layers in C# and create methods but not reusing

Question

enter image description hereI create new project with 3 layers one for presentation layer and one for data layer and one for business layer presentation layer contains 3 forms with controls business layer contains class with the method which down what I want is those method is validation for each text box is empty or not but I got the error while running strong text

namespace Teacher_Assistant.BL
{
    class Class1
    {
        public IEnumerable<Control> Controls { get; private set; }

        public void checktextboxisempty()
        {
            foreach (Control c in this.Controls)
            {
                if ( c is TextBox)
                {
                    TextBox textBox = c as TextBox;
                    if (textBox.Text == string.Empty)
                    {
                        MessageBox.Show("Please fill the boxes");
                        textBox.BackColor = Color.Red;
                        break;
                    }
                    else
                    {
                        textBox.BackColor = Color.White;
                    }
                }
            }
        }
        public void cleanalltextbox()
        {
            foreach (Control c in this.Controls)
            {
                if (c is TextBox)
                {
                    TextBox textBox = c as TextBox;
                    if (textBox.Text != string.Empty)
                    {
                        textBox.Clear();
                    }

                }
            }
        }
    }
}