Required field validtion in xml using xsd

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C#

XML

i want the xsd file for the XML as below mentions

<ClaimsCollection>
  <Claim ClientType="General">
    <Metadata>
      <Field Name="Audit Year">2010</Field>
      <Field Name="Auditor Name">abc</Field>
      <Field Name="Audit Number">22</Field>
      <Field Name="Category">xyz</Field>
    </Metadata>
  </Claim>
</ClaimsCollection>

So here I want to checck validation for 'Audit Year' field that it should be present in xml and value provided for this node is required one. so if Name='Audit year' is not exist then it should throw error and its node value is blank it also should throw error message. i want to make few fields optional and few required also so please help me regarding this.