Create Table logs
(
id int primary key identity(1,1),
msg nvarchar(MAX)
)
Go

insert into logs(msg)values('qaz')
insert into logs(msg)values('qaz')
insert into logs(msg)values('edc')
insert into logs(msg)values('rfv')

CREATE TRIGGER dbo.AI_CheckSingleRow
   ON  [dbo].[logs]
   AFTER INSERT, UPDATE
AS 
BEGIN
	-- SET NOCOUNT ON added to prevent extra result sets from
	-- interfering with SELECT statements.
	SET NOCOUNT ON;

    -- Insert statements for trigger here
	DECLARE @numrows int;
	SET @numrows = 0;

	Select @numrows = count(*) from inserted 

	IF @numrows > 1
	BEGIN
		ROLLBACK TRAN;
		RAISERROR('Msg cannot be added', 16, 1);
		RETURN;
	END

END
GO

Update logs
Set msg = '123'
where msg = 'qaz'
Go
Select SCOPE_IDENTITY()

static void Main(string[] args)
        {
            string connectionString = "Data Sou...";

            SqlDataReader rdr = null;
            SqlConnection con = null;
            SqlCommand cmd = null;

            try
            {
                con = new SqlConnection(connectionString);
                con.Open();

                string commandText = "Select SERVERPROPERTY('productversion'), @@servername";

                cmd = new SqlCommand(commandText);
                cmd.Connection = con;

                rdr = cmd.ExecuteReader();

                while (rdr.Read())
                {
                    Console.WriteLine(rdr[0].ToString());
                    Console.WriteLine(rdr[1].ToString());
                }

                rdr.Close();

                ////=============

                commandText = "Update logs Set msg = '123' where msg = 'qaz'; Select SCOPE_IDENTITY()";

                objid = cmd.ExecuteScalar();
                if (objid != DBNull.Value)
                    Console.WriteLine(objid.ToString());
                else
                    Console.WriteLine("NULL!!!"); 

                ////=============


            }
            catch (Exception e)
            {
                Console.WriteLine(e.Message);
            }
            finally
            {
                if (rdr != null)
                    rdr.Close();

                if (con != null && con.State == ConnectionState.Open)
                    con.Close();
            }

            Console.Read();

        }