// This example concatenates using StringBuilder object then writes contents of the 
// StringBuilder object out to a file using streamwriter

StringBuilder textSB = new StringBuilder();
string newLine;

for(int i = 0; i < 50; i++)
{
	newLine = "Line # " + i;
	textSB.Append(newLine).Append("|").Append("\r\n");
}

string filePath = @"c:\output.txt";
using (StreamWriter outFile = new StreamWriter(filePath))
    outFile.Write(textSB.ToString());

StringBuilder class is a much better way to concatenate strings.  Unlike a string object it can be changed as needed, and is much more efficient than using +=

C#: Using StringBuilder for Concatnation

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