Could not allocate a new page for database 'TEMPDB' because of insufficient disk space in filegroup 'DEFAULT'. Create the necessary space by dropping objects in the filegroup, adding additional files to the filegroup, or setting autogrowth on for existing files in the filegroup

Right click on database name, go to Properties.Go to files and see auto growth column and click on ... button and check enabled auto growth and checked unrestricted option and get solved the error.

How to serialize object to Json string

Crate Object and use JavascriptSerializer to Convert your object to Json string.
Example is as below.

Import assembly System.Web.Script.Serialization to your page.

JavaScriptSerializer jsonSerializer = new JavaScriptSerializer();
string jsonstring = jsonSerializer.Serialize(yourObject);