When is null not a null?

Question

4.88/5 (9 votes)

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Hi,

Was wondering if someone can explain this:

The following returns null.

C#

Convert.ToString(null)

The following returns null.

C#

Request.QueryString["InvalidQSParamName"]

But the following returns an empty string.

C#

Convert.ToString(Request.QueryString["InvalidQSParamName"])

As does

C#

string myString = Request.QueryString["InvalidQSParamName"];
Convert.ToString(myString)

But this returns null

C#

string myString = null;
Convert.ToString(myString)

Can anyone explain why the difference in behaviour between two?

Posted 20-Jun-13 3:07am

Stephen Hewison

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Comments

sudipta biswas 20-Jun-13 9:29am

I am getting null in both cases.

7 solutions

Solution 4

To add to the two above:

MSDN Convert.ToString(object)[^]

Quote:
Return Value
Type: System.String
The string representation of value, or String.Empty if value is null.

And MSDN Convert.ToString(string)[^]

Quote:
Return Value
Type: System.String
value is returned unchanged.

Posted 20-Jun-13 3:27am

Ron Beyer

Solution 6

it has something to do with the convert.toXX method mechanism , convert method will forcelly change the null object to the default value of XX . But when you use XX.Parse(null) it will run error.

Posted 7-Oct-13 23:23pm

blachsmith

Updated 7-Oct-13 23:26pm

v2

Solution 7

Both the cases returns null values

Posted 7-Jan-14 22:07pm

Suk@nta

Solution 5

In this case u are using a Parameter in "ToString" Method, so it will return "Empty String"

Convert.ToString(Request.QueryString["InvalidQSParamName"])

but, In this case u are using null as a Parameter in "ToString" Method, so it will return "Null"

C#

string myString = null;
Convert.ToString(myString)

Posted 20-Jun-13 3:29am

Ravi Kumar Shakya

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CHill60 · Accepted Answer · 2013-06-20T03:22:00

Solution 1

Couple of good explanations in the responses to this stackoverflow post[^]

Posted 20-Jun-13 3:22am

CHill60

Comments

Leo Chapiro 20-Jun-13 9:25am

LOL, CHill60, we have found the same answer on stackoverflow ^^

Karthik Harve 20-Jun-13 9:25am

good one. +5!!

Stephen Hewison 20-Jun-13 9:29am

Thanks

jemaritn80 20-Jun-13 9:40am

I must be missing something. What you're saying is accurate, but HttpRequest.QueryString is a NameValueCollection. NameValueCollection "Represents a collection of associated String keys and String values that can be accessed either with the key or with the index." So, it would be Convert.ToString(string s) which should return null when s=null and not "". My test is consistent with this, but I am down voted. I'm curious to know what is wrong with my findings.

Leo Chapiro · Accepted Answer · 2013-06-20T03:23:00

As written here http://stackoverflow.com/questions/10355736/why-does-convert-tostringnull-return-a-different-value-if-you-cast-null[^] :

There are 2 overloads of ToString that come into play here

C#

Convert.ToString(object o);
Convert.ToString(string s);

The C# compiler essentially tries to pick the most specific overload which will work with the input. A null value is convertible to any reference type. In this case string is more specific than object and hence it will be picked as the winner.

In the null as object you've solidified the type of the expression as object. This means it's no longer compatible with the string overload and the compiler picks the object overload as it's the only compatible one remaining.

jemaritn80 · Accepted Answer · 2013-06-20T03:25:00

What test are you using to make this conclusion? If InvalidQSParamName is not a defined query string name, then my testing indicates that

C#

Convert.ToString(Request.QueryString["InvalidQSParamName"])== null

I arrived at this conclusion by creating a page named testpage.aspx.

Markup:

ASP.NET

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="testpage.aspx.cs" Inherits="TestSpace.testpage" %>



<html>
<head runat="server">
    <title>Null Test</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:label id="TestLabel" runat="server" xmlns:asp="#unknown" />
    </div>
    </form>
</body>
</html>

Code-Behind:

C#

using System;

namespace TestSpace
{
    public partial class testpage : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (Convert.ToString(Request.QueryString["emptyquery"]) == null)
            {
                this.TestLabel.Text = "Null";
            }
            else if (Convert.ToString(Request.QueryString["emptyquery"]) == "")
            {
                this.TestLabel.Text = "Empty String";
            }
        }
    }
}

The page output is

Null

Perhaps you have inadvertently created a query string name of InvalidQSParamName with an empty-string value. Try changing the parameter name and see if that gets you the expected results.