namespace ClassLibrary1
{
    public partial class Form1 : Form
    {
        public const int _Id = 6;
        public const string field1 = "1";
        public const string field2 = "2";
        public const string field3 = "3";

        public Form1()
        {
            InitializeComponent();
            label1.Text = _Id.ToString() + field1 + field2 + field3;
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            MessageBox.Show(field1 + field2 + field3);
        }
    }
}

private void button1_Click(object sender, EventArgs e)
        {
            Assembly testAssembly = Assembly.LoadFrom("ClassLibrary1.dll");

            Type curType = null;
            foreach (Type type in testAssembly.GetTypes())
            {
                if (type.IsClass)
                {
                    curType = type;
                }
            }
            Form form = (Form)Activator.CreateInstance(curType);
            form.ShowDialog(); // Or Application.Run(form)
        
        }

    [ReportDll]
    interface IReportable
    {
        //SqlConnection Connection { get; }

        
        ReportsType ReportsType { get; }

        string ReportsGroup { get; }
        string ReportName { get; }
        int ReportId { get; }

        void PrepareParams(FlowLayoutPanel flpParams);
        void Generate();

    }
    public enum ReportsType {Отчёты, Справочники}

    public class ReportDllAttribute : Attribute
    {
        
    }

  public partial class Form1 : Form, IReportable
    {
        public ReportsType ReportsType
        {
            get { return ReportsType.Справочники; }
        }
        public string ReportsGroup
        {
            get { return "Производство"; }
        }
        public string ReportName
        {
            get { return "Клиенты"; }
        }
        public int ReportId
        {
            get { return 6; }
        }

        public SqlConnection Connection;

        public void PrepareParams(FlowLayoutPanel flpParams)
        {            
            Form1_Load(null, null);
            CheckBox c = new CheckBox();
            flpParams.Controls.Add(c);
        }

        public void Generate()
        {
            
        }

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

    }

            Assembly testAssembly = Assembly.LoadFrom("ClassLibrary1.dll");

            var t = testAssembly.GetTypes();
            Type mytype = testAssembly.GetType("ClassLibrary1.Form1");
            object myobj = Activator.CreateInstance(mytype);

            var t3 = mytype.GetFields();
            var t2 = mytype.GetMethods();
            MethodInfo mymethod = mytype.GetMethod("Prepare2");
            mymethod.Invoke(myobj, new object[] {flowLayoutPanel1});