UserControl1 UC = new UserControl1();
string outputJSON = JsonConvert.SerializeObject(UC);
File.WriteAllText("mifile.json", outputJSON);

public class PersonDto
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    // ...
}

public PersonDto Data
{
    get
    {
        return new PersonDto
        {
            FirstName = this.FirstNameTextBox.Text,
            LastName = this.LastNameTextBox.Text,
            // ...
        }
    }
}

UserControl1 UC = new UserControl1();
// ...
string outputJSON = JsonConvert.SerializeObject(UC.Data);