NULL Reference Error

Question

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What is a NullReferenceException in .NET?

When i am trying to access value stored in object it shows null reference error when the value is null.How to check for null values???

str = "<html>" + objName.Name + "</html>"

Thanks in Advance

i tried something like str = ""+ if(objName.name.count>0){objName.name} +""..but if cannot be used like this — Rachel, Aug 31 '11 at 06:14

score 4 · Answer 1 · answered Aug 31 '11 at 06:12

4

How about:

if(objName != null)
{
    str = "<html>" + objName.Name + "</html>"
}

answered Aug 31 '11 at 06:12

marc_s

675,133
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Its unclear if they intend to add the even if it doesn't have name value. I would assume if its and html tag that they would want to include it to be complete if its an html page. – Celess Aug 31 '11 at 06:27

score 4 · Answer 2 · answered Aug 31 '11 at 06:13

4

something like:

if(objName != null && !string.IsNullOrEmpty(objName.Name))
{
  str = "<html>" + objName.Name + "</html>"
}

answered Aug 31 '11 at 06:13

Davide Piras

42,120
10
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140

This is the best answer for checking all edge conditions. – Erik Philips Aug 31 '11 at 06:16
there is no need to check null or empty for objName.Name cause if it is null then also it will concate, not give any error due to string. – Upendra Chaudhari Aug 31 '11 at 06:20
yup I was also thinking this, but my defensive programming approach came first :) – Davide Piras Aug 31 '11 at 06:22
Depends on the situation. Per the question, this is acceptable, and for future readers, it's helpful to understand why the output might be blank without any exception. – Erik Philips Aug 31 '11 at 06:32
Thanks..but after using str=""+ objName != null ? objName.Name : string.empty + "" the formatting i did under html tag disappeared – Rachel Aug 31 '11 at 07:00

score 1 · Answer 3 · answered Aug 31 '11 at 06:14

1

Check null for object like below :

str="<html>"+ objName != null ? objName.Name : string.empty + "</html>"

answered Aug 31 '11 at 06:14

Upendra Chaudhari

6,197
5
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thanks it worked ..but rest of the formatting disappeared ..all the object after first are not showing any values – Rachel Aug 31 '11 at 06:31
can you please specify your problem in details ? – Upendra Chaudhari Aug 31 '11 at 06:35
str= "" + "
" + "" + "
Test Name
" + objTest.name.ToString() + "
" +objname.name.ToString() != null ? string.Join(", ", objname.name.ToArray()) : String.Empty +
";
– Rachel Aug 31 '11 at 06:40
after applying condition on null object the rest of formatting like image heading everything has disappeared. – Rachel Aug 31 '11 at 06:41
after applying i am getting just objTest.name values ...no image heading anything – Rachel Aug 31 '11 at 06:45
Check objname != null rather than objname.name.ToString() != null – Upendra Chaudhari Aug 31 '11 at 06:45
but i am trying to take different values like objTest.name ,objTest.class etc... – Rachel Aug 31 '11 at 06:48
str= "" + "
" + "" + "
Test Name
" + objTest != null ? objTest.name : string.empty + "
" + objname != null ? objname.name + objname.class : String.Empty +
";
– Upendra Chaudhari Aug 31 '11 at 06:51
even after checking like objName != null i don know where my rest of the formatting disappears..when i remove that part everything comes back like heading border image – Rachel Aug 31 '11 at 06:55
Give your whole code for str assignment line, so I can find problem – Upendra Chaudhari Aug 31 '11 at 06:57
it exceeds word limit i cant paste here..but its like what i have pasted – Rachel Aug 31 '11 at 06:59

Celess · Answer 4 · 2011-08-31T07:43:32.520

Simple case:

str = "<html>" + (objName != null ? objName.Name : "") + "</html>";

Little more performant:

if (objName == null)
   str = "<html></html>";
else
   String.Concat("<html">, objName.Name, "</html>");

If you are doing this a lot, consider using StringBuilder, which might look someting like:

var html = new StringBuilder();
html.Append("<html>");
html.Append(objName != null ? objName.Name : "");
...
html.Append("</html>";

If you are adding several tags together, and not using StringBuilder you need to do someting like this:

str = "<html>" + (objName != null ? objName.Name : "") + "</html>";
str += "<script>" + (objName2 != null ? objName2.Name : "") + "</script>";

... where you put a += to concantinate the new part with the last. Note also that "" is effectively the same as String.Empty. You might benifit greatly from using "" instead of String.Empty, until you learn to make the code shorter and more readable.

For your code:

string str;

str = "<body lang=EN-US style='tab-interval:.5in'>";
str += "<div class=Section1>";
str += "<img img width=100 height=50 id='myImg' src='C:/test.jpg'>";
str += "<p style='font-weight:bold; color:green;'>";
str += "<u> Test Name </u></p>" + objTest.name.ToString() + "<br/>";
str += objname.name.ToString() != null ? string.Join(", ", objname.name.ToArray()) : String.Empty;
str += "</body> </html> ";

... is going to make this much easier to figure out. You shoudl try the StringBuilder aproach which is nearly the same as this. Should make it much easier overall, I promise :)

So specfically here are a few problems:

`... + objTest.name.ToString() + ...`

This will fail if objTest is null, and makes the

`objname.name.ToString() != null`

not mean much.

You probably intend to do something like this (for both your objTest and objname):

str += "<u> Test Name </u></p>" + (objname != null && objname.name != null ? objname.name.toString() : "") + "<br/>";

This protects both objname and objname.name.

Even better would be:

string str;
var objNameName = (objname != null) ? objname.name : null;
var objTestName = (objTest != null) ? objTest.name : null;

str = "<body lang=EN-US style='tab-interval:.5in'>";
str += "<div class=Section1>";
str += "<img img width=100 height=50 id='myImg' src='C:/test.jpg'>";
str += "<p style='font-weight:bold; color:green;'>";
str += "<u> Test Name </u></p>" + (objTestName != null ? objTestName.ToString() : "") + "<br/>";
str += objNameName != null ? string.Join(", ", objNameName.ToArray()) : "";
str += "</body> </html> ";

Celess after using str=""+ objName != null ? objName.Name : string.empty + "" the formatting i did under html tag disappeared — Rachel, Aug 31 '11 at 07:11
I added more. See how that works for you and maybe helps it all make more sense. This part is hard at first, gets better. — Celess, Aug 31 '11 at 07:49

NULL Reference Error

4 Answers4