C# Interface

We have two Classes ClassOne & ClassTwo and two interfaces InterfaceOne and InterfaceTwo.
Is this code valid?

interface InterfaceOne

    {

        void InterfaceOneMethod();

    }



    interface InterfaceTwo

    {

        void InterfaceTwoMethod();

    }



    class ClassOne :  InterfaceOne, ClassTwo 

    {

        public void InterfaceOneMethod()

        {

            //Some Code

        }

    }



    class ClassTwo

    {        

    }

Ans: Base class must come before interface.

class ClassOne :   ClassTwo, InterfaceOne

    {

        public void InterfaceOneMethod()

        {

            //Some Code

        }

    }

WinForm Interview Question(C#)

Q. There is a WinForm(Form1) with a Button(Open Form2). If the user clicks the button it should open another WinForm(Form2). If the user again click on the Button in Form1, he should get a message saying that Form2 is already open.

Ans: In Form1's Button click event add this code.

 private void Form1Bttn_Click(object sender, EventArgs e)

        {

            bool FormOpen = false;



            //Application.OpenForms propety: Gets a collection of open forms 

            //owned by the application. 

            foreach (Form S in Application.OpenForms)

            {

                if (S is Form2)

                {

                    FormOpen = true;

                }

            }



            if (FormOpen == true)

            {

                MessageBox.Show("Form already open.");

            }



            else

            {

                Form MyForm = new Form2();

                MyForm.Show();

            }

        }