C#; Merging XML files

You forgot to specify the Type; e.g. -

private static history history = new history();

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-saige-

Erm why do I need history to be staic?

I've changed the program to bellow

It didn't like history += history.Load(File); ??
So wont 'history' get overwritten by the next file?

Also after loading each file why can't I access history.post.id?

To do this
history.post.id.Sort((x, y) => x.Name.CompareTo(y.Name));

    class Program
    {
        private static history history = new history();

        static void Main(string[] args)
        {
           string RootDir = @"C:\Users\user\AppData\Roaming\AppName\user";
            
            if (Directory.Exists(RootDir))
            {
                var Files = Directory.EnumerateFiles(RootDir, "history.xml", SearchOption.AllDirectories);
                foreach (string File in Files)
                {
                    history = history.Load(File);
                }
                // no post after the dot
               history.
            }
        }
    }

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To answer your first question, it is static because you are accessing it via a static method.  In order to not require the static keyword, you would either call it via a non-static method or create a new instance of the Program class and use this instance to access the history field.

As for your implementation, I believe this is the direction you are heading:

using System.Collections.Generic;
using System.IO;
using System.Xml.Serialization;

namespace EE_Q29135911
{
    class Program
    {
        static List<History> history = new List<History>();

        static void Main(string[] args)
        {
            string directory = @"C:\Users\user\AppData\Roaming\AppName\user";

            if (Directory.Exists(directory))
            {
                var files = Directory.EnumerateFiles(directory, "history.xml", SearchOption.AllDirectories);
                foreach (string file in files)
                {
                    history.Add(History.Load(file));
                }
            }
        }
    }

    class History
    {
        public List<Post> post { get; set; } = new List<Post>();

        public static History Load(string file)
        {
            var deserializer = new XmlSerializer(typeof(History));
            using (TextReader reader = new StreamReader(file))
            {
                return deserializer.Deserialize(reader) as History;
            }
        }

        public void Save(string file)
        {
            var serializer = new XmlSerializer(typeof(History));
            using (TextWriter writer = new StreamWriter(file))
            {
                serializer.Serialize(writer, this);
            }
        }
    }

    public class Post
    {
        public string Name { get; set; }
        public string Url { get; set; }
        public int Id { get; set; }
        public int Number { get; set; }
        public int ImageCount { get; set; }
        public int DownloadedImagesCount { get; set; }
        public string Finished { get; set; }
    }
}

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-saige-

I'm going to post what I've got so far because I don't understand a couple off things

Why can't I access "History.post" or "history.post" AT ###1

and

I tried to run this I get AT ###2

System.InvalidOperationException was unhandled
  HResult=-2146233079
  Message=MergeXML.History is inaccessible due to its protection level. Only public types can be processed.
  Source=System.Xml
  StackTrace:
       at System.Xml.Serialization.TypeDesc.CheckSupported()
       at System.Xml.Serialization.TypeScope.GetTypeDesc(Type type, MemberInfo source, Boolean directReference, Boolean throwOnError)
       at System.Xml.Serialization.ModelScope.GetTypeModel(Type type, Boolean directReference)
       at System.Xml.Serialization.XmlReflectionImporter.ImportTypeMapping(Type type, XmlRootAttribute root, String defaultNamespace)
       at System.Xml.Serialization.XmlSerializer..ctor(Type type, String defaultNamespace)
       at System.Xml.Serialization.XmlSerializer..ctor(Type type)
       at MergeXML.History.Load(String file) in d:\VB\Test\MergeXML\MergeXML\Program.cs:line 39
       at MergeXML.Program.Main(String[] args) in d:\VB\Test\MergeXML\MergeXML\Program.cs:line 24
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

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using System.IO;
using System.Xml.Serialization;


namespace MergeXML
{
    class Program
    {
        static List<History> history = new List<History>();
        static void Main(string[] args)
        {
            string RootDir = @"C:\Users\user\AppData\Roaming\AppName\user";

            if (Directory.Exists(RootDir))
            {
                var files = Directory.EnumerateFiles(RootDir, "history.xml", SearchOption.AllDirectories);
                foreach (string file in files)
                {
                    history.Add(History.Load(file));
                }
            }
           // ###1
            History.Save(RootDir + "\\history_out.xml");
            
        }
    }
    class History
    {
        public List<Post> post { get; set; } = new List<Post>();

        #region Loading/Saving
        public static History Load(string file)
        {
            var deserializer = new XmlSerializer(typeof(History)); // ###2
            using (TextReader reader = new StreamReader(file))
            {
                return deserializer.Deserialize(reader) as History;
            }
        }

        internal static void Save(string file)
        {
            var serializer = new XmlSerializer(typeof(History));
            using (TextWriter writer = new StreamWriter(file))
            {
                serializer.Serialize(writer, file);
            }
        }
        #endregion
    }

    public class Post
    {
        public string name { get; set; }
        public string url { get; set; }
        public int id { get; set; }
        public int number { get; set; }
        public int imageCount { get; set; }
        public int downloadedImagesCount { get; set; }
        public string finished { get; set; }
    }
}

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History needs to be made public; e.g. -

using System.IO;
using System.Xml.Serialization;


namespace MergeXML
{
    class Program
    {
        static List<History> history = new List<History>();
        static void Main(string[] args)
        {
            string RootDir = @"C:\Users\user\AppData\Roaming\AppName\user";

            if (Directory.Exists(RootDir))
            {
                var files = Directory.EnumerateFiles(RootDir, "history.xml", SearchOption.AllDirectories);
                foreach (string file in files)
                {
                    history.Add(History.Load(file));
                }
            }
           // ###1
            History.Save(RootDir + "\\history_out.xml");
            
        }
    }

    public class History
    {
        public List<Post> post { get; set; } = new List<Post>();

        #region Loading/Saving
        public static History Load(string file)
        {
            var deserializer = new XmlSerializer(typeof(History)); // ###2
            using (TextReader reader = new StreamReader(file))
            {
                return deserializer.Deserialize(reader) as History;
            }
        }

        internal static void Save(string file)
        {
            var serializer = new XmlSerializer(typeof(History));
            using (TextWriter writer = new StreamWriter(file))
            {
                serializer.Serialize(writer, file);
            }
        }
        #endregion
    }

    public class Post
    {
        public string name { get; set; }
        public string url { get; set; }
        public int id { get; set; }
        public int number { get; set; }
        public int imageCount { get; set; }
        public int downloadedImagesCount { get; set; }
        public string finished { get; set; }
    }
}

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-saige-

Wow  this  is getting really frustrating
I really don't understand why can't I access "History.post" or "history.post"?

I ran your your code against the test file (I even moved it thinking may be permissions)  

I get  the error bellow claiming invalid xml

I also attempted to create a post to save the XML so I could compare

      // ###1

            Post post = new Post()
            {
                name = "Hello World",
                url = "https://www.experts-exchange.com/questions/29135911/C-Merging-XML-files.html",
                id= 45678,
                number =7,
                imageCount =99,
                downloadedImagesCount = 99,
                finished ="true"

            };
            history.Add(post);
          History.Save(RootDir + "\\history_out.xml");

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I get

Severity	Code	Description	Project	File	Line	Suppression State
Error	CS1503	Argument 1: cannot convert from 'MergeXML.Post' to 'MergeXML.History'	MergeXML	D:\Vb\Test\MergeXML\MergeXML\Program.cs	41	Active

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Test XML (I added the DTDs thinking this might be the issue)

<?xml version="1.0" encoding="windows-1250"?>
<history xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <post>
    <name>My Collection </name>
    <url>https://example.com/hread.php?p=12345</url>
    <id>12345</id>
    <number>1</number>
    <imageCount>132</imageCount>
    <downloadedImagesCount>0</downloadedImagesCount>
    <finished>true</finished>
  </post>
</history>

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Error

System.InvalidOperationException was unhandled
  HResult=-2146233079
  Message=There is an error in XML document (2, 2).
  Source=System.Xml
  StackTrace:
       at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader xmlReader, String encodingStyle, XmlDeserializationEvents events)
       at System.Xml.Serialization.XmlSerializer.Deserialize(TextReader textReader)
       at MergeXML.History.Load(String file) in D:\Vb\Test\MergeXML\MergeXML\Program.cs:line 44
       at MergeXML.Program.Main(String[] args) in D:\Vb\Test\MergeXML\MergeXML\Program.cs:line 19
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: 
       HResult=-2146233079
       Message=<history xmlns=''> was not expected.
       Source=Microsoft.GeneratedCode
       StackTrace:
            at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationReaderHistory.Read4_History()
       InnerException: 

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You can't add a Post type to a List of History.  In order to add the post to the History instance you would access the Posts list; e.g. -

var history = new History();
Post post = new Post()
{
    name = "Hello World",
    url = "https://www.experts-exchange.com/questions/29135911/C-Merging-XML-files.html",
    id= 45678,
    number =7,
    imageCount =99,
    downloadedImagesCount = 99,
    finished ="true"
};
history.post.Add(post);

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-saige-

I was hopping that would work
So  all I'm doing is trying to create an xml file with a single post if I can do that maybe I can read it back in again

When I try and save I now get

There was an error generating the XML document.

  InnerException: 
       HResult=-2147467262
       Message=Unable to cast object of type 'System.String' to type 'MergeXML.History'.

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at

        internal static void Save(string file)
        {
            var serializer = new XmlSerializer(typeof(History));
            using (TextWriter writer = new StreamWriter(file))
            {
                serializer.Serialize(writer, file); // Errors here
            }
        }

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Thank you this is now working

I need to understand why

For instance why do you need to declare XmlElement

        [XmlElement(ElementName = "name")]
        public string Name { get; set; }

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also I get this create a new history object then add a new post to it than add the history to histories

           History history = new History();
            Post post = new Post()
            {
                Name = "Hello World",
                Url = "https://www.experts-exchange.com/questions/29135911/C-Merging-XML-files.html",
                Id = 45678,
                Number = 7,
                ImageCount = 99,
                DownloadedImagesCount = 99,
                Finished = "true"
            };
            history.Posts.Add(post);
            histories.Add(history);
            History result = new History();

            foreach (var history in histories.Distinct())
            {
                result.Posts.AddRange(history.Posts);
            }

            
            History.Save(RootDir + "\\history_out.xml", result);

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But why can't  you save histories strait to file
History.Save(RootDir + "\\history_out.xml", histories);
gives

Severity	Code	Description	Project	File	Line	Suppression State
Error	CS1503	Argument 2: cannot convert from 'System.Collections.Generic.List<MergeXML.History>' to 'MergeXML.History'	MergeXML	D:\Vb\Test\MergeXML\MergeXML\Program.cs	42	Active

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1.  The elements in your XML files are all lower cased elements and XML serialization is not case agnostic.  I could have used all lower case names for the classes and properties but that goes against the coding standards/naming conventions of most of the industry.  For more information you can refer to - https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/inside-a-program/coding-conventions

2.  Because your static method as a black box should do one thing.  In this case, you are, by definition and assumption, saving a single history item.

-saige-

Thank you for your time  and Patience

There are aspects I don't totally understand

"static List<History> histories = new List<History>();"

I think histories Is a list of multiple instances of History so 1 history.xml makes 1 History

This

        History result = new History();

            foreach (var history in histories.Distinct())
            {
                result.Posts.AddRange(history.Posts);
            }

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Converts all posts into 1 unique History posts this in turn allows manipulation of the single list eg (Saving, adding a single post and Querying ect)

Correct.  Per your requirements, you wanted to take multiple history files and merge them into one.  So you need to added each history instance to a collection.  

But Distinct really isn't needed.  In order to get rid of duplicates, the best thing to do would be to create a custom post comparer and use the Except method when adding posts; e.g. -

using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Xml.Serialization;


namespace MergeXML
{
    class Program
    {
        static List<History> histories = new List<History>();
        static void Main(string[] args)
        {
            var files = Directory.EnumerateFiles(".", "*history.xml", SearchOption.AllDirectories);
            foreach (string file in files)
            {
                histories.Add(History.Load(file));
            }
            // Create a singular out history
            History result = new History();
            foreach (var history in histories)
            {
                result.Posts.AddRange(history.Posts.Except(result.Posts, new PostComparer()));
            }
            History.Save(@"history_out.xml", result);
        }
    }

    [XmlRoot(ElementName = "history", DataType = "string", IsNullable = true)]
    public class History
    {
        [XmlElement(ElementName = "post")]
        public List<Post> Posts { get; set; } = new List<Post>();
        public static History Load(string file)
        {
            var deserializer = new XmlSerializer(typeof(History));
            using (TextReader reader = new StreamReader(file))
            {
                return deserializer.Deserialize(reader) as History;
            }
        }

        internal static void Save(string file, History source)
        {
            var serializer = new XmlSerializer(typeof(History));
            using (TextWriter writer = new StreamWriter(file))
            {
                serializer.Serialize(writer, source);
            }
        }
    }

    public class Post
    {
        [XmlElement(ElementName = "name")]
        public string Name { get; set; }
        [XmlElement(ElementName = "url")]
        public string Url { get; set; }
        [XmlElement(ElementName = "id")]
        public int Id { get; set; }
        [XmlElement(ElementName = "number")]
        public int Number { get; set; }
        [XmlElement(ElementName = "imageCount")]
        public int ImageCount { get; set; }
        [XmlElement(ElementName = "downloadedImagesCount")]
        public int DownloadedImagesCount { get; set; }
        [XmlElement(ElementName = "finished")]
        public string Finished { get; set; }
    }

    class PostComparer : IEqualityComparer<Post>
    {
        public bool Equals(Post x, Post y)
        {
            if (ReferenceEquals(x, y))
            {
                return true;
            }

            if (ReferenceEquals(x, null) || ReferenceEquals(y, null))
            {
                return false;
            }

            return Equals(x.DownloadedImagesCount, y.DownloadedImagesCount) &&
                Equals(x.Finished, y.Finished) &&
                Equals(x.Id, y.Id) &&
                Equals(x.ImageCount, y.ImageCount) &&
                Equals(x.Name, y.Name) &&
                Equals(x.Number, y.Number) &&
                Equals(x.Url, y.Url);
        }

        public int GetHashCode(Post obj)
        {
            if (ReferenceEquals(obj, null))
            {
                return 0;
            }
            return obj.DownloadedImagesCount.GetHashCode() ^ obj.Finished.GetHashCode() ^ 
                obj.Id.GetHashCode() ^ obj.ImageCount.GetHashCode() ^ obj.Name.GetHashCode() ^
                obj.Number.GetHashCode() ^ obj.Url.GetHashCode();
        }
    }
}

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With the same files above and one additional one:

3history.xml -

<history>
  <post>
    <name>My Collection 4</name>
    <url>https://example.com/hread.php?p=54321</url>
    <id>54321</id>
    <number>4</number>
    <imageCount>135</imageCount>
    <downloadedImagesCount>0</downloadedImagesCount>
    <finished>true</finished>
  </post>
  <post>
    <name>My Collection 5</name>
    <url>https://example.com/hread.php?p=23456</url>
    <id>23456</id>
    <number>5</number>
    <imageCount>136</imageCount>
    <downloadedImagesCount>0</downloadedImagesCount>
    <finished>true</finished>
  </post>
</history>

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Produces the following output -

<?xml version="1.0" encoding="utf-8"?>
<history xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <post>
    <name>My Collection </name>
    <url>https://example.com/hread.php?p=12345</url>
    <id>12345</id>
    <number>1</number>
    <imageCount>132</imageCount>
    <downloadedImagesCount>0</downloadedImagesCount>
    <finished>true</finished>
  </post>
  <post>
    <name>My Collection 2</name>
    <url>https://example.com/hread.php?p=67890</url>
    <id>67890</id>
    <number>2</number>
    <imageCount>133</imageCount>
    <downloadedImagesCount>0</downloadedImagesCount>
    <finished>true</finished>
  </post>
  <post>
    <name>My Collection 3</name>
    <url>https://example.com/hread.php?p=9876</url>
    <id>9876</id>
    <number>3</number>
    <imageCount>134</imageCount>
    <downloadedImagesCount>0</downloadedImagesCount>
    <finished>true</finished>
  </post>
  <post>
    <name>My Collection 4</name>
    <url>https://example.com/hread.php?p=54321</url>
    <id>54321</id>
    <number>4</number>
    <imageCount>135</imageCount>
    <downloadedImagesCount>0</downloadedImagesCount>
    <finished>true</finished>
  </post>
  <post>
    <name>My Collection 5</name>
    <url>https://example.com/hread.php?p=23456</url>
    <id>23456</id>
    <number>5</number>
    <imageCount>136</imageCount>
    <downloadedImagesCount>0</downloadedImagesCount>
    <finished>true</finished>
  </post>
</history>

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-saige-