.net – way to install the .NET 3.5 Framework without rebooting

System.Reflection.Assembly.GetExecutingAssembly().Location¹

Combine that with System.IO.Path.GetDirectoryName if all you want is the directory.

¹As per Mr.Mindor's comment:
System.Reflection.Assembly.GetExecutingAssembly().Location returns where the executing assembly is currently located, which may or may not be where the assembly is located when not executing. In the case of shadow copying assemblies, you will get a path in a temp directory. System.Reflection.Assembly.GetExecutingAssembly().CodeBase will return the 'permanent' path of the assembly.

Updated NOTE on this solution: Checking with FileAccess.ReadWrite will fail for Read-Only files so the solution has been modified to check with FileAccess.Read.

ORIGINAL: I've used this code for the past several years, and I haven't had any issues with it.

Understand your hesitation about using exceptions, but you can't avoid them all of the time:

protected virtual bool IsFileLocked(FileInfo file)
{
    try
    {
        using(FileStream stream = file.Open(FileMode.Open, FileAccess.Read, FileShare.None))
        {
            stream.Close();
        }
    }
    catch (IOException)
    {
        //the file is unavailable because it is:
        //still being written to
        //or being processed by another thread
        //or does not exist (has already been processed)
        return true;
    }

    //file is not locked
    return false;
}