How to declare optional date field in XSD - asp.net

Question

I was using XSD to validate my incoming XML elements.
XSD File
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.rg/2001/XMLSchema"
xmlns:tns="http://xxxy/ervices/V0"
targetNamespace="http://xxxy/ervices/V" elementFormDefault="qualified"
attributeFormDefault="unqualified" >
<xs:complexType name="FailType">
<xs:sequence>
<xs:element name="ConDt" type="xs:date" minOccurs="0" maxOccurs="1" nillable="true"/>
<xs:element name="PreEn" type="xs:string" minOccurs="1" maxOccurs="1"/>
</xs:sequence>
</xs:complexType>
</xs:schema>
My XML File
<Pat xmlns="http://xxxy/ervices/V">
<ConDt></ConDt>
</Pat>
I was getting element is invalid - The value '' is invalid according to its datatype 'http://org/2001/XMLSchema:date' - The string '' is not a valid Date value.

Answer 1

Specifying nillable="true" allows your instance to contain
<ConDt xsi:nil="true"></ConDt>
but it doesn't allow you to write
<ConDt></ConDt>
(Please don't ask me what they were thinking...)
If you want to allow
<ConDt></ConDt>
Then two possible approaches are
define it as a union type that allows either an xs:date or a
zero-length string, or
(my preferred approach) define it as a list type: a list of xs:date values with maxLength="1".

Answer 2

Thanks a lot Michael. I really appreciate your help.
Below code will allow us to declare optional date field.
<xs:element name="ConDt" maxOccurs="unbounded" minOccurs="0">
<xs:simpleType>
<xs:union>
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:minLength value="0" />
<xs:maxLength value="0" />
</xs:restriction>
</xs:simpleType>
<xs:simpleType>
<xs:restriction base="xs:date" />
</xs:simpleType>
</xs:union>
</xs:simpleType>
</xs:element>