Stop node containing subnodes and text in schema

If I have some xml like this:

<mynode>
    <mysubnode>
        <mysubsubnode>hello world</mysubsubnode>
        some more text
    </mysubnode>
</mynode>

As you can see, mysubnode contains both a subnode and some text data.

What I want to know is, is it possible to prevent this happening in a schema? I don't want nodes to contain subnodes and text, just subnodes or text.

Is there an option in my xs开发者_高级运维d I can specify to force this?

My program to that uses this xml is written in .NET, so I'll tag it as well incase there's anything of use in .net that I can utilise for this, though I'd much rather that the issue was fixed in the schema itself.

Cheers

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Edit: In re-reading the question, the OP expects to use XSD, not DTD (which btw is a good thing!). I intially though that DTD was requested; here is the XML Schema (XSD) version

<?xml version="1.0" encoding="ISO-8859-1" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:complexType name="mysubnodetype" mixed="false">
    <xs:sequence>
      <xs:element name="mysubsubnode" type="xs:string"/>
    </xs:sequence>
  </xs:complexType>

  <xs:complexType name="mynodeType" mixed="false">
    <xs:sequence>
      <xs:element name="mysubnode" type="mysubnodetype"/>
    </xs:sequence>
  </xs:complexType>

  <xs:element name="mynodeType" type="mynodeType"/>
</xs:schema>

Notes: the mixed="false" attribute added to the complexTypes is redundant, since by default complexType mixed mode is false (hence, by default, preventing the mixing of elements and text between elements.)

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(original answer, with DTD instead)
The following DTD would prevent this. Note the fact that mysubnode can only contain a subsubnode, there is no reference of PCDATA w/r mysubnode and therefore the "some more text" of the XML snippet in the question would be invalid.

<!ELEMENT mynode (mysubnode)>
<!ELEMENT mysubnode  (mysubsubnode)>
<!ELEMENT mysubsubnode (#PCDATA)>

To make the XML snippet of the question valid, something like this would be needed

<!ELEMENT mynode (mysubnode)>
<!ELEMENT mysubnode   (#PCDATAT | mysubsubnode)>
<!ELEMENT mysubsubnode (#PCDATA)>