I have one legacy app running for more than 7 years and it is no long working now since 28/02/2019 , error
error Exception of type 'CTCT.Exceptions.CtctException' , developed C# and the library
https://github.com/constantcontact/Constant-Contact-Dot-Net-Wrapper , I also tried to start to use the OAuth2 authentication no success.
Thank you for reaching out to Constant Contact API Developer Support.
Unfortunately the C# wrapper you are using is deprecated and no longer supported. Also, that wrapper is for our V1 API which is also deprecated. Right now we have a wrapper available for our V2 API, but you will want to familiarize yourself with our V2 API before jumping into it. You can find our full V2 API documentation at http://developer.constantcontact.com/docs/developer-guides/api-documentation-index.html, and if you would like to view our .NET V2 wrapper library on Github, you can find it here: https://github.com/constantcontact/.net-sdk
Please have a look and let me know if you have any other questions!
Tier II API Support Engineer
Tanks for your feedback , I'm still having issues to run the V2 app you sent for me this is a 5 years old app everything has changed from there till today , in order to use the api direct I developed one simple app for test the constant contact using simple get interaction, works well for me.
I'm sharing my solution https://github.com/americoa/ConstantContactV2 .