我猜您的问题与for循环有关.您应该在循环中定义局部变量，如下所示:

I guess your issue is related to for loop. You should define a local variable in your loop as follows:

    <div>
    @for(int i = 0; i < userInfo.SelectedDiagnosisIdList.Count; i++)
      {
        int local = i;
        <select id=$"dropdown{local+1}" 
               @bind="@userInfo.SelectedDiagnosisIdList[local]">
            @foreach (var item in diagnoses)
            {
                <option value="@item.Id">@item.Name</option>
            }
        </select>
        <p>@userInfo.SelectedDiagnosisIdList[local]</p>
    }
    </div>

希望这对您有帮助...

Hope this helps...