C# :: Read Data From More Than One File At Once - Not All Code Paths Return A Value

I am trying to read data from more than one file at once. The files are different types e.g. one is a text file one is an xml file like so, StudentInformation.txt, CollegeInformation.xml. The files are all stored in one place, in this case on the D drive of a local computer. I am trying to locate any files in the D drive with a file extension of .txt or of .xml (there may be more than two of these files in the future, so I'm trying to allow for that). Then I want to open all of these files, extract the information and output all the information in one display window. I want all the information from these two or more files to be displayed together in the display window.

Here is the code so far. It is throwing up errors.

//Load from txt files
private void btnLoad_Click(object sender, RoutedEventArgs e)
{
IEnumerable<string> fileContents = Directory.EnumerateFiles("D:\", "*.*", SearchOption.TopDirectoryOnly)
.Select(x => new FileInfo(x))
.Where(x => x.Extension == ".xml" || x.Extension == ".txt")
.Select(file => ParseFile(file));}

[Code] ....

The error it throws up is:
Error 1 'BookList.Mainwindow.ParseFile(System.IO.FileInfo)': not all code paths return a value