I assume that you are using your code on a computer where SharePoint is not hosted. And you want to remotely check file (s). In this case, use the SharePoint ListService form for this.
Here is a small example:
Web_Reference_Folder.Lists listService = new Web_Reference_Folder.Lists(); listService.Credentials = System.Net.CredentialCache.DefaultCredentials; string fileCheckout = "http://Server_Name/sites/Subsite/Shared Documents/MyFile.txt"; bool myResults = listService.CheckOutFile(fileCheckout, "true", "20 Jun 2006 12:00:00 GMT");
Here you can see all the SharePoint services:
http://msdn.microsoft.com/en-us/library/dd878586%28v=office.12%29.aspx
If you use the code on the computer where SharePoint is installed, in this case you can add a link to Microsoft.SharePoint.dll and continue. If you want to deploy your code on a machine that will also host the SharePoint server, you need to compile the code in a similar environment.
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