Question

Please, help me understand, what’s wrong with this code. (I am trying to build a string, taking parts of it line by line from a text file).

I get a runtime error “In the instance of the object reference not set to an object” on the line strbuild.Append(str);

        StreamReader reader = new StreamReader("buf.txt", System.Text.Encoding.ASCII);
        StringBuilder strbuild = new StringBuilder();
        strbuild = null;

        while (reader.Peek() >= 0)
        {
            string str = null;
            str = reader.ReadLine().ToString();

            string segment = str.Substring(0, 1);

            if (segment == "A")
            {
                strbuild.Append(str); //here  i get an error
            }
            else if (segment == "B")
            {
                strbuild.Append("BET");
            }

        }
        printstr = strbuild.ToString();
        reader.Close();

        MessageBox.Show(printstr);

Answer 1

Look at these lines:

StringBuilder strbuild = new StringBuilder();
strbuild = null;

What do you expect to happen when you then call strbuild.Append(...)? Why are you setting strbuild to null at all?

You seem to be fond of two-line variable initialization – here’s another example:

string str = null;
str = reader.ReadLine().ToString();

This would be more easily readable as just:

string str = reader.ReadLine();

(ReadLine already returns a string, so you don’t need to call ToString() on the result.)

However, I do suggest that you use a using statement for the StreamReader – otherwise when an exception is thrown, you’ll be leaving the reader open.

One nice thing about TextReader.ReadLine() is that it returns null when you’re done. You don’t need to peek and then read.

Finally, if you’re only testing a single character you don’t need a substring – just use the string indexer to get a char. So, you could have:

StringBuilder builder = new StringBuilder();

// Consider using File.OpenText
using (StreamReader reader = new StreamReader("buf.txt", Encoding.ASCII))
{
    string line;
    // Normally side-effect + test is ugly, but this is a common and
    // neat idiom
    while ((line = reader.ReadLine()) != null)
    {
        // TODO: What do you want to happen for empty lines?
        char segment = str[0];
        if (segment == 'A')
        {
            builder.Append(line);
        }
        else if (segment == 'B')
        {
            builder.Append("BET");
        }
    }
}
MessageBox.Show(builder.ToString());