Getting "Object Reference Error" while adding String.Format in StringBuilder

Question

I am getting "object reference error" while putting string.format in stringbuilder. I have below code in C#2.0

public const string PageLinkGoogleMarkup = "<link rel=\"alternate\" hreflang=\"{0}\" href=\"{1}www.mysite.com{2}{3}\" />\r\n";

Now when I am adding this to HtmlTextWriter it is working fine, as below:

HtmlTextWriter writer (Object);
writer.write(string.format(PageLinkGoogleMarkup,str[1],header,links,querystr)); //This works perfect

And when I tries to add this above code to StringBuilder instead of HtmlTextWriter it gives error:

As there is big loop so I decided to go with appending everything to StringBuilder first and at last rendering using writer.Write (Whole html in a bunch), please below code

StringBuilder sb = new StringBuilder();
sb.Append(string.format(PageLinkGoogleMarkup,str[1],header,links,querystr)); //here i get object reference error

writer.Write(sb.ToString());

Please suggest!!

Thanks.

Answer 1

One of the following variables is null when you are using it in the StringBuilder code (but was not null when you used it in the working code):

str (or the item in index [1])
header
links
querystr

Use the debugger to figure out which one is null, fix that, and the code should work fine.