Answer by DK. for How can I merge XML files?

"Automatic XML merge" sounds like a relatively simple requirement, but when you go into all the details, it gets complex pretty fast. Merge with c# or XSLT will be much easier for more specific task, like in the answer for EF model. Using tools to assist with a manual merge can also be an option (see this SO question).

For the reference (and to give an idea about complexity) here's an open-source example from Java world: XML merging made easy

Back to the original question. There are few big gray-ish areas in task specification: when 2 elements should be considered equivalent (have same name, matching selected or all attributes, or also have same position in the parent element); how to handle situation when original or merged XML have multiple equivalent elements etc.

The code below is assuming that

• we only care about elements at the moment
• elements are equivalent if element names, attribute names, and attribute values match
• an element doesn't have multiple attributes with the same name
• all equivalent elements from merged document will be combined with the first equivalent element in the source XML document.

.

// determine which elements we consider the same//private static bool AreEquivalent(XElement a, XElement b){    if(a.Name != b.Name) return false;    if(!a.HasAttributes && !b.HasAttributes) return true;    if(!a.HasAttributes || !b.HasAttributes) return false;    if(a.Attributes().Count() != b.Attributes().Count()) return false;    return a.Attributes().All(attA => b.Attributes(attA.Name)        .Count(attB => attB.Value == attA.Value) != 0);}// Merge "merged" document B into "source" A//private static void MergeElements(XElement parentA, XElement parentB){    // merge per-element content from parentB into parentA    //    foreach (XElement childB in parentB.DescendantNodes())    {        // merge childB with first equivalent childA        // equivalent childB1, childB2,.. will be combined        //        bool isMatchFound = false;        foreach (XElement childA in parentA.Descendants())        {            if (AreEquivalent(childA, childB))            {                MergeElements(childA, childB);                isMatchFound = true;                break;            }        }        // if there is no equivalent childA, add childB into parentA        //        if (!isMatchFound) parentA.Add(childB);    }}

It will produce desired result with the original XML snippets, but if input XMLs are more complex and have duplicate elements, the result will be more... interesting:

public static void Test(){    var a = XDocument.Parse(@"<Root><LeafA><Item1 /><Item2 /><SubLeaf><X/></SubLeaf></LeafA><LeafB><Item1 /><Item2 /></LeafB></Root>");    var b = XDocument.Parse(@"<Root><LeafB><Item5 /><Item1 /><Item6 /></LeafB><LeafA Name=""X""><Item3 /></LeafA><LeafA><Item3 /></LeafA><LeafA><SubLeaf><Y/></SubLeaf></LeafA></Root>");    MergeElements(a.Root, b.Root);    Console.WriteLine("Merged document:\n{0}", a.Root);}

Here's merged document showing how equivalent elements from document B were combined together:

<Root><LeafA><Item1 /><Item2 /><SubLeaf><X /><Y /></SubLeaf><Item3 /></LeafA><LeafB><Item1 /><Item2 /><Item5 /><Item6 /></LeafB><LeafA Name="X"><Item3 /></LeafA></Root>