I dont even know if this will compile (Im on my Mac at the moment) - but I'd be trying something like
string folderPath = "Your File Folder Here";
if (Directory.Exists(folderPath))
{
DirectoryInfo di = new DirectoryInfo(folderPath);
List<string> xlsxFiles = di.GetFiles("*.xlsx")
.Where(file => !(file.Name.EndsWith("testFile.xlsx")))
.Select(file => file.Name).ToList();
foreach (string filenameToDelete in xlsxFiles)
{
File.Delete(filenameToDelete);
}
}