public int InsertUser(UserInfo usin)
    {
        using (SqlConnection userconn = new SqlConnection(ConnectionString))
        {
            using (SqlCommand usercomm = new SqlCommand("dbo.InsertUser")) 
            {
                usercomm.CommandType = CommandType.StoredProcedure;
                SqlParameter parID = new SqlParameter("@UserID", SqlDbType.Int,  4 );
                parID.Direction = ParameterDirection.Output;
                SqlParameter parUser = new SqlParameter("@Username", SqlDbType.NVarChar,  50 );
                parUser.Value = usin.UserName_field;
                SqlParameter parPass = new SqlParameter("@Password", SqlDbType.NVarChar,  20 );
                parPass.Value = usin.Password_field;
                SqlParameter parFio = new SqlParameter("@Fio", SqlDbType.NVarChar,  150 );
                parFio.Value = usin.FIO_field;
                usercomm.Parameters.Add(parID);
                usercomm.Parameters.Add(parUser);
                usercomm.Parameters.Add(parPass);
                usercomm.Parameters.Add(parFio);
                try
                {
                    userconn.Open();
                    usercomm.Connection = userconn;
                    usercomm.ExecuteNonQuery();
                    return (int)usercomm.Parameters["@UserID"].Value;
                }
                catch (SqlException sqlerror)
                {

                    throw new ApplicationException("Ошибка соединения с базой!");
                }
                finally
                {
                    userconn.Close();
                }
        }   
      }
    }

ALTER PROCEDURE [dbo].[InsertUser]
@UserID int OUTPUT,
@Username varchar( 50 ),
@Password varchar( 20 ),
@Fio varchar( 150 )
AS
BEGIN
DECLARE @error int, @Guest bit
IF @Password IS NULL SET @Guest =  1  ELSE SET @Guest =  0 
INSERT INTO dbo.Users (UserName, Password, FIO, Guest) VALUES(@Username, @Password, @Fio, @Guest)
SET @error = @@ERROR
IF @error != 0  BEGIN
	IF @error =  515  RAISERROR ('Фамилия не может являться пустой. Проверьте правильность данных!',  1 ,  1 ) 
	ELSE RAISERROR ('Неизвестная ошибка, обратитесь к администратору!',  1 ,  1 )
						END
SET @UserID = @@IDENTITY
END

    Store_DB_Executor sde;
    ...
    protected void parsing(object Sender, EventArgs e)
    {
        UserInfo hypouser = new UserInfo(TextBox1.Text, TextBox2.Text, TextBox3.Text);
        sde.InsertUser(hypouser);
    }

public class UserInfo
{
    int UserID;
    string UserName, Password, FIO;
    DateTime RegistrationDate;
    bool Guest;

    public UserInfo(string UserName, string Password, string FIO)
	{
        this.UserName = UserName;
        this.Password = Password;
        this.FIO = FIO;
	}

    public int UserID_field
    {
        get { return UserID; }
        set { UserID = value; }
    }

    public string UserName_field
    {
        get { return UserName; }
        set { UserName = value; }
    }

    public string Password_field
    {
        get { return Password; }
        set { Password = value; }
    }

    public string FIO_field
    {
        get { return FIO; }
        set { FIO = value; }
    }

    public DateTime RegistrationDate_field
    {
        get { return RegistrationDate; }
        set { RegistrationDate = value; }
    }

    public bool Guest_field
    {
        get { return Guest; }
        set { Guest = value; }
    }