3tier application using c#

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For web app:
building 3tier app[^]
a-simple-3-tier-layers-application-in-asp.net[^]

For windows app:
3-tier architecture in C#[^]
http://social.msdn.microsoft.com/Forums/en-IE/winformsapplications/thread/2c15af26-7758-4268-9568-5932225cca32[^]

Create a class library - BusinessLogic. Add a reference of it in UI layer project.
Create a class library - DataAccess. Add a reference of it in BL layer project.

Now, from your UI, use the object model and pass on to BusinessLogic project class. This class is a Business logic class. Do the changes as per your need here.
Now, pass on the changed data from business logic class to dataAccess project class. In this class, use ADO.NET and pass on the needed values to Stored Procedure.

For getting back data, it will be transferred from DA to BL and then BL to UI layer.
Have a look at these, explaination with samples:
3-tier architecture in C#[^]
3-Tier Architecture Examples[^]
3-Tier Architecture in ASP.NET with C#[^]
3-Tier Architecture in asp.net using c#[^]

you can used mvc framework for implement Three-tier architecture.
and you can used web form than build data application layer , presentation layer ,business application layer
and you want to more help than used
http://social.msdn.microsoft.com/Forums/pl-PL/netfx64bit/thread/86b50b3d-97a8-4ef0-81ee-0c63d19b483f[^]
http://msdn.microsoft.com/en-us/library/aa286485.aspx[^]

3tier in the sense you need 3 -layers as follows
1-user interface & validation(console app,asp.net etc..(then in this project add two dll files by right click on the project name and then later add the reference between them ))
2-dll or reusable component.
3-dll or reusable component.
then 3rd layer will interact with SQL SERVER Database.