Convert XML to CSV

Some of the values contain spaces so to make the csv file more robust and easier to read you should quote them or just everything. I also noticed that your csv does not contain the header row. Is there a reason for that?

I suggest using a more generic approach with some more linq that will first extract header names and then the values. Everything is joined together with the Aggregate extension that uses a StringBuilder initialized with the header and then appends each line to it.

var headers =
    xDocument
        .Descendants("Item")
        .First()
        .Elements().Select(e => e.Name.LocalName);

var delimiter = ";";

var entries =
    xDocument
        .Descendants("Item")
        .Select(d => string.Join(delimiter, d.Elements().Select(e => $"\"{e.Value}\"")))
        .Aggregate(
            new StringBuilder().AppendLine(string.Join(delimiter, headers)), 
            (current, next) => current.AppendLine(next));

var csv = entries.ToString();

There is no need to hardcode the field names.

Your code has several issues, which may or may not apply to your case.

• You don't sanitize any of the input, so if for example one of the XML tags contains a semicolon or a new line character you will end up with invalid CSV.
• You load the whole document into memory (the XDocument) and then aggregate all the output into memory before writing it to a file.

I suggest a streaming approach using XmlReader and C# iterators. The example I'm giving below is more efficient, but may be too complex for the task you require (small files).

First the reader and converter

class InventoryItem 
{
    // Properties for title, amount, price, year, etc..
}

private IEnumerable<InventoryItem> LoadInventoryItemsFromXML(string xmlFile) 
{
    // Open the source XML file
    using (var stream = new FileStream(xmlFile, FileMode.Open, FileAccess.Read, FileShare.Read))
    using (var reader = XmlReader.Create(stream)) 
    {
        // Read each of the tags
        // MoveToContent skips any whitespace and comments that may reside in the XML

        reader.MoveToContent();
        reader.ReadStartElement(); // Inventory
        reader.MoveToContent();

        while (reader.NodeType == XmlNodeType.Element)
        {
            reader.ReadStartElement(); // Item

            // Read the individual properties of the inventory item
            var item = new InventoryItem();
            while (reader.NodeType == XmlNodeType.Element)
            {
                // You could use a switch, possibly a dictionary if the amount of fields increase
                // Or you could use reflection
                switch (reader.LocalName)
                {
                    case "Name":
                        item.Name = reader.ReadElementContentAsString();
                        break;

                    case "Count":
                        item.Count = reader.ReadElementContentAsInt();
                        break;

                    // etc.  

                    // Skip any unkown properties
                    default:
                      reader.Skip();
                      break;
                }
            }

            reader.ReadEndElement(); // Item

            yield return item;
        }

        reader.ReadEndElement(); // Inventory
    }
}

Then write result out to CSV

private void WriteInventoryItemsToCSV(string csvFile, IEnumerable<InventoryItem> items)
{
    // Open the output CSV file
    using (var stream = new FileStream(csvFile, FileMode.Create, FileAccess.Write, FileShare.None))
    using (var writer = new StreamWriter(stream)) 
    {
        // Loop through each of the items we read from the source CSV file.
        foreach (var item in items) 
        {
            writer.Write(CSVEscape(item.Title));
            writer.Write(';');

            // Asumming Amount and Price are numbers, we know they won't contain semicolons.
            // This means we don't have to escape them.
            writer.Write(item.Amount);
            writer.Write(';');
            writer.Write(item.Price);          
            writer.WriteLine();
        }
    }
}

// This methods escapes a string using quotes
private string CSVEscape(string str) 
{
    // Surround content in quotes and replace any quotes inside the string
    // with two double quotes
    return "\"" + str.Replace("\"", "\"\"") + "\"";
}

Then you can call this where you need it

var items = LoadInventoryItemsFromXML(FilePath_CSVToXML);
WriteInventoryItemsToCSV(AppDomain.CurrentDomain.BaseDirectory + @"frMediaShop\test.csv", items);

You can do it with a series of Regex.Replaces:

// 1) Replace closing and opening tags with commas.
//    Include quotes in case any values have commas in them.
var result = Regex.Replace(input, @"(\S)<\/[^>]*>\s*<[^>]*>(\S)", "$1\",\"$2");

// 2) Put in CSV line breaks and remove xml delimiters, include leading and trailing quotes
result = Regex.Replace(result, @"<\/[^>]*>\s*<\/Item>\s*<Item>\s*<[^>]*>", "\r\n");

// 3) Remove remaining tags and trim any whitespace
result = Regex.Replace(result, @"\s*<.*>\s*", "");

// 4) put in header row and first and last quotes
result = "Name,Count,Price,Comment,Artist,Publisher,Genre,Year,ProductID,\r\n\"" + 
         result + "\"";

is this a better solution than other solutions? No. It's brittle and would break easily.

Here are the Regex steps visually:

1. https://regex101.com/r/2jmvhc/1
2. https://regex101.com/r/0XyvZe/1
3. https://regex101.com/r/Sj6GsP/1