Read fixed width record from text file

Use FileHelpers.

Example:

[FixedLengthRecord()] 
public class MyData
{ 
  [FieldFixedLength(8)] 
  public string someData; 

  [FieldFixedLength(16)] 
  public int SomeNumber; 

  [FieldFixedLength(12)] 
  [FieldTrim(TrimMode.Right)]
  public string someMoreData;
}

Then, it's as simple as this:

var engine = new FileHelperEngine<MyData>(); 

// To Read Use: 
var res = engine.ReadFile("FileIn.txt"); 

// To Write Use: 
engine.WriteFile("FileOut.txt", res); 

Substring sounds good to me. The only downside I can immediately think of is that it means copying the data each time, but I wouldn't worry about that until you prove it's a bottleneck. Substring is simple :)

You could use a regex to match a whole record at a time and capture the fields, but I think that would be overkill.