Generally constraining valid values of one element based on the presence or 
absense of another is not something that XSD can describe.  Typically 
described as co-constraints this is something you will have to enforce by 
code or by some other layer or system.

But in this case you could change your schema so that the XML looks like 
this:

<PrimaryApplicant/>
<CoApplicant/>

or

<PrimaryApplicant/>

And not

<PrimaryApplicant/>
<PrimaryApplicant/>

and not

<CoApplicant/>

And this is something XSD can define, since it is a simple sequence as 
follows:

<xs:complexType name="Applicant">
  </xs:complexType>

  <xs:element name="root">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="PrimaryApplicant" type="Applicant" />
        <xs:element name="CoApplicant" type="Applicant" minOccurs="0"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

"Nathan Alden" <visualcsharpcoder@h...> wrote in message 
news:ejbLraYfFHA.3912@t......
> Using XSD, how can I force an element appearing in an XML document to
> contain an attribute with a specific value?
>
> Here is the complexType I've defined:
>
> <xs:complexType name="Applicant">
> <xs:attribute name="type">
>  <xs:simpleType>
>   <xs:restriction base="xs:string">
>    <xs:enumeration value="PrimaryApplicant"/>
>    <xs:enumeration value="CoApplicant"/>
>   </xs:restriction>
>  </xs:simpleType>
> </xs:attribute>
> </xs:complexType>
>
> I want to require the XML to only allow this:
>
> <Applicant type="PrimaryApplicant"/>
> <Applicant type="CoApplicant"/>
>
> or this:
>
> <Applicant type="PrimaryApplicant"/>
>
> but not this:
>
> <Applicant type="PrimaryApplicant"/>
> <Applicant type="PrimaryApplicant"/>
>
> and not this:
>
> <Applicant type="CoApplicant"/>
>
> Is this possible with XML Schema?
>
> Thanks,
>
> Nate
>