcome I am trying to achieve is

UPDATED: 4th AUG, I've moved on a little further, but I offer a bounty for the solution. Here is what I have now:

interface IFormInterface
{
    DialogResult ShowDialog();
}


public class FormInterface<TObject> : SubForm, IFormInterface where TObject : class
{ }

public partial class Form1 : Form
{
    private FormController<Parent> _formController;

    public Form1()
    {
        InitializeComponent();
            _formController = new FormController<Parent>(this.btnShowSubForm, new DataController<Parent>(new MeContext()));   
    }
}

public class FormController<TObject> where TObject : class
{
    private DataController<TObject> _dataController;
    public FormController(Button btn, DataController<TObject> dataController)
    {
        _dataController = dataController;
        btn.Click += new EventHandler(btnClick);
    }

    private void btnClick(object sender, EventArgs e)
    {
        showSubForm("Something");
    }

    public void showSubForm(string className)
    {
        //I'm still stuck here because I have to tell the interface the Name of the Class "Child", I want to pass <TObject> here.
        // Want to pass in the true Class name to FormController from the MainForm only, and from then on, it's generic.

        IFormInterface f2 = new FormInterface<Child>();
        f2.ShowDialog();
    }
}

class MeContext : DbContext
{
    public MeContext() : base(@"data source=HAZEL-PC\HAZEL_SQL;initial catalog=MCL;integrated security=True;MultipleActiveResultSets=True;App=EntityFramework") { }
    public DbSet<Parent> Child { get; set; }
}

public class DataController<TObject> where TObject : class
{
    protected DbContext _context;

    public DataController(DbContext context)
    {
        _context = context;
    }
}

public class Parent
{
    string Name { get; set; }
    bool HasChildren { get; set; }
    int Age { get; set; }
}

public class Child
{
    string Name { get; set; }
    int Age { get; set; }
}

Answer 1

Maybe you've tried this, but you can create a custom class:

public class GenericForm<TObject> : Form where TObject : class
{
    // Here you can do whatever you want,
    // exactly like the example code in the
    // first lines of your question
    public TObject GenericType { get; set; }

    public GenericForm()
    {
        // To show that this actually works,
        // I'll handle the Paint event, because
        // it is executed AFTER the window is shown.
        Paint += GenericForm_Paint;
    }

    private void GenericForm_Paint(object sender, EventArgs e)
    {
        // Let's print the type of TObject to see if it worked:
        MessageBox.Show(typeof(TObject).ToString());
    }
}

If you create an instance of it like that:

var form = new GenericForm<string>();
form.Show();

The result is:

Going further, you can create an instance of type TObject from within the GenericForm class, using the Activator class:

GenericType = (TObject)Activator.CreateInstance(typeof(TObject));

In this example, since we know that is a string, we also know that it should throw an exception because string does not have a parameterless constructor. So, let's use the char array (char[]) constructor instead:

GenericType = (TObject)Activator.
         CreateInstance(typeof(TObject), new char[] { 'T', 'e', 's', 't' });

MessageBox.Show(GenericType as string);

The result:

Let's do the homework then. The following code should achieve what you want to do.

public class Parent
{
    string Name { get; set; }
    bool HasChildren { get; set; }
    int Age { get; set; }
}

public class Child
{
    string Name { get; set; }
    int Age { get; set; }
}

public class DataController<TObject> where TObject : class
{
    protected DbContext _context;

    public DataController(DbContext context)
    {
        _context = context;
    }
}

public class FormController<TObject> where TObject : class
{
    private DataController<TObject> _dataController;

    public FormController(Button btn, DataController<TObject> dataController)
    {
        _dataController = dataController;
        btn.Click += new EventHandler(btnClick);
    }

    private void btnClick(object sender, EventArgs e)
    {
        GenericForm<TObject> form = new GenericForm<TObject>();
        form.ShowDialog();
    }
}

public class GenericForm<TObject> : Form where TObject : class
{
    public TObject GenericType { get; set; }

    public GenericForm()
    {
        Paint += GenericForm_Paint;
    }

    private void GenericForm_Paint(object sender, EventArgs e)
    {
        MessageBox.Show(typeof(TObject).ToString());

        // If you want to instantiate:
        GenericType = (TObject)Activator.CreateInstance(typeof(TObject));
    }
}

However, looking to your current example, you have two classes, Parent and Child. If I understand correctly, those are the only possibilities to be the type of TObject.

If that is the case, then the above code will explode if someone pass a string as the type parameter (when the execution reaches Activator.CreateInstance) - with a runtime exception (because string does not have a parameterless constructor):

To protect your code against that, we can inherit an interface in the possible classes. This will result in a compile time exception, which is preferable:

The code is as follows.

// Maybe you should give a better name to this...
public interface IAllowedParamType { }

// Inherit all the possible classes with that
public class Parent : IAllowedParamType
{
    string Name { get; set; }
    bool HasChildren { get; set; }
    int Age { get; set; }
}

public class Child : IAllowedParamType
{
    string Name { get; set; }
    int Age { get; set; }
}

// Filter the interface on the 'where'
public class DataController<TObject> where TObject : class, IAllowedParamType
{
    protected DbContext _context;

    public DataController(DbContext context)
    {
        _context = context;
    }
}

public class FormController<TObject> where TObject : class, IAllowedParamType
{
    private DataController<TObject> _dataController;

    public FormController(Button btn, DataController<TObject> dataController)
    {
        _dataController = dataController;
        btn.Click += new EventHandler(btnClick);
    }

    private void btnClick(object sender, EventArgs e)
    {
        GenericForm<TObject> form = new GenericForm<TObject>();
        form.ShowDialog();
    }
}

public class GenericForm<TObject> : Form where TObject : class, IAllowedParamType
{
    public TObject GenericType { get; set; }

    public GenericForm()
    {
        Paint += GenericForm_Paint;
    }

    private void GenericForm_Paint(object sender, EventArgs e)
    {
        MessageBox.Show(typeof(TObject).ToString());

        // If you want to instantiate:
        GenericType = (TObject)Activator.CreateInstance(typeof(TObject));
    }
}

UPDATE

As RupertMorrish noted, you can still compile the following code:

public class MyObj : IAllowedParamType
{
    public int Id { get; set; }

    public MyObj(int id)
    {
        Id = id;
    }
}

And that should still rise an exception, because you just removed the implicit parameterless constructor. Of course, if you know what you are doing, this is hard to happen, however we can forbidden this by using new() on the 'where' type filtering - while also getting rid of the Activator.CreateInstance stuff.

The entire code:

// Maybe you should give a better name to this...
public interface IAllowedParamType { }

// Inherit all the possible classes with that
public class Parent : IAllowedParamType
{
    string Name { get; set; }
    bool HasChildren { get; set; }
    int Age { get; set; }
}

public class Child : IAllowedParamType
{
    string Name { get; set; }
    int Age { get; set; }
}

// Filter the interface on the 'where'
public class DataController<TObject> where TObject : new(), IAllowedParamType
{
    protected DbContext _context;

    public DataController(DbContext context)
    {
        _context = context;
    }
}

public class FormController<TObject> where TObject : new(), IAllowedParamType
{
    private DataController<TObject> _dataController;

    public FormController(Button btn, DataController<TObject> dataController)
    {
        _dataController = dataController;
        btn.Click += new EventHandler(btnClick);
    }

    private void btnClick(object sender, EventArgs e)
    {
        GenericForm<TObject> form = new GenericForm<TObject>();
        form.ShowDialog();
    }
}

public class GenericForm<TObject> : Form where TObject : new(), IAllowedParamType
{
    public TObject GenericType { get; set; }

    public GenericForm()
    {
        Paint += GenericForm_Paint;
    }

    private void GenericForm_Paint(object sender, EventArgs e)
    {
        MessageBox.Show(typeof(TObject).ToString());

        // If you want to instantiate:
        GenericType = new TObject();
    }
}

Answer 2

I think you can add a new type argument to FormController:

public class FormController<TParent, TChild>
  where TParent : class
  where TChild : class 
{
    ...

    public void showSubForm(string className)
    {
        IFormInterface f2 = new FormInterface<TChild>();
        f2.ShowDialog();
    }
}

Answer 3

So as I understand it, you want a Form<T> to open upon some action in the MainForm, with your MainForm using a FormController, as a manager of all your forms, relaying the generic type information to your Form<T>. Furthermore, the instantiated object of your Form<T> class should request an instance of a DatabaseController<T> class from your FormController.

If that is the case, the following attempt might work:

MainForm receives a reference to the FormController instance upon constructor initialization or has another way to interact with the FormController, e.g. a CommonService of which both know, etc.

This allows MainForm to call a generic method of the FormController to create and show a new Form object:

void FormController.CreateForm<T> () 
{
    Form<T> form = new Form<T>();
    form.Show();
    // Set potential Controller states if not stateless
    // Register forms, etc.
}

with Form<T> along the lines of:

class Form<T> : Form where T : class
{
    DatabaseController<T> _dbController;
    Form(FormController formController)
    {
        _dbController = formController.CreateDatabaseController<T>();
    }
}

Now you have a couple of ways for the Form to receive a DatabaseController instance:

1. Your Form<T> receives a reference of the FormController or has another way to communicate with it to call a method along the lines of:

DatabaseController<T> FormController.CreateDatabaseController<T> () 
{
    return new DatabaseController<T>();
}

Your FormController does not need to be generic, otherwise you'd need a new FormController instance for every T there is. It just needs to supply a generic method.

2. Your Form<T> receives an instance of the DatabaseController from the FormController upon constructor initialization:

   void FormController.CreateForm () { Form form = new Form(new DatabaseController()); form.Show(); }

with Form<T> being:

class Form<T> : Form where T : class
{
    DatabaseController<T> _dbController;
    Form(DatabaseController<T> controller) 
    {
         _dbController = controller;
    }
}

3. Like with 2 but your Form<T> and DatabaseController<T> provide static FactoryMethods to stay true to the Single Responsibility Priciple. e.g.:

public class Form<T> : Form where T : class
{
    private DatabaseController<T> _dbController;

    public static Form<T> Create<T>(DatabaseController<T> controller)
    {
        return new Form<T>(controller);
    }

    private Form(DatabaseController<T> controller) 
    {
         _dbController = controller;
    }
}

4. You can also use an IoC Container to register and receive instances of a specific type at runtime. Every Form<T> receives an instance of the IoC Container at runtime and requests its corresponding DatabaseController<T>. This allows you to better manage the lifetime of your controller and form objects within the application.