object reference not set to an instance of object exception coming at runtime

I am getting this error at runtime: object reference not set to an instance of object

my question is that am i using strin开发者_开发知识库gbuilder array correctly here. Because I am new in C#. and i think its the problem with my stringbuilder array. Below is the code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Services;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Text;
using System.Web.Script.Serialization;
using System.Web.Script.Services;
using System.Collections;


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

    }


    [WebMethod]
    public static string SendMessage()
    {
        try
        {
            al2c00.ldap ws = new al2c00.ldap();
            Hashtable htPeople = new Hashtable();

            //DataTable dt = ws.GetEmployeeDetailsBy_NTID("650FA25C-9561-430B-B757-835D043EA5E5", "john");

            StringBuilder[] empDetails = new StringBuilder[100];


            string num = "ambreen";
            empDetails[0].Append("amby");
            num = empDetails[0].ToString();

            htPeople.Add("bellempposreport", num);


            JavaScriptSerializer jss = new JavaScriptSerializer();

            string output = jss.Serialize(htPeople);
            return output;
        }
        catch(Exception ex)
        {
            return ex.Message + "-" + ex.StackTrace;
        }
    }
}

please reply me what i am doing wrong here.

---

empDetails is currently an array of StringBuilders, and empDetails[0] (the first StringBuilder in the array) is not instantiated, hence the error you're getting. What you should do is instead of making an array of StringBuilders (unless that's what you intend...), you should just make a single StringBuilder.

StringBuilder empDetails = new StringBuilder();

Then to append you can just do:

empDetails.Append("stringhere");

---

This line creates an array of 100 null references:

StringBuilder[] empDetails = new StringBuilder[100];

If you want to use empDetails[0], you have to assign something to it first:

empDetails[0] = new StringBuilder();

It's not clear to me why you are creating an array of 100 StringBuilders though. Couldn't you just do this?

htPeople.Add("bellempposreport", "ambreen");

---

Don't bother assigning this a value:

string num = "ambreen";

as it's going to be lost when you reassign it.

This:

StringBuilder[] empDetails = new StringBuilder[100]; 
//no need for creating an array of string builders.

Should be this:

StringBuilder empDetails = new StringBuilder();

AND

empDetails.Append("amby");

string num = empDetails.ToString();

http://msdn.microsoft.com/en-us/library/2839d5h5(VS.71).aspx

---

I think he might want to be pre-sized to 100

StringBuilder empDetails = new StringBuilder(100);
empDetails.Append("stringhere");

---

Only use stringbuilder if you are combining a large document of text. If you are just joining a few (<10) items simply using string.Format or just + them together such as

string myEmployee = string.Format("First:{0} Last: {1} Middle: {2}", "Stack", "Overflow", "C#");

---

Thanks everyone. My problem solved using below code:

        //Creating stringbuilder array for storing key values
        StringBuilder[] empDetails = new StringBuilder[70];

        for (int i = 0; i < empDetails.Length; i++) 
        { 
            empDetails[i] = new StringBuilder(); 
        }