<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="DocumentElement">
    <xs:complexType>
      <xs:sequence>
        <xs:element maxOccurs="unbounded" name="entities">
          <xs:complexType>
            <xs:sequence>
              <xs:element minOccurs="0" name="name" type="xs:string" />
              <xs:element minOccurs="0" name="surname" type="xs:string" />
            </xs:sequence>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
    <xs:unique name="unique1">
      <xs:selector xpath="entities" />
      <xs:field xpath="surname" />
    </xs:unique>
  </xs:element>
</xs:schema>

<?xml version="1.0" standalone="yes"?>
<DocumentElement>
  <entities>
    <name>aaa</name>
    <surname>bbbbb</surname>
  </entities>
  <entities>
    <name>ccc</name>
  </entities>
  <entities>
    <name>ddd</name>
  </entities>
</DocumentElement>

DataTable dt = new DataTable();
DataColumn dc = new DataColumn("Blah", System.Type.GetType("System.String"));
dc.Unique = true;
dc.AllowDBNull = true;
dt.Columns.Add(dc);
for(int i=0;i<5;i++)
{
 DataRow dro = dt.NewRow();
 dt.Rows.Add(dro);
}