MVC C#: System.InvalidOperationException

Question

I am new to MVC C#. I am building my project based on the MVC Music Store tutorial.

I have the following code which throw the following error message during runtime. Please tell me how to handle the null value in my code.

[error message]

An exception of type 'System.InvalidOperationException' occurred in System.Core.dll but was not handled in user code

[/error message]

[code]

public ActionResult SubCategory(int? s_catid)

{

var subcategories = db.Categories.Include("Sub_Categories").Single(s => s.C_Catid == s_catid);

return View(subcategories);

}

[/code]

Thanks and regards.

TK

TK Lim · Answer

Thanks for the reply. May i know when do i need to do the changes or checking? I am confused because i knew that there will be a value passing over but still the program throw the error message. Thanks and regards.

Anupam Singh · Answer

There are many ways to do this  1. you can use optional parameter like : public ActionResult SubCategory( int s_catid=1) // set default category. 2. you can use FirstOrDefault() instead of Single to get default values instead of null. var subcategories = db.Categories.Include( Sub_Categories ).FirstOrDefault(s => s.C_Catid == s_catid ); 3. or you can check null or count using if(subcategories!=null)