Make String concatenation faster in C#

Possible Duplicate:
What's the best string concatenation method using C#?

Hi,

I have a code snippet like this where a large content of data is read from a file and check each position for some value and concat a string.

This string concatenation takes large amounts of time and processing power. Is there an approach where I can reduce the execution time?

Important: Reading Content file syntax is incorrect just need to give a idea

string x;

while (var < File.Length)
{
  if (File.Content[var] == "A")
  {
       x += 1;    
  }
  else
  {
     x += 0;
  }
  var++;
}