Eval()、XPath() 和 Bind() 这类数据绑定方法只能在数据绑定控件的上下文中使用。 （FromView)

在formview里面放3个联动到dropdownlist到时候会出现这样到错误信息。

查找了中文博客，没有任何到解释，终于在找国外资料到时候找到了这个问题到完全解决方案，非常到不错。

Demo for 2-way databinding cascading lists within a FormView

You might have wanted to use inside a databound templated server control (e.g. the FormView or the GridView) several cascading dropdownlists. The listitems in each dropdownlist would change based on the selection of the previous dropdownlist, e.g. the city selections would change based on the province and the latter would change based on the country selection. Sounds easy? Well, you added to your webform a datasource object linked for each dropdownlist where the SelectParameters collection contains a ControlParameter pointing to the previous list.

But you encounter any of those 2 error messages:

Databinding methods such as Eval(), XPath(), and Bind() can only be used in the context of a databound control
'DropDownList1' has a SelectedValue which is invalid because it does not exist in the list of items.

"What is the problem?", you ask. Well, it is not a bug by Microsoft's judgement (look at this Bug Report)

When you have a dropdownlist that is dependent on another within a FormView server control, it becomes populated on SelectedIndexChanged event of the first dropdown list. However, in this case the template was not rebound to the data, only the dropdown list. The FormView was reconstructed from the saved ViewState. This causes the error message: "Databinding methods such as Eval(), XPath(), and Bind() can only be used in the context of a databound control".

The solution is to remove the 2-way databinding from the cascading dropdownlists and replace it with customized code during processing the ItemUpdating event.

Let me expand a bit on the last sentence because people who visited the site asked for such an explanation. What are the problems again?

The problem of populating the dependant lists: When one list changes the second list needs to be re-bound to the data based on the previous list selection. To do that using the BIND statement, the container must be databound. But upon postback from the first list the container is not databound rather it is reconstructed from the viewstate
The problem of saving the values of the dependent lists after the user makes a selection: When updating the results without the BIND statement on the SelectedValue property of the cascading lists, you need to handle the ItemUpdating event to update the values manually to the data store.

Click on the Edit link (of the FormView display above) to activate the EditItemTemplate of the FormView. Change the country selection to watch the province/State and city selections change accordingly. Then you can view the same concepts applied to the GridView by clicking on the tab for the GridView above. The source code is available in 4 versions based on the type of datasource and the programming language that you use.

Notice that every button click on this demo posts the entire page back to the server and causes the entire screen to flash back. This distracts from the viewing experience and it has a cost on band width where the entire navigational controls are being posted back and refreshed. Compare this to the same sample but while using MS ASP.NET AJAX.

完整代码：

1<%@ Import Namespace="System.Data" %>

2

3<%@ Page Language="C#" AutoEventWireup="false" MasterPageFile="~/WEBSWAPP.master" %>

4

5<script language="C#" runat="server">

6

7 //As the FormView becomes databound, the DataBound events of the dropdownlists

8 //will trigger. At this stage the page has not been rendered yet and we can

9 //manipulate the displayed values on the dropdownlists

10 protected void ddlProvince_DataBound(object sender, EventArgs e)

11

38 void FormView1_PageIndexChanging(object sender, FormViewPageEventArgs e)

39

55 protected void ddlCity_DataBound(object sender, EventArgs e)

56 //==============================================================================================

70 //The concept is basically to intercept the databinding within the ItemUpdating event

71 //by passing into the FormViewUpdateEventArgs the new values that we need to update.

72 //This interception gives us the opportunity to set the selections for the cascading dropdownlist

73 //without relying on the Bind method to effect the 2-way databinding

74 //==============================================================================================

75 protected void FormView1_ItemUpdating(object sender, FormViewUpdateEventArgs e)

76

90

91 void FormView1_ItemCommand(object sender, FormViewCommandEventArgs e)

92

96 protected void FormView1_ItemInserting(object sender, FormViewInsertEventArgs e)

97

112 void odsAddresses_Inserted(object sender, ObjectDataSourceStatusEventArgs e)

113

120 void AddEmptyItem(DropDownList ddl)

121

127

128</script>

129

130<asp:Content ID="Content2" runat="server" ContentPlaceHolderID="ContentPlaceHolder1">

131

132 <table>

133 <tr>

134 <td>

135 <h3>

136 Demo for 2-way databinding cascading lists within a FormView</h3>

137 <asp:FormView ID="FormView1" runat="server" DataSourceID="odsAddresses" AllowPaging="True"

138 OnItemUpdating="FormView1_ItemUpdating" OnItemInserting="FormView1_ItemInserting" DataKeyNames="PK_ID"

139 OnItemCommand="FormView1_ItemCommand" OnPageIndexChanging="FormView1_PageIndexChanging" >

140 <EditItemTemplate>

141 <%---------------------EditItemTemplate Object datasources definitions-----------------------------------

142 Notice that the object datasources are specific to this template and are declared withint its boundaries

143 --------------------------------------------------------------------------------------------------------%>

144 <asp:ObjectDataSource ID="odsCountries" runat="server" SelectMethod="CountryList"

145 TypeName="WEBSWAPP_BLL.Demos"></asp:ObjectDataSource>

146 <asp:ObjectDataSource ID="odsProvinces" runat="server" SelectMethod="ProvincesList"

147 TypeName="WEBSWAPP_BLL.Demos">

148 <SelectParameters>

149 <asp:ControlParameter ControlID="ddlCountry" Name="CountryID" PropertyName="SelectedValue"

150 Type="String" />

151 </SelectParameters>

152 </asp:ObjectDataSource>

153 <asp:ObjectDataSource ID="odsCities" runat="server" SelectMethod="CitiesList" TypeName="WEBSWAPP_BLL.Demos">

154 <SelectParameters>

155 <asp:ControlParameter ControlID="ddlProvince" Name="ProvinceID" PropertyName="SelectedValue"

156 Type="String" />

157 </SelectParameters>

158 </asp:ObjectDataSource>

159 <%-------------- end of the object datasource difinitions for the EditItemTempalte ----------%>

160 <%------------- Begin of markup for the EditItemTemplate -----------------------------------%>

161 <%-- create a panel to be able to mark the Update hyperlink as the default button so that if

162 the user presses the Enter key while editing the Update button will clicked --%>

163 <asp:Panel ID="PanelForEditTemplate" runat="server" DefaultButton="UpdateButton">

164 <table class="CascadingDemo1">

165 <caption>

166 FormView in Edit mode</caption>

167 <thead>

168 <tr>

169 <th>

170 ID

171 </th>

172 <th>

173 <asp:Label ID="lblID" runat="server" Text='<%# Eval("PK_ID") %>'></asp:Label>

174 </th>

175 <th class="Origin">

176 Original Values</th>

177 </tr>

178 </thead>

179 <tr>

180 <td class="label2">

181 Company

182 </td>

183 <td colspan="2">

184 <asp:TextBox MaxLength="50" Width="300" ID="txtCompany" runat="server" Text='<%# Bind("Company") %>'></asp:TextBox>

185 <%-- Let's add a requiredfieldvalidator to ensure that edited values are not empty

186 but notice to set the CausesValidation=false on the Cancel CommandButton --%>

187 <asp:RequiredFieldValidator ID="valCompany" runat="Server" ControlToValidate="txtCompany"

188 ForeColor="white" ErrorMessage="Cannot leave the company name empty" Text="*"

189 SetFocusOnError="true" Display="Dynamic"></asp:RequiredFieldValidator>

190 </td>

191 </tr>

192 <tr>

193 <td class="label2">

194 Street Address

195 </td>

196 <td colspan="2">

197 <asp:TextBox ID="txtStreet" MaxLength="50" Width="300" runat="server" Text='<%# Bind("Street") %>'></asp:TextBox>

198 <asp:RequiredFieldValidator ID="valStreet" runat="Server" ControlToValidate="txtStreet"

199 ForeColor="white" ErrorMessage="Cannot leave the street address empty" Text="*"

200 SetFocusOnError="true" Display="Dynamic"></asp:RequiredFieldValidator>

201 </td>

202 </tr>

203 <tr>

204 <td class="label2">

205 Country

206 </td>

207 <td>

208 <asp:DropDownList ID="ddlCountry" runat="server" DataSourceID="odsCountries" AutoPostBack="True"

209 SelectedValue='<%# Eval("CountryID") %>' DataTextField="Description" DataValueField="PK_ID">

210 </asp:DropDownList>

211 </td>

212 <td class="Origin">

213 <asp:Label ID="lblCountry" runat="server" Text='<%# Eval("Country") %>'></asp:Label>

214 </td>

215 </tr>

216 <tr>

217 <td class="label2">

218 Province/State

219 </td>

220 <td>

221 <asp:DropDownList ID="ddlProvince" runat="server" DataSourceID="odsProvinces" AutoPostBack="true"

222 OnDataBound="ddlProvince_DataBound" DataTextField="Description" DataValueField="PK_ID">

223 </asp:DropDownList>

224 <asp:RequiredFieldValidator ForeColor="white" ID="valddlProvince" runat="server"

225 ErrorMessage="Cannot leave the province selection blank" Text="*" Display="Dynamic"

226 ControlToValidate="ddlProvince"></asp:RequiredFieldValidator>

227 </td>

228 <td class="Origin">

229 <asp:Label ID="lblProvince" runat="server" Text='<%# Eval("Province") %>'></asp:Label>

230 </td>

231 </tr>

232 <tr>

233 <td class="label2">

234 City

235 </td>

236 <td>

237 <asp:DropDownList ID="ddlCity" runat="server" DataSourceID="odsCities" OnDataBound="ddlCity_DataBound"

238 DataTextField="Description" DataValueField="PK_ID">

239 </asp:DropDownList>

240 <asp:RequiredFieldValidator ForeColor="white" ID="valddlCity" runat="server" ErrorMessage="Cannot leave the city selection blank"

241 Text="*" Display="Dynamic" ControlToValidate="ddlCity" ></asp:RequiredFieldValidator>

242 </td>

243 <td class="Origin">

244 <asp:Label ID="lblCity" runat="server" Text='<%# Eval("City") %>'></asp:Label>

245 </td>

246 </tr>

247 </table>

248 <asp:LinkButton ID="UpdateButton" runat="server" CommandName="Update" Text="Update"></asp:LinkButton>

249 <asp:LinkButton ID="btnCancel" CausesValidation="false" runat="server" CommandName="Cancel"

250 Text="Cancel"></asp:LinkButton>

251 </asp:Panel>

252 <%---------------------- end of markup for the EditItemTemplate ---------------------------%>

253 </EditItemTemplate>

254 <ItemTemplate>

255 <table class="CascadingDemo1">

256 <caption >

257 FormView in ReadOnly mode</caption>

258 <tr>

259 <td>

260 Company

261 </td>

262 <td>

263 <asp:Label ID="lblCompany" runat="server" Text='<%# Eval("Company") %>'></asp:Label>

264 </td>

265 </tr>

266 <tr>

267 <td>

268 Street Address

269 </td>

270 <td>

271 <asp:Label ID="lblStreet" runat="server" Text='<%# Eval("Street") %>'></asp:Label>

272 </td>

273 </tr>

274 <tr>

275 <td>

276 Country

277 </td>

278 <td>

279 <asp:Label ID="lblCountry" runat="server" Text='<%# Eval("Country") %>'></asp:Label>

280 </td>

281 </tr>

282 <tr>

283 <td>

284 Province/State

285 </td>

286 <td>

287 <asp:Label ID="lblProvince" runat="server" Text='<%# Eval("Province") %>'></asp:Label>

288 </td>

289 </tr>

290 <tr>

291 <td>

292 City

293 </td>

294 <td>

295 <asp:Label ID="lblCity" runat="server" Text='<%# Eval("City") %>'></asp:Label>

296 </td>

297 </tr>

298 </table>

299 <asp:LinkButton ToolTip="Click here to Edit the record" Text="Edit" runat="server"

300 ID="Edit" CommandName="Edit"></asp:LinkButton>

301 <asp:LinkButton ToolTip="Click here to insert a new Record" Text="Insert" runat="server"

302 ID="Insert" CommandName="New"></asp:LinkButton>

303 </ItemTemplate>

304 <InsertItemTemplate>

305 <%-----------------------------------------------------------------------------------------------------

306 Object Datasources defintions for the InsertItemTemplate

307 Notice that these definitions are specific to this tempalte and are only visible within its defintition

308 --------------------------------------------------------------------------------------------------------%>

309 <asp:ObjectDataSource ID="odsCountries" runat="server" SelectMethod="CountryList"

310 TypeName="WEBSWAPP_BLL.Demos"></asp:ObjectDataSource>

311 <asp:ObjectDataSource ID="odsProvinces" runat="server" SelectMethod="ProvincesList"

312 TypeName="WEBSWAPP_BLL.Demos">

313 <SelectParameters>

314 <asp:ControlParameter ControlID="ddlCountry" Name="CountryID" PropertyName="SelectedValue"

315 Type="String" />

316 </SelectParameters>

317 </asp:ObjectDataSource>

318 <asp:ObjectDataSource ID="odsCities" runat="server" SelectMethod="CitiesList" TypeName="WEBSWAPP_BLL.Demos">

319 <SelectParameters>

320 <asp:ControlParameter ControlID="ddlProvince" Name="ProvinceID" PropertyName="SelectedValue"

321 Type="String" />

322 </SelectParameters>

323 </asp:ObjectDataSource>

324 <asp:Panel ID="PanelForInsertTemplate" runat="server" DefaultButton="btnInsert">

325 <table class="CascadingDemo1">

326 <caption >

327 FormView in Insert mode</caption>

328 <tr>

329 <td class="label2">

330 Company

331 </td>

332 <td>

333 <asp:TextBox MaxLength="50" Width="300" ID="txtCompany" runat="server" Text='<%# Bind("Company") %>'></asp:TextBox>

334 <%-- Let's add requiredfieldvalidators to ensure that newly inserted records receive values

335 but notice to set the CausesValidation=false on the Cancel CommandButton --%>

336 <asp:RequiredFieldValidator ID="valCompany" runat="Server" ControlToValidate="txtCompany"

337 ForeColor="white" ErrorMessage="Cannot leave the company selection empty" Text="*"

338 SetFocusOnError="true" Display="Dynamic"></asp:RequiredFieldValidator>

339 </td>

340 </tr>

341 <tr>

342 <td class="label2">

343 Street Address

344 </td>

345 <td>

346 <asp:TextBox ID="txtStreet" MaxLength="50" Width="300" runat="server" Text='<%# Bind("Street") %>'></asp:TextBox>

347 <asp:RequiredFieldValidator ID="valStreet" runat="Server" ControlToValidate="txtStreet"

348 ForeColor="white" ErrorMessage="Cannot leave the street selection empty" Text="*"

349 SetFocusOnError="true" Display="Dynamic"></asp:RequiredFieldValidator>

350 </td>

351 </tr>

352 <tr>

353 <td class="label2">

354 Country

355 </td>

356 <td>

357 <%-- when we insert a new record there is no country value to be bound upon

358 displaying the empty record. Therefore let's add a default listitem with a null

359 value so that we can validate it using a RequiredFieldValidator to ensure that

360 the user will enter a value in it. I will use here the AppendDataBoundItems

361 property of the dropdownlist so that any databound items to be created will not

362 replace the default listitem that I just created. --%>

363 <asp:DropDownList ID="ddlCountry" runat="server" DataSourceID="odsCountries" AutoPostBack="True"

364 SelectedValue='<%# Eval("CountryID") %>' DataTextField="Description" DataValueField="PK_ID" AppendDataBoundItems="true">

365 <asp:ListItem Value="">Select a Country</asp:ListItem>

366 </asp:DropDownList>

367 <asp:RequiredFieldValidator ForeColor="white" ID="valddlCountry" runat="server" ErrorMessage="Cannot leave the country blank"

368 Text="*" Display="Dynamic" ControlToValidate="ddlCountry"></asp:RequiredFieldValidator>

369 </td>

370 </tr>

371 <tr>

372 <td class="label2">

373 Province/State

374 </td>

375 <td>

376 <asp:DropDownList ID="ddlProvince" runat="server" DataSourceID="odsProvinces" AutoPostBack="true"

377 DataTextField="Description" DataValueField="PK_ID" OnDataBound="ddlProvince_DataBound">

378 <asp:ListItem Value="">Select a Province</asp:ListItem>

379 </asp:DropDownList>

380 <asp:RequiredFieldValidator ForeColor="white" ID="valddlProvince" runat="server"

381 ErrorMessage="Cannot leave the province selection blank" Text="*" Display="Dynamic"

382 ControlToValidate="ddlProvince"></asp:RequiredFieldValidator>

383 </td>

384 </tr>

385 <tr>

386 <td class="label2">

387 City

388 </td>

389 <td>

390 <asp:DropDownList ID="ddlCity" runat="server" DataSourceID="odsCities" OnDataBound="ddlCity_DataBound"

391 DataTextField="Description" DataValueField="PK_ID">

392 <asp:ListItem Value="">Select a City</asp:ListItem>

393 </asp:DropDownList>

394 <asp:RequiredFieldValidator ForeColor="white" ID="valddlCity" runat="server" ErrorMessage="Cannot leave the city selection blank"

395 Text="*" Display="Dynamic" ControlToValidate="ddlCity"></asp:RequiredFieldValidator>

396 </td>

397 </tr>

398 </table>

399 <asp:LinkButton ID="btnInsert" runat="server" CommandName="Insert" Text="Insert"></asp:LinkButton>

400 <asp:LinkButton ID="btnCancel" runat="server" CausesValidation="false" CommandName="Cancel"

401 Text="Cancel"></asp:LinkButton>

402 </asp:Panel>

403 </InsertItemTemplate>

404 </asp:FormView>

405 <%-- The object Datasource definition for the FormView is declared outside of the FormView

406 --------------------------------------------------------------------------------------------------%>

407 <asp:ObjectDataSource ID="odsAddresses" runat="server" SelectMethod="Contacts" TypeName="WEBSWAPP_BLL.Demos"

408 UpdateMethod="UpdateContact" InsertMethod="InsertContact" OnInserted="odsAddresses_Inserted"

409 >

410 <UpdateParameters>

411 <asp:Parameter Name="PK_ID" Type="Int32" />

412 <asp:Parameter Name="Company" Type="String" />

413 <asp:Parameter Name="Street" Type="String" />

414 <asp:Parameter Name="CityID" Type="Int32" />

415 </UpdateParameters>

416 <InsertParameters>

417 <asp:Parameter Name="Company" Type="String" />

418 <asp:Parameter Name="Street" Type="String" />

419 <asp:Parameter Name="CityID" Type="Int32" />

420 </InsertParameters>

421 </asp:ObjectDataSource>

422 <%-- let's add a summary validation control to display a pop up message --%>

423 <asp:ValidationSummary ID="valSumm" runat="server" ShowMessageBox="true" ShowSummary="false" />

424 <%-- Let's add a label to display any status--%>

425 <asp:Label ID="lblStatus" runat="server" EnableViewState="false" CssClass="ErrMessage"></asp:Label>

426 </td>

427 </tr>

428

429 </table>

430</asp:Content>

431