**当我试图将我的网格视图数据转换为PDF时，我得到了空引用异常网格视图标题行但数据在网格视图中可用。请帮助我解决此错误。我在这使用asp.net web API ..我试图在另一个网格视图绑定数据..它显示相同的错误空引用异常，但当我尝试直接按钮单击它工作正常..只是它显示错误当我尝试使用Web API时..请帮我解决这个问题 我已将我的编程附加到数据加载和绑定
之下public void GetData()
{
string conn = ConfigurationManager.ConnectionStrings["con"].ToString();
SqlConnection con = new SqlConnection(conn);
con.Open();
SqlCommand cmd = new SqlCommand("select * from api", con);
SqlDataAdapter da = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
da.Fill(ds, "api");
//DataTable dta = new DataTable();
//dta = ds.Tables["api"];
//da.Fill(dta);
GridView2.DataSource = ds;
GridView2.DataBind();
}
convert to pdf
public string Button3_Click()
{
PdfPTable pdfTable = new PdfPTable(GridView2.HeaderRow.Cells.Count);
foreach (TableCell headercell in GridView2.HeaderRow.Cells)
{
iTextSharp.text.Font font = new iTextSharp.text.Font();
font.Color = new Basecolor(GridView2.HeaderRow.ForeColor);
PdfPCell pdfcell = new PdfPCell(new Phrase(headercell.Text, font));
pdfcell.BackgroundColor = new Basecolor(GridView2.HeaderStyle.BackColor);
pdfTable.AddCell(pdfcell);
}
foreach (GridViewRow gridviewrow in GridView2.Rows)
{
foreach (TableCell tablecell in gridviewrow.Cells)
{
iTextSharp.text.Font font = new iTextSharp.text.Font();
font.Color = new Basecolor(GridView2.RowStyle.ForeColor);
PdfPCell pdfcell = new PdfPCell(new Phrase(tablecell.Text));
pdfcell.BackgroundColor = new Basecolor(GridView2.RowStyle.BackColor);
pdfTable.AddCell(pdfcell);
}
}
Document pdfdoc = new Document(PageSize.A4, 10f, 10f, 10f, 10f);
PdfWriter.GetInstance(pdfdoc, Response.OutputStream);
pdfdoc.Open();
pdfdoc.Add(pdfTable);
pdfdoc.Close();
Response.ContentType = "application/pdf";
Response.AppendHeader("content-disposition", "attachment;filename=Employyes.pdf");
Response.Write(pdfdoc);
Response.Flush();
Response.End();
return "success";
}
ajax function call
$(document).ready(function () {
$("#Button2").click(function () {
alert("ajax call");
$.ajax
({
method: "GET",
url: "api/Employees",
contentType: "application/json; charset=utf-8",
dataType: "json",
success: function (response) {
alert("success");
},
error: function (responce) {
alert("error");
},
});
});
});
<asp:Button ID="Button1" runat="server" Style="width: 136px" Text="Export to Excel" Width="175px" />      
<asp:Button ID="Button2" runat="server" Text="Export to PDF" Width="175px" />
<br />
<br />
<div class="auto-style1" id="NewData">
<asp:GridView ID="GridView2" runat="server" AutoGenerateColumns="true">
</asp:GridView>
</div>
here is my controller class
public class EmployeesController : ApiController
{
// success s = new success();
BusinessLogicLayer bal = new BusinessLogicLayer();
public string Get(string username, string password)
{
return bal.login(username, password);
}
[System.Web.Mvc.HttpPost]
public string POST(string firstname, string lastname, string username, string password, string email)
{
return bal.Register(firstname, lastname, username, password, email);
}
[System.Web.Http.HttpGet]
public string buttonclick()
{
// MessageBox.Show("come");
return bal.ExportConvert();
}
here is my business logic layer class method
public string ExportConvert()
{
success s = new success();
return s.Button3_Click();
}[enter image description here][1]
这是我的登录页面代码
**<!DOCTYPE html>
<html>
<head>
<title></title>
<meta charset="utf-8" />
<link href="Content/bootstrap.min.css" rel="stylesheet" />
</head>
<body style="padding-top:20px;">
<div class="col-md-10 col-md-offset-1">
<div class="well">
<table class="table table-bordered">
<thead>
<tr class="success">
<th colspan="2">
Existing User Login
<a class="btn btn-success pull-right" href="Registration.html">Register</a>
</th>
</tr>
</thead>
<tbody>
<tr>
<td>UserName</td>
<td>
<input type="text" id="username" placeholder="username"/>
</td>
</tr>
<tr>
<td>Password</td>
<td>
<input type="password" id="password" placeholder="Password" />
</td>
</tr>
<tr class="success">
<td colspan="2">
<input type="button" id="btnlogin" class="btn btn-success" value="login" />
</td>
</tr>
</tbody>
</table>
<div class="modal fade" tabindex="-1" id="successmodel"
data-keyboard="false" data-backdrop="static">
<div class="modal-dialog modal-sm">
<div class="modal-content">
<button type="button" class="close" data-dismiss="modal">
×
</button>
<h4>Success</h4>
</div>
<div class="modal-body">
<h2>Registraton success</h2>
</div>
<div class="modal-footer">
<button type="button" data-dismiss="modal" class="btn btn-success">
Close
</button>
</div>
</div>
</div>
</div>
<div id="divError" class="alert alert-danger collapse">
<a id="linkClose" class="close" href="#" >×</a>
<div id="divErrorText"></div>
</div>
</div>
<script src="Scripts/jquery-1.10.2.min.js"></script>
<script src="Scripts/bootstrap.min.js"></script>
<script type="text/javascript" >
$(document).ready(function () {
$('#linkClose').click(function () {
$('divError').hide('fade');
})
});
$('#btnlogin').click(function () {
$.ajax({
url: '/api/Employees/?username=' + $("#username").val() + '&password=' + $("#password").val(),
method: 'Get',
contentType: 'application/json',
data: { id: 1 },
dataType: 'json',
success: function (data) {
console.log(data);
//var data = data.d;
//alert("success " + data);
if (data == 'success') {
window.location = "/success.aspx";
}
else {
alert("Wrong Username and Password");
}
/* if (data != null) {
$.each(data, function (i, obj) {
var c = new WorkflowsEntity(data[i]); //logic
self.WorkflowList.push;
});
alert("success");
//window.location.href = "/success.html";
}*/
},
error: function (jqXHR, textStatus, errorThrown) {
alert(" un correct username password ");
$('#divErrorText').text(jqXHR.responceText);
$('#divError').show('fade');
}
});
});
</script>
</body>
</html>**