The selecteditem of dropdownlist cannot get the selected value due to improper datavaluefield settings.

Selecteditem of dropdownlist takes the selected value. No matter what you select, it is always the value of the first listitem.

For example:

Dropdownlist1

Datatextfield datavaluefield

Test1 value1

Test2 value1

Test3 value1

Dropdownlist1 no matter which value you choose dropdownlist1.selecteditem. Text is test1

I finally found out that it was caused by this reason.
When binding dropdownlist data, datavaluefield and datatextfield are set.
Because datavaluefield has the same value, no matter which listitem you choose after submitting the webpage
Selecteditem obtains the same value. In. net, the selecteditem may be determined based on datavaluefield.
Although datatextfield is different, as long as they are the same datavaluefield,. Net considers them to be the same value.