Do I really have to give an initial value to all my variables?

Question

Possible Duplicate:
Why do I have to assign a value to an int in C# when defaults to 0?

I'm just starting to learn C# by writing a contrived application called Journal. In a function for parsing journal files I've declared the variable DateTime currentEntryDate. It won't get a value until I reach a line that defines a new entry. The second time I reach an entry line, the variable will be used to create an instance of class JournalEntry for the previous entry.

The problem is that the code for the usage of the variable won't compile:

Use of unassigned local variable 'currentEntryDate'

This makes no sense to me. Do I really have to give a wasted initial value to my variables just to keep the compiler happy? Surely I've misunderstood something, or there's an error in my code somewhere.

The code on Pastebin: Journal.cs. I've highlighted the relevant lines.

The code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.IO;

namespace Journal
{
    class Journal
    {
        public List<JournalEntry> Entries;

        private static readonly string EntryLineRegex =
            @"-- Entry: (?<title>.*) \((?<year>\d{4})-(?<month>\d{2})" +
            @"-(?<day>\d{2})\)";

        public static Journal FromFile(string filePath)
        {
            Journal returnValue = new Journal();

            StreamReader fileReader = new StreamReader(filePath);

            // Prepare variables for parsing the journal file.
            bool hitFirstEntry = false;
            DateTime currentEntryDate;
            string currentEntryTitle;
            StringBuilder currentEntryText = new StringBuilder();

            // Prepare a regular expression for the entry lines.
            Regex entryLineRegex = new Regex(EntryLineRegex);

            while (!fileReader.EndOfStream)
            {
                string line = fileReader.ReadLine();

                if (line.StartsWith("--"))
                {
                    // Is this the first entry encountered? If so, don't try to
                    // process the previous entry.
                    if (!hitFirstEntry)
                    {
                        hitFirstEntry = true;
                    }
                    else
                    {
                        // Create a JournalEntry with the current entry, then
                        // reset for the next entry.
                        returnValue.Entries.Add(
                            new JournalEntry(
                                currentEntryText.ToString(), currentEntryDate
                            )
                        );

                        currentEntryDate = new DateTime();
                        currentEntryText.Clear();
                    }

                    // Extract the new entry title and date from this line and
                    // save them.
                    Match entryMatch = entryLineRegex.Match(line);
                    GroupCollection matches = entryMatch.Groups;

                    currentEntryDate = new DateTime(
                        Convert.ToInt16(matches["year"].Value),
                        Convert.ToInt16(matches["month"].Value),
                        Convert.ToInt16(matches["day"].Value)
                    );

                    currentEntryTitle = matches["title"].Value;
                }
                else
                {
                    currentEntryText.Append(line);
                }
            }

            return returnValue;
        }
    }

    class JournalEntry
    {
        public string Text;
        public DateTime EntryDate;

        public JournalEntry(string text, DateTime entryDate)
        {
            this.Text = text;
            this.EntryDate = entryDate;
        }
    }
}

Answer 1

I think the concern here is the compiler is not smart enough to grasp your way of reading the input, and that there is a path of execution for which the variable will NOT be initialized, i.e if it goes through the else first, before the if. To avoid this you might need to initialize as you define it.

Answer 2

How about restructuring your loop like this? This will ensure that currentEntryDate has a value before you use it:

string line = fileReader.ReadLine();
while (line != null)
{
    // Extract the new entry title and date from this line and
    // save them.
    Match entryMatch = entryLineRegex.Match(line);
    GroupCollection matches = entryMatch.Groups;

    currentEntryDate = new DateTime(
        Convert.ToInt16(matches["year"].Value),
        Convert.ToInt16(matches["month"].Value),
        Convert.ToInt16(matches["day"].Value)
    );

    currentEntryTitle = matches["title"].Value;

    while ((line = fileReader.ReadLine()) != null && !line.StartsWith("--"))
    {
        currentEntryText.Append(line);
    }

    // Create a JournalEntry with the current entry, then
    // reset for the next entry.
    returnValue.Entries.Add(
        new JournalEntry(
            currentEntryText.ToString(), currentEntryDate
        )
    );

    currentEntryText.Clear();
}

Answer 3

In this case the compiler doesn't realize the "dependence" between hitFirstEntry and currentEntryDate.

Even if you can "prove" that whenever hitFirstEntry is changed to true, then the currentEntryDate will be assigned soon after that, and the currentEntryDate won't be read for the first time until (at the earliest) in next iteration of the loop, the compiler is not that sophisticated. Maybe you can re-write your code.

Edit: Here's a "minimal" version of your code:

        bool isFirstTime = true;
        DateTime localVarToBeAssigned;

        while (true)
        {
            if (isFirstTime)
            {
                isFirstTime = false;
            }
            else
            {
                // this reads the variable
                Console.WriteLine(localVarToBeAssigned);
            }

            // this assigns the variable
            localVarToBeAssigned = DateTime.Now;
        }

Answer 4

If you really, really don't want to instantiate it:

DateTime? currentEntryDate = null

The question mark makes DateTime nullable, which it usually isn't.

Answer 5

You declare local variable this variables dont have default values and take their memory from stack you should initial local variables befor useing them with change line of this code :

currentEntryDate = new DateTime();

too upper lines your code will be work,