Text file processing line by line with | as delimiter - How to place Null between two pipe signs when found adjacent to each other

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C#

C#4.0

Anyone knows how to parse out a text file line by line and when two pipe signs found to be adjacent to each other then the program would place a string (for example the qord Null) in between. I am using the following code to pen and read the text file to begin with:

 using (StreamReader readFile = new StreamReader(tmpFilename.FullName.ToString()))
{
string line = null;
    while ((line = readFile.ReadLine()) != null)
              {
             // Code here....process out certain fields in each line }
}

Need to know the best appraoch to do so without impacting performance. Thanks.