为什么“对象引用”没有设置为对象的实例?这里出现错误
本文关键字：对象引用 实例 这里 错误 为什么 设置 对象 | 更新日期: 2023-09-27 17:49:29
我正在一个asp.net mvc web应用程序工作，我有以下视图:-
@model MvcApplication4.Models.SelectedCustomers
<h3>Select Customers Detials</h3>
@using (Html.BeginForm("Export", null))
{ <table>
<tr>
<th>
NAME @Html.CheckBox("IncludeName", true)
</th>
<th>
Description @Html.CheckBox("IncludeDescription", true)
</th>
<th>
Address @Html.CheckBox("IncludeAddress", true)
</th>
</tr>
@foreach (var item in Model.Info) {
<tr>
<td>
@Html.DisplayFor(modelItem => item.ORG_NAME)
</td>
<td>
@Html.DisplayFor(modelItem => item.LOGIN_URI)
</td>
<td>
@Html.DisplayFor(modelItem => item.SUPPORT_EMAIL)
</td>
@Html.HiddenFor(modelItem => item.ORG_ID)
</tr>
}
</table>
<p>
@Html.ActionLink("Back","customer","Home") |
<button type="submit" name="Format" value="xls">extract to excel</button> |
<button type="submit" name="Format" value="csv">extract to text file</button>
</p>
}
和以下动作方法:-
[HttpPost]
public ActionResult Export(ExportViewModel exportOptions, IList<AccountDefinition> accounts)
{
// var accounts = GetAccount();
if (exportOptions.Format == "csv")
{
return accounts.AsCsvResult(exportOptions);
}
else if (exportOptions.Format == "xls")
{
return accounts.AsXlsResult(exportOptions);
}
throw new NotSupportedException(
string.Format("Unsupported format: {0}", exportOptions.Format)
);
}
然后我在我的AccountDefinition模型中定义了以下方法:-
public partial class AccountDefinition
{
public long ORG_ID { get; set; }
public string ORG_NAME { get; set; }
public string LOGIN_URI { get; set; }
public string SUPPORT_EMAIL { get; set; }
public virtual SDOrganization SDOrganization { get; set; }
}
public static class ActionResultextensions
{
public static ActionResult AsCsvResult(this IEnumerable<AccountDefinition> accounts, ExportViewModel exportOptions)
{
return new CsvResult(accounts, exportOptions);
}
public static ActionResult AsXlsResult(this IEnumerable<AccountDefinition> accounts, ExportViewModel exportOptions)
{
return new XlsResult(accounts, exportOptions);
}
}
}
然后我有以下模型类:-
1。
public class CsvResult : ExportAccountsResult
{
public CsvResult(IEnumerable<AccountDefinition> accounts, ExportViewModel exportOptions)
: base(accounts, exportOptions)
{
}
protected override string ContentType
{
get { return "text/csv"; }
}
protected override string Filename
{
get { return "accounts.csv"; }
} }
2,
public class XlsResult : ExportAccountsResult
{
public XlsResult(IEnumerable<AccountDefinition> accounts, ExportViewModel exportOptions)
: base(accounts, exportOptions)
{
}
protected override string ContentType
{
get { return "application/vnd.ms-excel"; }
}
protected override string Filename
{
get { return "accounts.csv"; }
}
}
3。
public class ExportViewModel
{
public string Format { get; set; }
public bool IncludeName { get; set; }
public bool IncludeDescription { get; set; }
public bool IncludeAddress { get; set; }
}
4。
public abstract class ExportAccountsResult : ActionResult
{
protected ExportAccountsResult(IEnumerable<AccountDefinition> accounts, ExportViewModel exportOptions)
{
this.Accounts = accounts;
this.ExportOptions = exportOptions;
}
protected IEnumerable<AccountDefinition> Accounts { get; private set; }
protected ExportViewModel ExportOptions { get; private set; }
protected abstract string ContentType { get; }
protected abstract string Filename { get; }
public override void ExecuteResult(ControllerContext context)
{
var response = context.HttpContext.Response;
response.ContentType = ContentType;
var cd = new ContentDisposition
{
FileName = this.Filename,
Inline = false
};
response.AddHeader("Content-Disposition", cd.ToString());
// TODO: Use a real CSV parser here such as https://github.com/JoshClose/CsvHelper/wiki/Basics
// and never roll your own parser as shown in this oversimplified
// example. Here's why: http://secretgeek.net/csv_trouble.asp
using (var writer = new StreamWriter(response.OutputStream))
{
foreach (var account in this.Accounts)
{
var values = new List<object>();
if (this.ExportOptions.IncludeName)
{
values.Add(account.ORG_NAME);
}
if (this.ExportOptions.IncludeDescription)
{
values.Add(account.LOGIN_URI);
}
if (this.ExportOptions.IncludeAddress)
{
values.Add(account.SUPPORT_EMAIL);
}
writer.WriteLine(string.Join(", ", values));
}
}
}
}
但是当我运行点击"提取到excel"或"提取到文本文件"按钮内的视图，我得到以下例外:-
System.NullReferenceException was unhandled by user code
HResult=-2147467261
Message=Object reference not set to an instance of an object.
Source=MvcApplication4
StackTrace:
at MvcApplication4.Models.ExportAccountsResult.ExecuteResult(ControllerContext context) in c:'Users'Administrator'Desktop'MvcApplication4'MvcApplication4'Models'ExportAccountsResult.cs:line 41
at System.Web.Mvc.ControllerActionInvoker.InvokeActionResult(ControllerContext controllerContext, ActionResult actionResult)
at System.Web.Mvc.ControllerActionInvoker.<>c__DisplayClass1c.<InvokeActionResultWithFilters>b__19()
at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func`1 continuation)
InnerException:
对ExportAccountsresults.cs中的以下代码进行修改:-
foreach (var account in this.Accounts)
{
::::::
我已经更新了我的观点如下:-
@model MvcApplication4.Models.SelectedCustomers
<h3>Select Customers Detials</h3>
@using (Html.BeginForm("Export", null))
{
Int32 c = -1;
<table>
<tr>
<th>
NAME @Html.CheckBox("IncludeName", true)
</th>
<th>
Description @Html.CheckBox("IncludeDescription", true)
</th>
<th>
Address @Html.CheckBox("IncludeAddress", true)
</th>
</tr>
@foreach (var item in Model.Info) {
c++;
<tr>
<td>
@Html.DisplayFor(modelItem => item.ORG_NAME)
</td>
<td>
@Html.DisplayFor(modelItem => item.LOGIN_URI)
</td>
<td>
@Html.DisplayFor(modelItem => item.SUPPORT_EMAIL)
</td>
<td>@Html.Hidden(String.Format("Info[{0}].ORG_ID", c), item.ORG_ID)</td>
</tr>
}
</table>
<p>
@Html.ActionLink("Back","customer","Home") |
<button type="submit" name="Format" value="xls">extract to excel</button> |
<button type="submit" name="Format" value="csv">extract to text file</button>
</p>
}
和我的动作方法到下面:-
[HttpPost]
public ActionResult Export(ExportViewModel exportOptions, SelectedCustomers sc)
{
// var accounts = GetAccount();
var accounts = sc.Info.ToList();
if (exportOptions.Format == "csv")
{
return accounts.AsCsvResult(exportOptions);
}
else if (exportOptions.Format == "xls")
{
return accounts.AsXlsResult(exportOptions);
}
throw new NotSupportedException(
string.Format("Unsupported format: {0}", exportOptions.Format)
);
}
现在csv文件将打开，但它将是空的，它将只包含"，，"每条记录，例如，如果在excel工作表或文本文件中应该有7条记录，那么。csv文件将有以下没有数据的内容:-
, ,
, ,
, ,
, ,
, ,
, ,
, ,
下面运行(请在视图中查看HiddeFor和Hidden的区别):
控制器namespace SomeApp.Controllers {
public class TestModel {
public IEnumerable<TestModelItem> Items { get; set; }
}
public class TestModelItem {
public int Id { get; set; }
public String Name { get; set; }
}
public class TestsController : Controller {
public ActionResult Index() {
TestModel tm = new TestModel {
Items = new List<TestModelItem> {
new TestModelItem { Id = 1, Name = "first"},
new TestModelItem { Id = 2, Name = "second"}
}
};
return View(tm);
}
[HttpPost]
public ActionResult Index(TestModel tm) {
return View(tm);
}
}
}
和视图
@model SomeApp.Controllers.TestModel
@{
Layout = null;
Int32 c = -1;
}
<!DOCTYPE html>
<html>
<head>
<title>Index</title>
</head>
<body>
<form method="post">
<table>
@foreach (var i in Model.Items) {
c++;
<tr>
@*<td>@Html.HiddenFor(x => i.Id)</td>*@
<td>@Html.Hidden(String.Format("Items[{0}].Id", c), i.Id)</td>
<td>@Html.DisplayFor(x => i.Name)</td>
</tr>
}
</table>
<button type=submit>go</button>
</form>
</body>
</html>
问题是IList<AccountDefinition>不能转换为IEnumerable<AccountDefinition>，因此在这里接收IEnumerable<T>:
public ActionResult Export(ExportViewModel exportOptions, IList<AccountDefinition> accounts)
或将其他所有内容更改为IList<T>。
另一个方法是:
public ActionResult Export(ExportViewModel exportOptions, IList<AccountDefinition> accounts)
{
var enumerableAccounts = accounts.AsEnumerable();
if (exportOptions.Format == "csv")
{
return enumerableAccounts.AsCsvResult(exportOptions);
}
else if (exportOptions.Format == "xls")
{
return enumerableAccounts.AsXlsResult(exportOptions);
}
throw new NotSupportedException(
string.Format("Unsupported format: {0}", exportOptions.Format)
);
}