How to checkin to foursquare Using dotnet

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ASP.NET

Hi

I have created a venue and i wanted all user to leave a comment (check in) about that venue in foursquare.
So in my application i am able to send the user to authenticate page and redirecting him to the callback url which i specified. But there i am unable to get Access token and further not able to checkin.

Please help me in this.

This is the code i am using "objSharpSquare.GetAccessToken("http://localhost:5161/DreamForce_Sample/testpage.aspx", Request("code"))"

but it gives error:The remote server returned an error: (400) Bad Request.

Plz help me in this

Thanks in advance