Thinking out aloud - Dave Hunter's SharePoint Blog

Usually in SharePoint there are several ways to implement some functionality.  With experience you learn which is the best method.  This is true of retrieving a document from a document library.  At first you think the files must be part of a collection so you look at how to access the SPFileCollection and from that access the document you are interested in.

Approach 1

Typically in .NET when access a collection and retrieve a single item either by name, guid or integer you check the item has been returned before using it. This returns an empty structure and you test if it isn't NULL.  The following example tries this with retrieving a document (SPFile) from a document library.

SPWeb currentSite = SPContext.Current.Web;
SPDocumentLibrary docLib = (SPDocumentLibrary)currentSite.GetList(currentSite.Url + "/Documents"); 

if (docLib.RootFolder.Files[fileName] != null) 
{ 
  // do something 

}

This throws an Argument Exception

Approach 2

SharePoint has a "Exists" property of the SPFile, this returns a boolean flag to indicate if the file exists.

SPWeb currentSite = SPContext.Current.Web;
SPDocumentLibrary docLib = (SPDocumentLibrary)currentSite.GetList(currentSite.Url + "/Documents"); 

if (docLib.RootFolder.Files[fileName].Exists == true) 
{ 
  // do something 
} 

Again this throws a Argument Exception

Approach 3 

The correct way of checking if an item exists is using the GetFile method of the SPWeb.  The example below illustrates this.

if (currentSite.GetFile(fileName).Exists == true)
{
  // do something
} 

Summary

When retrieving a SPFile from a document library use the SPWeb.GetFile() method and test if it exists using SPFile.Exists.