Question

I have two xml files that both have the same schema and I would like to merge into a single xml file. Is there an easy way to do this?

For example,

<Root>
    <LeafA>
        <Item1 />
        <Item2 />
    </LeafA>
    <LeafB>
        <Item1 />
        <Item2 />
    </LeafB>
</Root>

+

<Root>
    <LeafA>
        <Item3 />
        <Item4 />
    </LeafA>
    <LeafB>
        <Item3 />
        <Item4 />
    </LeafB>
</Root>

= new file containing

<Root>
    <LeafA>
        <Item1 />
        <Item2 />
        <Item3 />
        <Item4 />
    </LeafA>
    <LeafB>
        <Item1 />
        <Item2 />
        <Item3 />
        <Item4 />
    </LeafB>
</Root>

Answer 1

“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>