删除此行中设置参数值的部分：

mysqlcmd6.Parameters.Add("@res", MySqlDbType.Int32).Value=HITEM.Text;

看起来你希望这能约束 @res 到 HITEM 文本框，不是这样的。 HITEM.Text 只是一个字符串，当您将该值赋给int参数时，您告诉mysql您希望它能够将该字符串解析为int。
相反，只创建参数，如下所示：

mysqlcmd6.Parameters.Add("@res", MySqlDbType.Int32);

您还需要告诉ado.net这是一个输出参数。然后在查询运行后，通过将参数值指定给hitem.text而不是从hitem.text检查参数值：

private void a_KeyDown(object sender, KeyEventArgs e)
{
    //You can re-use the *names* of these variables, since their scopes are limited to the method
    //You can also stack them to share the same scope block and reduce nesting/indentation
    using (var con = new MySqlConnection(connectString))
    using (var cmd = new MySqlCommand("thamer1", con))
    {
        cmd.CommandType = CommandType.StoredProcedure;
        //  mysqlcmd6.CommandText = "thamer1"; //you already did this in constructor. Don't need to do it again
        cmd.Parameters.Add("@main_items_id_", MySqlDbType.Int32).Value = a.Text;
        //DON'T assign to the Value, but DO make sure ADO.Net understands this is an OUTPUT parameter
        cmd.Parameters.Add("@res", MySqlDbType.Int32).Direction = ParameterDirection.Output;

        //wait as long as possible to call Open()
        con.Open();
        cmd.ExecuteNonQuery();

        //Now you can assign**to**HITEM.Text, rather than from it.
        HITEM.Text = cmd.Parameters["@res"].Value;
    }
    //End the scope as soon as possible, so the connection can be disposed faster
    // MessageBox.Show("saved");
    // GridFill();
}

这里又是没有任何额外的评论：

private void a_KeyDown(object sender, KeyEventArgs e)
{
    using (var con = new MySqlConnection(connectString))
    using (var cmd = new MySqlCommand("thamer1", con))
    {
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.Parameters.Add("@main_items_id_", MySqlDbType.Int32).Value = a.Text;
        cmd.Parameters.Add("@res", MySqlDbType.Int32).Direction = ParameterDirection.Output;

        con.Open();
        cmd.ExecuteNonQuery();
        HITEM.Text = cmd.Parameters["@res"].Value;
    }
}

更好的做法是将所有sql方法从事件处理程序移到一个单独的类中。事件处理程序应该只需要调用新类中的方法，如下所示：

public static class DB
{
    private static string connectionString = "...";

    public static int thamer(int main_item_id)
    {
        using (var con = new MySqlConnection(connectString))
        using (var cmd = new MySqlCommand("thamer1", con))
        {
            cmd.CommandType = CommandType.StoredProcedure;
            cmd.Parameters.Add("@main_items_id_", MySqlDbType.Int32).Value = main_item_id;
            cmd.Parameters.Add("@res", MySqlDbType.Int32).Direction = ParameterDirection.Output;

            con.Open();
            cmd.ExecuteNonQuery();
            return (int)cmd.Parameters["@res"].Value;
        }
    }
}

private void a_KeyDown(object sender, KeyEventArgs e)
{
    HITEM.Text = DB.thamer(int.Parse(a.Text)).ToString();
}

改变这个

mysqlcmd6.Parameters.Add("@main_items_id_", 
MySqlDbType.Int32).Value = a.Text;
            mysqlcmd6.Parameters.Add("@res", MySqlDbType.Int32).Value = 
HITEM.Text;

至

int value1 = 0;
int value2 = 0;

if (!Int32.Text.TryParse(a.Text) || !Int32.TryParse(HITEM.Text))
{
    return;
}
 mysqlcmd6.Parameters.Add("@main_items_id_",  MySqlDbType.Int32).Value =  value1;
 mysqlcmd6.Parameters.Add("@res", MySqlDbType.Int32).Value = value2;