XMLUnit for Java provides a Hamcrest matcher named CompareMatcher and XMLUnit.NET has an NUnit Constraint¹ named CompareConstraint that can be used to compare two XML sources with each other.

This example will throw an AssertionError: "Expected attribute value 'abc' but was 'xyz'".

final String control = "<a><b attr=\"abc\"></b></a>";
final String test = "<a><b attr=\"xyz\"></b></a>";

assertThat(test, CompareMatcher.isIdenticalTo(control));

The corresponding NUnit test code is

string control = "<a><b attr=\"abc\"></b></a>";
string test = "<a><b attr=\"xyz\"></b></a>";

Assert.That(test, CompareConstraint.IsIdenticalTo(control));

You can compare any kind of Objects with each other which can be used as a XML sources.

See: Providing Input To XMLUnit - Input.from(Object)

The Difference between identical and similar is decided by the default DifferenceEvaluator unless you override it.

The matcher and constraint can be configured via a fluent API:

CompareMatcher.isSimilarTo(control)
    .ignoreComments() // [1]
    .ignoreWhitespace() // [2]
    .normalizeWhitespace() // [3]
    .throwComparisonFailure() // [4]
    .withComparisonFormatter(comparisonFormatter) // [5]
    .withComparisonListeners(comparisonListeners) // [6]
    .withDifferenceEvaluator(differenceEvaluator) // [7]
    .withDifferenceListeners(comparisonListeners) // [8]
    .withNodeMatcher(nodeMatcher) // [9]
    .withAttributeFilter(attributeFilter) // [10]
    .withNodeFilter(nodeFilter) // [11]
    .withDocumentBuilderFactory(dbf) // [12]
    .ignoreElementContentWhitespace(); // [13]

and

CompareConstraint.IsSimilarTo(control)
    .IgnoreComments() // [1]
    .IgnoreWhitespace() // [2]
    .NormalizeWhitespace() // [3]
    .WithComparisonFormatter(comparisonFormatter) // [5]
    .WithComparisonListeners(comparisonListeners) // [6]
    .WithDifferenceEvaluator(differenceEvaluator) // [7]
    .WithDifferenceListeners(comparisonListeners) // [8]
    .WithNodeMatcher(nodeMatcher) // [9]
    .WithAttributeFilter(attributeFilter) // [10]
    .WithNodeFilter(nodeFilter) // [11]
    .IgnoreElementContentWhitespace(); // [13]

1. ignoreComments():
   will stripping all comments from the test- and control-XML before comparing.
2. ignoreWhitespace():
   will removing all empty text nodes and trimming the non-empty ones from the test- and control-XML before comparing. If all you need is to remove text nodes solely consisting of whitespace, then you want to use ignoreElementContentWhitespace.
3. normalizeWhitespace():
   will removing all empty text nodes and normalizing the non-empty ones from the test- and control-XML before comparing. With "normalized" in this context means all whitespace characters are replaced by space characters and consecutive whitespace characters are collapsed.
4. throwComparisonFailure():
   This will throw a org.junit.ComparisonFailure in case of differences. Eclipse, IntelliJ and NetBeans can than show you a nice Diff-View of the differences.
   See throwComparisonFailure. This is only available for XMLUnit for Java's CompareMatcher
5. withComparisonFormatter():
   Use a custom Formatter for the Error Messages.
   See ComparisonFormatter.
6. withComparisonListeners():
   Registers a listener that is notified of each comparison.
   See ComparisonListener.
7. withDifferenceEvaluator():
   Provide your own custom DifferenceEvaluator implementation.
   See DifferenceEvaluator.
8. withDifferenceListeners():
   Registers a listener that is notified of each comparison with outcome other than ComparisonResult.EQUAL.
   See ComparisonListener.
9. withNodeMatcher():
   Sets the strategy for selecting nodes to compare.
   See NodeMatcher.
10. withAttributeFilter(): Optional strategy that allows certain attributes to be ignore completely. See AttributeFilter.
11. withNodeFilter(): Optional strategy that allows certain nodes to be ignore completely. See NodeFilter.
12. withDocumentBuilderFactory(): Use the given DocumentBuilderFactory when creating a document from the test or control source. Only available in XMLUnit for Java - since XMLUnit 2.2.0.
13. ignoreElementContentWhitespace(): will removing all text nodes solely consisting of whitespace (AKA as element content whitespace) - since XMLUnit 2.6.0.

Instead of Matcher returning false a org.junit.ComparisonFailure will be thrown.

The advantage over the standard Matcher behavior is, that a ComparisonFailure can provide the effected Control-Node and Test-Node in separate Properties. Eclipse, NetBeans and IntelliJ can provide a nice DIFF-View for the two values. ComparisonFailure is also used in org.junit.Assert.assertEquals(Object, Object) if both values are Strings:

The only disadvantage is, that you can't combine the CompareMatcher with other Matchers (like org.hamcrest.CoreMatchers.not(Object)) anymore. The following code will NOT WORK properly: assertThat(test, not(isSimilarTo(control).throwComparisonFailure()))

1: Actually there are two of them, one for NUnit 2.x and one for NUnit 3.x.