Getting file names without extensions

When getting file names in a certain folder:

DirectoryInfo di = new DirectoryInfo(currentDirName);
FileInfo[] smFiles = di.GetFiles("*.txt");
foreach (FileInfo fi in smFiles)
{
    builder.Append(fi.Name);
    builder.Append(", ");
    ...
}

fi.Name gives me a file name with its extension: file1.txt, file2.txt, file3.txt.

How can I get the file names without the extensions? (file1, file2, file3)