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Given this tag, <a:TagName> What is the "a:"? RRS feed

  • Question

  • I'm sure this is a simple question, but it's one of those ones that's difficult to search for in a search engine.  I have a WCF service and just today my tags changed from <TagName></TagName> to <a:TagName></a:TagName>.  I'm really not sure what I did to cause this, but I'm afraid some of my clients who use the service may be affected.  Can someone just tell me what those "a:" things are called or what specification they are a part of?  Or, certainly if you could tell me why they would appear that would be great too, but really just need to know what they're called so I can do a google search lol.

    Thanks a lot for any help.

    Monday, October 24, 2011 7:55 PM

Answers

  • They're the prefix of the XML element. A XML element has three properties: a local name (in your case, "TagName"), a prefix (in your example, either "" or "a") and a namespace, which is dependent on the namespace declarations which are in scope (see examples below). The specification for this behavior is at http://www.w3.org/TR/xml-names/. A client shouldn't rely on the prefix for making decisions regarding the semantics of a XML element; it should only rely on the element local name and namespace for that.

    Some examples to help you understand this concept:

    <root xmlns="http://default.namespace">
        <!-- here any elements declared without a prefix have namespace equal to "http://default.namespace" -->
        <foo>The namespace of -foo- is "http://default.namespace"</foo>
        <newDecl xmlns:a="http://another.namespace">
            <!-- the declaration above associated "http://another.namespace" to the prefix "a" -->
            <item1>The namespace of -item1- is still "http://default.namespace"</item1>
            <a:item2>The namespace of -item2- is the one associated with the prefix "a": "http://another.namespace"</a:item2>
            <item3 xmlns="http://redeclaring.default">We redeclared the default namespace, now the namespace of item3 is "http://redeclaring.default"</item3>
            <item4>But item3 is now out of scope, so the namespace of -item4- is "http://default.namespace"</item4>
        </newDecl>
    </root>
    
    

    Now, to understand exactly what "a:" means in your <a:TagName>, you need to find the namespace declaration which binds the prefix "a" to any namespace. I don't know exactly what caused the change, but as I mentioned before, if the client is written correctly, these two documents are supposed to be equivalent:

    Document 1: with prefixes

    <a:root xmlns:a="http://my.namespace">
        <a:TagName>Some value</a:TagName>
        <a:TagName>Other value</a:TagName>
    </a:root>
    
    

    Document 2: without prefixes:

    <root xmlns="http://my.namespace">
        <TagName>Some value</TagName>
        <TagName>Other value</TagName>
    </root>
    

    Notice that the local name and the namespace of all elements in the documents are the same (as are the text nodes), so they're supposed to be treated the same way by WCF clients. Now, there are some clients which aren't implemented correctly, and sometimes they're not under our control, so WCF provides some extensibility points where you can change the prefixes of your XML documents prior to returning it to the client. The post at http://blogs.msdn.com/b/carlosfigueira/archive/2010/06/13/changing-prefixes-in-xml-responses.aspx shows one way of doing that.


    Carlos Figueira
    • Marked as answer by kstevens715 Monday, October 24, 2011 8:40 PM
    Monday, October 24, 2011 8:36 PM
    Moderator

All replies

  • They're the prefix of the XML element. A XML element has three properties: a local name (in your case, "TagName"), a prefix (in your example, either "" or "a") and a namespace, which is dependent on the namespace declarations which are in scope (see examples below). The specification for this behavior is at http://www.w3.org/TR/xml-names/. A client shouldn't rely on the prefix for making decisions regarding the semantics of a XML element; it should only rely on the element local name and namespace for that.

    Some examples to help you understand this concept:

    <root xmlns="http://default.namespace">
        <!-- here any elements declared without a prefix have namespace equal to "http://default.namespace" -->
        <foo>The namespace of -foo- is "http://default.namespace"</foo>
        <newDecl xmlns:a="http://another.namespace">
            <!-- the declaration above associated "http://another.namespace" to the prefix "a" -->
            <item1>The namespace of -item1- is still "http://default.namespace"</item1>
            <a:item2>The namespace of -item2- is the one associated with the prefix "a": "http://another.namespace"</a:item2>
            <item3 xmlns="http://redeclaring.default">We redeclared the default namespace, now the namespace of item3 is "http://redeclaring.default"</item3>
            <item4>But item3 is now out of scope, so the namespace of -item4- is "http://default.namespace"</item4>
        </newDecl>
    </root>
    
    

    Now, to understand exactly what "a:" means in your <a:TagName>, you need to find the namespace declaration which binds the prefix "a" to any namespace. I don't know exactly what caused the change, but as I mentioned before, if the client is written correctly, these two documents are supposed to be equivalent:

    Document 1: with prefixes

    <a:root xmlns:a="http://my.namespace">
        <a:TagName>Some value</a:TagName>
        <a:TagName>Other value</a:TagName>
    </a:root>
    
    

    Document 2: without prefixes:

    <root xmlns="http://my.namespace">
        <TagName>Some value</TagName>
        <TagName>Other value</TagName>
    </root>
    

    Notice that the local name and the namespace of all elements in the documents are the same (as are the text nodes), so they're supposed to be treated the same way by WCF clients. Now, there are some clients which aren't implemented correctly, and sometimes they're not under our control, so WCF provides some extensibility points where you can change the prefixes of your XML documents prior to returning it to the client. The post at http://blogs.msdn.com/b/carlosfigueira/archive/2010/06/13/changing-prefixes-in-xml-responses.aspx shows one way of doing that.


    Carlos Figueira
    • Marked as answer by kstevens715 Monday, October 24, 2011 8:40 PM
    Monday, October 24, 2011 8:36 PM
    Moderator
  • I figured out what the "a:" was.  It's a namespace.  It started showing up because I moved one of my data contracts to a different namespace in my C# project and the "a" namespace was added automatically to prevent ambiguity.  Since it remains valid XML I would not think this would affect clients using it.  In theory.
    • Marked as answer by kstevens715 Monday, October 24, 2011 8:37 PM
    • Unmarked as answer by kstevens715 Monday, October 24, 2011 8:40 PM
    Monday, October 24, 2011 8:37 PM
  • Carlos,

    Thank you very much for your excellent response.  This is exactly what I was looking for.  I had actually just figured out that it meant namespace right before I saw you posted... but your explanation and your URL showing how to override the prefix is very helpful.

    Monday, October 24, 2011 8:42 PM