Asp.net 错误：不能在DropDownList中选择多个项目

这是我的代码，我收到一个错误..不能在DropDownList中选择多个项目。当页面加载时，在DDLMotality中选择ALL，但当我将DDLMotality.selectedvalue更改为任意值时，则无错误，但当我更改为ALL时，再次选择获取错误

更改表格后，请提交表格 表1.target=； 表1.action=； 表格1.提交

 ' USED TO ADD REFERRING PHYSICIAN IN THE DROPDOWN DYNAMICALLY 
    If CInt(Session("CenterID")) = 0 Then
        sql = "Select Ref_Phy_ID,Name from Ref_Phy_Master WHERE Ref_Phy_ID in(Select distinct Ref_Phy_ID from Patient_Details where Ref_Phy_ID <> '')"
    Else
        sql = "Select Ref_Phy_ID,Name from Ref_Phy_Master WHERE Ref_Phy_ID in(Select distinct Ref_Phy_ID from Patient_Details where Ref_Phy_ID <> '') And Center_ID = " & Session("CenterID")
    End If
    objDS = objFun.RunQuery(sql)
    ' USED TO REFRESH THE PAGE WHIN IT IS POSTED BACK 
    ' USED TO DISPLAY DEFAULT FIRST ITEM IN THE DROPDOWN 
    Dim Li1 As New ListItem()
    Li1.Text = "ALL"
    Li1.Value = ""
    cboRefPhy.Items.Add(Li1)
    ' USED TO COUNT THE STUDIES IN THE DROPDOWN 
    If (objDS.Tables(0).Rows.Count <> 0) Then
        ' USED TO CIRCULATE LOOP UPTO THE RECORD COUNT
        Dim i As Integer
        For i = 0 To objDS.Tables(0).Rows.Count - 1
            ' USED TO CREATE NEW ITEM IN THE DROPDOWN 
            Dim Li As New ListItem
            Li.Text = objDS.Tables(0).Rows(i)("Name").ToString()
            Li.Value = objDS.Tables(0).Rows(i)("Ref_Phy_ID").ToString()
            'USED TO ADD ITEMS IN THE DROPDOWN 
            cboRefPhy.Items.Add(Li)
        Next
    End If
    'USED TO SAVE THE CHANGES IN DATASET  
    objDS.AcceptChanges()
    ' USED TO CLOSE THE DATABASE CONNECTION 
    objDS.Dispose()
    cboRefPhy.ClearSelection()
    cboRefPhy.SelectedValue = Convert.ToString(Request.Form("cboRefPhy"))

    'USED TO ADD MODALITY IN THE DROPDOWN DYNAMICALLY
    If CInt(Session("CenterID")) = 0 Then
        sqlStudy = "Select Modality_ID,Modality from Hospital_Modality_Master WHERE Modality_ID in(Select distinct Study_ID from Patient_Details where Study_ID <> '')"
    Else
        sqlStudy = "Select Modality_ID,Modality from Hospital_Modality_Master WHERE Modality_ID in(Select distinct Study_ID from Patient_Details where Study_ID <> '') And Center_ID = " & Session("CenterID")
    End If
    'Dim objDS As New DataSet()
    objDS = objFun.RunQuery(sqlStudy)
    ' USED TO REFRESH THE PAGE WHIN IT IS POSTED BACK 
    ' USED TO DISPLAY DEFAULT FIRST ITEM IN THE DROPDOWN 
    'Dim Li1 As New ListItem()
    Li1.Text = "ALL"
    Li1.Value = ""
    ' Dim all As String
    ' all = "All"
    'Ddl_Modality.Items.Add(all)
    DDLModality.Items.Add(Li1)
    ' USED TO COUNT THE STUDIES IN THE DROPDOWN 
    If (objDS.Tables(0).Rows.Count <> 0) Then
        ' USED TO CIRCULATE LOOP UPTO THE RECORD COUNT
        Dim i As Integer
        For i = 0 To objDS.Tables(0).Rows.Count - 1
            ' USED TO CREATE NEW ITEM IN THE DROPDOWN 
            Dim Li As New ListItem
            Li.Text = objDS.Tables(0).Rows(i)("Modality").ToString()
            Li.Value = objDS.Tables(0).Rows(i)("Modality_ID").ToString()
            'USED TO ADD ITEMS IN THE DROPDOWN 
            DDLModality.Items.Add(Li)
        Next
    End If
    'USED TO SAVE THE CHANGES IN DATASET  
    objDS.AcceptChanges()
    ' USED TO CLOSE THE DATABASE CONNECTION 
    objDS.Dispose()
    DDLModality.ClearSelection()
    DDLModality.SelectedValue = Convert.ToString(Request.Form("DDLModality"))


    ' USED TO ADD STUDY IN THE DROPDOWN DYNAMICALLY 
    If CInt(Session("CenterID")) = 0 Then
        sqlStudy = "Select Study_ID,Study_Desc from Study_Master WHERE Study_ID in(Select distinct Study_ID from Patient_Details where Study_ID <> '')"
    Else
        sqlStudy = "Select Study_ID,Study_Desc from Study_Master WHERE Study_ID in(Select distinct Study_ID from Patient_Details where Study_ID <> '') And Center_ID = " & Session("CenterID")
    End If

    If (DDLModality.SelectedItem.Text <> "ALL") Then
        sqlStudy = sqlStudy & " AND Modality = '" & DDLModality.SelectedItem.Text & "'"

    End If

试一试

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在设置ddl.SelectedValue之前，我面临着同样的问题。如果将一个已定义的ListItem（在本例中为Li1）添加到两个或多个dropdownlist，则在将selectedvalue分配给任何dropdownlist时会发生此错误

简单的解决方案是为单独的下拉列表（如Li1、Li2、Li3等）定义单独的列表项

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我不明白背后的逻辑，但它是有效的

对不起，我不知道VB，但在C中，您可以在说DDLModality.SelectedValue=Convert.toString.Quest.FormDDLModality之前设置DDLModality.SelectedValue=-1。这将取消选择以前的任何选择，因此当您确实选择某个内容时，尚未选择值。在您编写代码时，很容易受到SQL注入攻击。请使用参数化查询。查看为什么需要DDLModality.SelectedValue=Convert.ToStringRequest.FormDDLModality设置DDLModality.SelectedValue=-1设置DDLModality后，我收到相同的错误。请为下拉列表发布呈现的标记好吗？下拉列表中是否有重复项？否//当页面加载时，请在DDLModality中选择全部，但当我更改时DDLModality.selectedvalue为任意值，然后无错误，但当我更改为ALL时再次出现错误..将Li2设置为新列表项Li2.Text=ALL Li2.value=DDLModality.Items.AddLi2

DDLModality.ClearSelection()

Dim Li2 As New ListItem()
      Li2.Text = "ALL"
      Li2.Value = ""  
      DDLModality.Items.Add(Li2)