Q: C# xml validation I defined a xsd:
Very similar to HTML table. rows has columns and columns has elements.
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:attributeGroup name="basics">
    <xs:attribute name="title" type="xs:string" use="required"/>
    <xs:attribute name="field_id" type="xs:string" use="required"/>
    <xs:attribute name="is_mandatory" type="xs:boolean" use="required"/>
  </xs:attributeGroup>

  <xs:element name="form">
    <xs:complexType>      
      <xs:sequence>
        <xs:element name="row">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="col">
                <xs:complexType>
                  <xs:sequence>
                    <!-- lable -->
                    <xs:element name="label" type="xs:string"/>

                    <!-- image -->
                    <xs:element name="image" >
                      <xs:complexType>
                        <xs:attribute name="src" type="xs:string" use="required"/>
                      </xs:complexType>
                    </xs:element>

                    <!-- textbox -->
                    <xs:element name="textbox">
                      <xs:complexType>
                        <xs:attributeGroup ref="basics"/>
                        <xs:attribute name="hint" type="xs:string" use="optional" default=""/>
                      </xs:complexType>
                    </xs:element>

                    <!-- yesno -->
                    <xs:element name="yesno">
                      <xs:complexType>
                        <xs:attributeGroup ref="basics"/>                        
                      </xs:complexType>
                    </xs:element>

                    <!-- calendar -->
                    <xs:element name="calendar">
                      <xs:complexType>
                        <xs:attributeGroup ref="basics"/>
                      </xs:complexType>
                    </xs:element>

                    <!-- Select/ multi select -->
                    <xs:element name="select">
                      <xs:complexType>
                        <xs:sequence>
                          <xs:element name="option"/>
                        </xs:sequence>
                        <xs:attributeGroup ref="basics"/>
                        <xs:attribute name="singleChoice" type="xs:boolean" use="required"/>
                      </xs:complexType>
                    </xs:element>

                  </xs:sequence>                  
                </xs:complexType>
              </xs:element>
            </xs:sequence>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
      <xs:attribute name="title" type="xs:string" use="required"/>
    </xs:complexType>
  </xs:element>
</xs:schema>

And created the following xml (for me is a valid xml):
<?xml version="1.0" encoding="utf-8" ?>
<form title="title">
    <row>
        <col>
            <textbox title="aaa" field="Name" mandatory="false" hint="aaa"/>
        </col> 
    </row>

    <row>
        <col>
            <textbox title="bbb" field="UID" mandatory="true" hint="bbb"/>
            <yesno title="dddddd" field="field-yesno" mandatory="true"/>
        </col> 
    </row>

    <row>
        <col>
            <lable>dddddd</lable>
        </col>
        </row>

    <row>
        <col>
            <calendar title="cccc" field="StartDate" mandatory="true"/>
        </col>
    </row>

    <row>
        <col>
            <select title="select" field="ffff" mandatory="true">
                <option value="1">option 1</option>
                <option value="2" selected="true">option 2</option>
                <option value="3">option 3</option>
            </select>
        </col>
    </row>
</form>

When I'm trying to validate that:
XmlReaderSettings settings = new XmlReaderSettings();
        settings.Schemas.Add(null, getAbsolutePath("xml\\form_schema.xsd"));
        settings.ValidationType = ValidationType.Schema;
        settings.CloseInput = true;
        settings.ValidationFlags = XmlSchemaValidationFlags.ReportValidationWarnings |
                                   XmlSchemaValidationFlags.ProcessIdentityConstraints |
                                   XmlSchemaValidationFlags.ProcessInlineSchema |
                                   XmlSchemaValidationFlags.ProcessSchemaLocation;

        // create xml reader
        XmlReader reader = XmlReader.Create(new StringReader(xml), settings);
        XmlDocument document = new XmlDocument();
        document.Load(reader);
        document.Validate(new ValidationEventHandler(ValidationHandler));

I'm getting the following exception:
The element 'col' has invalid child element 'textbox'. List of possible elements expected: 'label'

Whats wrong with my xsd or C# code ? (the xml is good example) ?

A: The problem looks to be with the XSD. You have specified a sequence within the col element and it's expecting col to look like this:
<col>
    <label />
    <image />
    <textbox />
    <yesno />
    <calendar />
    <select />
</col>

You either need to put minOccurs="0" on each element:
<xs:element name="label" type="xs:string" minOccurs="0" />

or use <xs:choice>

A: Here's your mistake:
<lable>dddddd</lable>

I think lable is not the same as label.
Additionally: I may be wrong, but aren't you supposed to keep the proper order? You've got a textbox, while your first column in xsd is supposed to be a label.

A: The reason appears to be that you have the incorrect order in your XML example when you have defined the following XSD:
                <xs:element name="label" type="xs:string"/>

                <!-- image -->
                <xs:element name="image" >
                  <xs:complexType>
                    <xs:attribute name="src" type="xs:string" use="required"/>
                  </xs:complexType>
                </xs:element>

                <!-- textbox -->
                <xs:element name="textbox">
                  <xs:complexType>
                    <xs:attributeGroup ref="basics"/>
                    <xs:attribute name="hint" type="xs:string" use="optional" default=""/>
                  </xs:complexType>
                </xs:element>

I am not an XSD expert but I was curious about your question and read the following on W3Schools.com.  I hope I am reading this correctly, I just wanted to show what I have found researching your question.
http://www.w3schools.com/Schema/schema_complex.asp
. The "employee" element can be declared directly by naming the element, like this:
<xs:element name="employee">
  <xs:complexType>
    <xs:sequence>
     <xs:element name="firstname" type="xs:string"/>
      <xs:element name="lastname" type="xs:string"/>
    </xs:sequence>
  </xs:complexType>
</xs:element>

If you use the method described above, only the "employee" element can use the specified complex type. Note that the child elements, "firstname" and "lastname", are surrounded by the  indicator. This means that the child elements must appear in the same order as they are declared. You will learn more about indicators in the XSD Indicators chapter.
