How to skip lines while reading in a file using compact framework 3.5

Question

I have a Windows Form app that I have rewritten using the Compact Framework 3.5. In the original app I had a block of code that I used to read in file and skip its first 4 lines.

Here is the code block that works fine:

var openFile = File.OpenText(fullFileName);
            var fileEmpty = openFile.ReadLine();
            if (fileEmpty != null)
            {
                var lines = File.ReadAllLines(fullFileName).Skip(4); //Will skip the first 4 then rewrite the file
                openFile.Close();//Close the reading of the file
                File.WriteAllLines(fullFileName, lines); //Reopen the file to write the lines
                openFile.Close();//Close the rewriting of the file
            } 

I had to rewrite the above code in since it can't be used like so in the Compact Framework.

Here is my code:

var openFile = File.OpenText(fullFileName);
            var fileEmpty = openFile.ReadLine();
            if (fileEmpty != null)
            {
                var sb = new StringBuilder();
                using (var sr = new StreamReader(fullFileName))
                {
                    string line1;
                    // Read and display lines from the file until the end of 
                    // the file is reached.
                    while ((line1 = sr.ReadLine().Skip(4).ToString()) != null) //Will skip the first 4 then rewrite the file
                    {
                        sb.AppendLine(line1);
                    }
                }

However when I run the above, I receive the error on (while ((line1 = sr.ReadLine().Skip(4).ToString()) != null)) ArgumentNullException was unhandled and Value can not be null.

Can someone please tell me how I can do this in the compact framework?

Answer 1

Since sr.ReadLine() returns a single string, this is going to skip the first four characters in your string, return the rest as a character array, and then call ToString() on it... not what you want.

sr.ReadLine().Skip(4).ToString()

The reason you're getting an ArgumentNullException is because sr.ReadLine() eventually returns a null string, and when you try to skip the first four characters of a null string, it throws, as you can see by looking at the implementation for Skip():

public static IEnumerable<TSource> Skip<TSource>(this IEnumerable<TSource> source, int count)
{
    if (source == null)
        throw new ArgumentNullException("source");

    return SkipIterator<TSource>(source, count);
}

Keeping most of your code the same, you could just read the first few lines and do nothing with them (assuming you'll definitely have at least 4 lines in the file).

using (var sr = new StreamReader(fullFileName))
{
    // read the first 4 lines but do nothing with them; basically, skip them
    for (int i=0; i<4; i++)
        sr.ReadLine();

    string line1;

    while ((line1 = sr.ReadLine()) != null)
    {
        sb.AppendLine(line1);
    }
}