A null value for parameter 'id' of non-nullable type 'System.Int32' for method 'Test1.Models.Event FormView1_GetItem(Int32)' in 'Test1.EventDetails'. An optional parameter must be a reference type or a nullable type.

Question

User380908829 posted

I am new at working with web api and getting data from there. 

this is my error

A null value for parameter 'id' of non-nullable type 'System.Int32' for method 'Test1.Models.Event FormView1_GetItem(Int32)' in 'Test1.EventDetails'. An optional parameter must be a reference type or a nullable type.

 

I've got one web application and one web api .

this is the code for the formview , can somebody tell me how to fix this error !

.aspx

<%@ Page Title="" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true" CodeBehind="EventDetails.aspx.cs" Inherits="Test1.EventDetails" %>

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


<asp:FormView ID="FormView1" runat="server" ItemType="Test1.Models.Event" SelectMethod="FormView1_GetItem" >
<ItemTemplate>
<div>
<h1>
<%#: Item.title %>
</h1>
</div>
<br />
<table>
<tr>
<td>

</td>
<td>&nbsp;</td>
<td style="vertical-align:top; text-align:left;">
<b>Description</b><br /><%#: Item.description %><br /><span><b>Event ID</b>&nbsp;<%#: Item.eventkey %></span></td>
</tr>
</table>
</ItemTemplate>
</asp:FormView>

</asp:Content>

and this is 

aspx,cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Net.Http;
using System.Net;
using System.Net.Http.Headers;
using System.Threading.Tasks;
using Test1.Models;
using System.Globalization;
using Newtonsoft.Json;
using System.Web.ModelBinding;

namespace Test1
{
public partial class EventDetails : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{

RunAsync().Wait();

}

async Task RunAsync()
{

using (var client = new HttpClient())
{
client.BaseAddress = new Uri("http://localhost/locator/");
client.DefaultRequestHeaders.Accept.Clear();
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
string v = Request.QueryString["eventkey"];
HttpResponseMessage response = await client.GetAsync("api/Event/getevdetails?id=" + v).ConfigureAwait(false);
if (response.IsSuccessStatusCode)
{
Event myEvent = await response.Content.ReadAsAsync<Event>();

FormView1.DataSource = myEvent;
FormView1.DataBind();

}

}
}

// The id parameter should match the DataKeyNames value set on the control
// or be decorated with a value provider attribute, e.g. [QueryString]int id
public Test1.Models.Event FormView1_GetItem([QueryString]int id)
{

var uri = new Uri("http://localhost/locator/api/Event/getevdetails?id=" + id);
using (var client = new WebClient())
{
return JsonConvert.DeserializeObject<Event>(
client.DownloadString(uri)
);
}
}

}

}

// The id parameter should match the DataKeyNames value set on the control
// or be decorated with a value provider attribute, e.g. [QueryString]int id

Answer 1

User-760709272 posted

try changing

public Test1.Models.Event FormView1_GetItem([QueryString]int id)

to

public Test1.Models.Event FormView1_GetItem([QueryString]int? id)

you might have to change some code inside the method too to handle the fact that id might be null.

Answer 2

User2008642861 posted

Hi Drenusha,

Thank you for your post. I suggest that you could add a "if" statement to prevent if your parameter is empty.

Please refer to below code.

public Test1.Models.Event FormView1_GetItem([QueryString]int id)
        {
            if (id.Equals(null))
            {
                var uri = new Uri("http://localhost/locator/api/Event/getevdetails?id=" + id);
                using (var client = new WebClient())
                {
                    return JsonConvert.DeserializeObject<Event>(
                    client.DownloadString(uri)
                    );
                }
            }
            else 
            {
                string msg = "";
                return msg="The parameter is null!";
            }
        }

Hope this could be helpful to you.

Best regards,

Archer

Answer 3

User380908829 posted

Error:  Cannot implicity convert type 'string' to \Test1.Models.Event'

here is the model that i have:

public class Event

{

public int eventkey {get;set;}

public int type_fk {get;set;}

public string title {get;set;}

}

:(

Answer 4

User1728944201 posted

change to look like this:

 var uri = new Uri("http://localhost/locator/api/Event/getevdetails?id=" + Convert.ToString(id));

Answer 5

User616014865 posted

I am telling everybody in this blog to put try..catch around Request.QueryString because the parameter you are expecting ain't defined in url string.

At Catch( ), you should put in a default value for v.