Open file ReadOnly

You can set the file attribute by calling File.SetAttributes

string path = Path.Combine(client._WorkLogFileLoc, "dump.txt");
FileAttributes curAttributes = File.GetAttributes(path);
File.SetAttributes(path, curAttributes | FileAttributes.ReadOnly);

The typical problem is that the other process has the file open for writing. All of the standard File methods and StreamReader constructors open the file with FileShare.Read. That cannot work, that denies write sharing. You cannot deny writing, the other process was first and got write access. So you'll be denied access instead.

You have to use FileShare.ReadWrite, like this:

var fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite); 
using (var sr = new StreamReader(fs))
{
    // etc...
}

Beware that you'll still have a tricky problem, you are reading a half-written file. The other process flushes data to the file at random points in time, you may well read only half a line of text. YMMV.

If you want to open the file read-only, try this:

using (TextReader reader 
   = new StreamReader(File.OpenRead(Path.Combine(client._WorkLogFileLoc, "dump.txt")))) 
{     
         //do stuff 
} 

Notice the call to File.OpenRead().