这段代码中为啥ltrhist.Text后用+=而不用=

这段代码中为什么ltrhist.Text后用+=而不用=？

C# code

        private void Hist(crd_query_msg credit)        {                       var hists = credit.crd_hist;            var format = "<tr><td>{0}</td><td>{1}</td><td>{2}</td><td>{3}</td><td>{4}</td><td>{5}</td><td>{6}</td></tr>";            int i = 0;            foreach (var hist in hists)            {                i++;                ltrhist.Text += string.Format(format, i, hist.QRY_DATE.NullToString(), hist.APPT_NAME, hist.CAUSE ?? "未知",                hist.INFO_TYPE, hist.QRY_MODE, hist.TIME_STAMP.NullToString());            }        }

C# code

ltrhist.Text += string.Format(format, i, hist.QRY_DATE.NullToString(), hist.APPT_NAME, hist.CAUSE ?? "未知",                hist.INFO_TYPE, hist.QRY_MODE, hist.TIME_STAMP.NullToString());等同于ltrhist.Text =ltrhist.Text + string.Format(format, i, hist.QRY_DATE.NullToString(), hist.APPT_NAME, hist.CAUSE ?? "未知",                hist.INFO_TYPE, hist.QRY_MODE, hist.TIME_STAMP.NullToString());
[解决办法]
ltrhist.Text += "aaa" 意思就是
ltrhist.Text = ltrhist.Text + "aaa"
[解决办法]
这是追加文本，
将hists里的所有文本都追加到ltrhist.Text