Simple CSV Parser/Reader Function Written in C#

• Download demo - 26.24 KB

Introduction

This is a simple parser function named CSVParser for parsing a CSV (comma separated values) file. It takes a string containing a single line in CSV file as input and returns ArrayList.

CSV follows the following rules:

1. All values are separated by comma
2. If comma is part of value, then enclose value in double quotes
   e.g. a,b,"12,000",c
3. If double quote is part of value, then replace it with two double quotes and enclose value in double quotes
   e.g. a,b,"He said ""Hi""",c

Usage

ArrayList alResult;
using (StreamReader objReader = new StreamReader(@"C:\Testfile.csv"))
{
    while ((strLineText = objReader.ReadLine()) != null)
    {
        alResult = CSVParser(strLineText);
        //do processing
    }
}

How Does It Work?

This function works based on Finite State Automata concept. Finite state automata has current state and an input. Based on transition table, it changes current state for an input and performs an action.

The state diagram is as follows:

The parser function maintains two objects, a string builder object (henceforth called as TEMPSTR) to temporarily store characters and an array list (henceforth called as ARRLIST).

INPUT

" (double quote) (Indicated by 0)
, (Comma) (Indicated by 1)
N (newline) (Indicated by 3)
O (character other than , " and N) (Indicated by 2)

TRANSITION TABLE
CUR_STATE>INPUT>NEXT STATE
0 > 0 > 2
0 > 1 > 0
0 > 2 > 1
0 > 3 > 5

1 > 0 > 6
1 > 1 > 0
1 > 2 > 1
1 > 3 > 5

2 > 0 > 4
2 > 1 > 3
2 > 2 > 3
2 > 3 > 6

3 > 0 > 4
3 > 1 > 3
3 > 2 > 3
3 > 3 > 6

4 > 0 > 2
4 > 1 > 8
4 > 2 > 6
4 > 3 > 7

5 > X > 5

6 > X > 6

7 > X > 5

8 > X > 0

(X = Any input)

The code is as follows:

The 9X4 aActionDecider array represents the above transition table. First dimension represents state (S0 to S8) while second dimension represents input character (0 to 3). The array gives the next state based on the current state and input character. For example, if the current state is 3 and next input character is quote (0), then the next state is aActionDecider[3][0] i.e. 4.

private static ArrayList CSVParser(string strInputString)
{
    int intCounter = 0, intLenght;
    StringBuilder strElem = new StringBuilder();
    ArrayList alParsedCsv = new ArrayList();
    intLenght = strInputString.Length;
    strElem = strElem.Append("");
    int intCurrState = 0;
    int[][] aActionDecider = new int[9][];
    //Build the state array
    aActionDecider[0] = new int[4] { 2, 0, 1, 5 };
    aActionDecider[1] = new int[4] { 6, 0, 1, 5 };
    aActionDecider[2] = new int[4] { 4, 3, 3, 6 };
    aActionDecider[3] = new int[4] { 4, 3, 3, 6 };
    aActionDecider[4] = new int[4] { 2, 8, 6, 7 };
    aActionDecider[5] = new int[4] { 5, 5, 5, 5 };
    aActionDecider[6] = new int[4] { 6, 6, 6, 6 };
    aActionDecider[7] = new int[4] { 5, 5, 5, 5 };
    aActionDecider[8] = new int[4] { 0, 0, 0, 0 };
    for (intCounter = 0; intCounter < intLenght; intCounter++)
    {
        intCurrState = aActionDecider[intCurrState]
                                  [GetInputID(strInputString[intCounter])];
        //take the necessary action depending upon the state 
        PerformAction(ref intCurrState, strInputString[intCounter], 
                     ref strElem, ref alParsedCsv);
    }
    //End of line reached, hence input ID is 3
    intCurrState = aActionDecider[intCurrState][3];
    PerformAction(ref intCurrState, '\0', ref strElem, ref alParsedCsv); 
    return alParsedCsv;
}

private static int GetInputID(char chrInput)
{
    if (chrInput == '"')
    {
        return 0;
    }
    else if (chrInput == ',')
    {
        return 1;
    }
    else
    {
        return 2;
    }
}
private static void PerformAction(ref int intCurrState, char chrInputChar, 
                    ref StringBuilder strElem, ref ArrayList alParsedCsv)
{
    string strTemp = null;
    switch (intCurrState)
    {
    case 0:
        //Separate out value to array list
        strTemp = strElem.ToString();
        alParsedCsv.Add(strTemp);
        strElem = new StringBuilder();
        break;
    case 1:
    case 3:
    case 4:
        //accumulate the character
        strElem.Append(chrInputChar);
        break;
    case 5:
        //End of line reached. Separate out value to array list
        strTemp = strElem.ToString();
        alParsedCsv.Add(strTemp);
        break;
    case 6:
        //Erroneous input. Reject line.
        alParsedCsv.Clear();
        break;
    case 7:
        //wipe ending " and Separate out value to array list
        strElem.Remove(strElem.Length - 1, 1);
        strTemp = strElem.ToString();
        alParsedCsv.Add(strTemp);
        strElem = new StringBuilder();
        intCurrState = 5;
        break;
    case 8:
        //wipe ending " and Separate out value to array list
        strElem.Remove(strElem.Length - 1, 1);
        strTemp = strElem.ToString();
        alParsedCsv.Add(strTemp);
        strElem = new StringBuilder();
        //goto state 0
        intCurrState = 0;
        break;
    }
}

About the Demo

The demo program reads a CSV file having 4 columns and displays data in datagrid. The demo program is just to show how to use the parser function. It has no practical use.

Download the zip file, run the Windows application. Select "temp.csv" file in "data" folder. Click on parse. The datagrid shows content of the CSV file.