There are two points to considerate.
1) This source code worked for me:
private static string Execute(string credentials, string scriptDir, string scriptFilename)
{
Process process = new Process();
process.StartInfo.UseShellExecute = false;
process.StartInfo.WorkingDirectory = scriptDir;
process.StartInfo.RedirectStandardOutput = true;
process.StartInfo.FileName = "sqlplus";
process.StartInfo.Arguments = string.Format("{0} @{1}", credentials, scriptFilename);
process.StartInfo.CreateNoWindow = true;
process.Start();
string output = process.StandardOutput.ReadToEnd();
process.WaitForExit();
return output;
}
I set the working directory to the script directory, so that sub scripts within the script also work.
Call it e.g. as Execute("usr/pwd@service", "c:\myscripts", "script.sql")
2) You have to finalize your SQL script with the statement EXIT;