Question

I have a problem with declaring a type as passed from parameter type variable.

public static void ShowThisForm(Type passXtraForm, Form parent)
    {
        var xfrm = passXtraForm.GetType();
        xfrm xfrmName = new xfrm();
        xfrmName.Show();
    }

Can I declare a variable as a type from passXtraForm.GetType() and declare it to another variable? Just passing the type of form to another class.

Thanks in advance for the response.

Answer 1

First of all, it doesn’t look like you need the parent parameter; I’d eliminate it entirely. Then I’d use generics to do what you’re trying to accomplish:

public static void ShowThisForm<T>() where T : Form, new()
{
    T xfrmName = new T();
    xfrmName.Show();
} 

The where T : Form, new() portion of this code is called a type constraint and it prevents you from calling the ShowThisForm method with a type that doesn’t have a default constructor and derive from Form.

By indicating that T must have a default constructor, the compiler knows how to resolve new T(); by indicating that T derives from Form, the compiler knows how to call the .Show() method.

So, if you have a form class called MyForm, you could use the following syntax to call this method:

ShowThisForm<MyForm>();

For more documentation, you should take a look at these articles on MSDN:

• Generic Methods
• Constraints on Type Parameters