C# - storing multiple values in a string

First, you need to fix your logic to handle the end correctly (it's a matter of execution order).

Then, use a StringBuilder to concatenate the strings:

StringBuilder sb = new StringBuilder();

foreach (string item in ReadLines) {
    if (item == "Start Details") {
        DetailsFound = true;
    }
    else if (item == "End Details") {
        break;
    }
    else if (DetailsFound) {
        Console.WriteLine("Details - " + item);
        sb.AppendLine(item);
        // VARIABLE / STRING REQUIRED HERE
    }
}

string output = sb.ToString();

If you want to have the lines a separate values, use a List<string>:

List<string> list = new List<string>();

foreach (string item in ReadLines) {
    if (item == "Start Details") {
        DetailsFound = true;
    }
    else if (item == "End Details") {
        break;
    }
    else if (DetailsFound) {
        Console.WriteLine("Details - " + item);
        list.Add(item);
        // VARIABLE / STRING REQUIRED HERE
    }
}

// list holds all lines

Few comments:

• You should look at MSDN for General Naming Conventions
• Using File.ReadAllLines will load all the lines in the file to memory. In this instance, I don't think we want to do it as we may stop after several lines. Consider using File.ReadLines

You were given plenty of answers that use the usual enumeration, which is great for a C# beginner as it should look familiar to other languages. I want to expose you to one powerful C# feature, LINQ.

You can perform the task in the following manner:

    public static void TakeBetween(string filePath, string startText, string endText)
    {
        var data = File.ReadLines(filePath).SkipWhile(line => !line.Contains(startText)).Skip(1).TakeWhile(line => !line.Contains(endText));
        // Do whatever that is needed with data. It is of type IEnumerable<string>
    }

you can replace .Contains with any other string finding method you desire. Note that this won't load unnecessary data to memory.

Relevant LINQ functions:

• SkipWhile
• Skip
• TakeWhile

You're not too far off.

With a few minor modifications to your code:

List<string> results = new List<string>();

foreach (string item in ReadLines) 
{
    if (item == "Start Details") 
    {
        DetailsFound = true;
    }
    else if (item == "End Details") {
        break;
    }
    else if (DetailsFound) 
    {
        Console.WriteLine("Details - " + item);
        results.Add(item);
    }

}

That will store the strings found in a list. Now you can process them as you wish. For instance, you can get them all together in a single comma-separated string:

var commaSeparated = String.Join(results, ", ");

Try following :

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


namespace Oppgave3Lesson1
{
    class Program
    {
        const string FILENAME = @"c:\temp\test.txt";
        static void Main(string[] args)
        {
            string input = "";
            List<string> lines = new List<string>();
            StreamReader reader = new StreamReader(FILENAME);

            Boolean foundStart = false;
            while ((input = reader.ReadLine()) != null)
            {
                if (!foundStart)
                {
                    if(input.StartsWith("Start")) foundStart = true;
                }
                else
                {
                    if (input.StartsWith("End")) break;
                    lines.Add(input);
                }
            }

        }
    }
}

item is defined in the foreach loop and refers to the read line. You must define before the loop a string variable and attach each item.

string WriteDataHere = String.Empty; // <- 

bool DetailsFound = false;
string[] ReadLines = File.ReadAllLines(path);
foreach (string item in ReadLines) {
    if (item == "Start Details") {
    DetailsFound = true;
}
if (DetailsFound) {
    Console.WriteLine("Details - " + item);
    // VARIABLE / STRING REQUIRED HERE
    WriteDataHere += item; // <--
    }
    if (item == "End Details") {
        break;
    }
}

But this solution would cause a string that looks like this:

I am line 1I am line 2I am line 3

A possible solution is to check if this string is empty and add a linebreak if notso the code line looks a little bit different:

if( String.isNullOrEmpty(WriteDataHere) == false)
   WriteDataHere += "\n";

WriteDataHere += item; // <--

A little bit better solution is to save all "items" in a List

List<string> listOfStrings = new List<string>();

and add each item on each iteration via the Add-Method.

But it depends on what you plan to do with your string.

If you are wanting to output the lines after some specific string, I'd do it like this

const string _startIdentifier = "Start Details"
const string _endIdentifier = "End Details"

private bool detailsFound
private List<string> details

foreach (string item in File.ReadAllLines(path)) {
    if (!detailsFound && item.Equals(_startIdentifier) {
        detailsFound = true;
    }

    if (item.Equals(__endIdentifier){
        break;
    }

    if (detailsFound) {
        details.Add(item);
        Console.WriteLine("Details - " + item);
    }
}

Then if you want to combine those items into a single string

var combinedItems = String.Join(", ", items)

That will create a string that looks something like "I am line 1, I am line 2, I am line 3". Of course you can change the deliminator from a comma to a new line with Environment.NewLine