How to avoid the error "{"startIndex cannot be larger than length of string.\r\nParameter name: startIndex"}" in a C# console application?

How to avoid the error:
{"startIndex cannot be larger than length of string.\r\nParameter name: startIndex"}

I get the error at the line 243 --->    checknum = s.Substring(246, 6);

I wrote this application using VS2010 with C#.

I included a sample input file.

The C# application is in the attached file as well.
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Michael FowlerSolutions ConsultantCommented:
The problem here is that the string "s" is less than 246 characters long.

One way to avoid this would be to test the string length before extracting the string length

if (s.Length >= 252) {
    s.Substring(246, 6);

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zimmer9Author Commented:
It seems to me that I am going into the loop too many times to process the 4 checks.

There are 4 checks in my sample file and the debugger shows the CheckOrderNum field has a value of 7 when the error occurs.

CheckOrderNum = CheckOrderNum + 1;
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