CODEWITHSUNDEEP
c#nullreferenceexceptionlexerindexoutofrangeexception
austanss
austanss

Reputation: 291

Multiple runtime exceptions when using string array in C# for unknown reason

I am writing a lexer, or tokenizer, for a programming language. One of the main functions is to split the line of source code into "tokens". I am achieving this by splitting on spaces to create string arrays. Therefore, when I want to preserve strings, I must temporarily change the contents to a keyword while the line is split, and then put the strings back in afterwards. This worked until I developed the variable system for the language, and I had to be able to preserve multiple strings. Then all exception hell broke loose.

Exceptions:

NullReferenceException (line 12) subStringArg[ini] = quotes[1];

IndexOutOfRangeException (line 34) value = value.Replace("STRING", subStringArg[ini]);

Minimum reproducible example:

public static string[] LexLine(string line)
        {
            string[] subStringArg = null;
            string[] quotes = null;
            string[] tokens = null;
            int ini = 0; // Random name
            while (line.Contains('\"'))
            {
                if (line.Contains('\"'))
                {
                    quotes = line.Split('\"');
                    subStringArg[ini] = quotes[1];
                }

                if (subStringArg != null)
                {
                    line = line.Replace(quotes[1], "STRING");
                    line = line.Replace("\\", "");
                    line = line.Replace("\"", "");
                }
                ini++;
            }
            tokens = line.Split(' ');
            if (tokens[0] == "Output" && tokens[1] != "STRING")
            {
                tokens[1] = IO.OutputVariable(tokens[1]);
            }
            ini = 0;
            foreach (string i in tokens)
            {
                if (i == "STRING" && subStringArg != null)
                {
                    string value = i;
                    value = value.Replace("STRING", subStringArg[ini]);
                    tokens[currentArrayEntry] = value;
                }
                currentArrayEntry++;
                ini++;
            }
            return tokens;
        }

Source code (from my lang):

Output "Defining variables..." to Console. // Exception thrown here
New string of name "sampleStringVar" with value "sample".
Output "enter your name:" to Console.
Get input.
Output sampleStringVar to Console.

I'm asking here because I am clueless as what to do. I should not be getting a NullReferenceException from assigning values.

Upvotes: 0

Views: 103

Answers (1)

jason.kaisersmith
jason.kaisersmith

Reputation: 9610

You set the following

string[] subStringArg = null;

then later you do this

subStringArg[ini] = quotes[1];

But you have not initialised subStringArg so it is still null, so you can't assign anything to it and you will get a NullReferenceError

You must first initialise your array before you can assign anything into it.

Also, you should not assume that you have a value in quotes[1] without first checking it. This will lead to a IndexOutOfRangeException

As another point. The first If statement inside your while loop checks for the same condition as the while loop, so it will always be true!

So the following would be better

string[] subStringArg = new String[enterKnownLengthHere_OrUseALIst];
...
 while (line.Contains('\"'))
 {
        quotes = line.Split('\"');
        if(quotes != null && quotes.Length >= 2)
            subStringArg[ini] = quotes[1];
        else
           //Error handling here!


        if (subStringArg.Length >= 1)
        {
                line = line.Replace(quotes[1], "STRING");
                line = line.Replace("\\", "");
                line = line.Replace("\"", "");
        }
        ini++;
    }

Upvotes: 3

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