Method error 500/12031 in ajax cascadingdropdown extender

If you are getting Method error 500 or method error 12031 in populating dropdown using ajax cascading dropdown, than you may have missed to include below mentioned line of code in code behind of your asmx (webservice) file

Add this line in webservice's code behind
[System.Web.Script.Services.ScriptService()]

It should look like this

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.Web.Script.Services.ScriptService()]
public class CascadingDropDown : System.Web.Services.WebService {

It should fix the error
This also occurs if there is any error in your sql statements or webservice code, you need to check them as well

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Implementing Ajax Cascading DropDownList in GridView with databse in ASP.NET

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