OpenGIS With .Net 2.0 And WCF

@YaronNaveh

Geospatial and location based services became very popular these days. One concern for developers of such applications is how to represent the data in the system. This includes simple data types such as points or coordinate systems and complex ones such as advanced topologies. The correct approach is of course to use types from the standard schema set of the Open Geospatial Consortium (OGC).

For a .Net developer this may imply the following needs:

Use xsd.exe to generate classes for serizliation

Create .Net 2.0 / WCF web services that utilize these schemas

Consume Wsdl's with these schemas

Unfortunetely the OpenGIS standard schemas are not interoperable with .Net. This has already been noticed by developers for older OpenGIS versions but newer versions seem to be equality incompatible.

If you only want the fix without the details download it here and see in the end of the post the fix details:




Here are some of the errors one gets when trying to consume Gml 3.1.1 and Gml 3.2.1 schemas in .Net:

Gml 3.1.1 - WCF

While using "add service reference" we get this error:

Custom tool error: Failed to generate code for the service reference 'ServiceReference1'. Please check other error and warning messages for details.

And after this an empty proxy code is generated:

namespace ConsoleApplication1.ServiceReference1 {

}

Gml 3.1.1 - .Net 2.0

The wsdl importing stage seems to work fine. Let's build a one line client:

WebReference.Service c = new WebReference.Service();

Such a simple client - what can possibly go wrong? Well nothing, except this:

"There was an error reflecting property '_ReferenceSystem'."

"There was an error reflecting type 'ConsoleApplication1.WebReference.AbstractReferenceSystemType'."
...

"There was an error reflecting property 'Item'."
...
"There was an error reflecting property 'Text'."

"Member 'Text' cannot be encoded using the XmlText attribute. You may use the XmlText attribute to encode primitives, enumerations, arrays of strings, or arrays of XmlNode."

And this is after I have omitted some inner exceptions.

In some cases (depending on .Net 2.0 patch level) the below exception will appear - not much improvement:

Unable to generate a temporary class (result=1).
error CS0029: Cannot implicitly convert type 'ConsoleApplication70.localhost.LineStringSegmentType' to 'ConsoleApplication70.localhost.LineStringSegmentType[]'
error CS0029: Cannot implicitly convert type 'ConsoleApplication70.localhost.LineStringSegmentType' to 'ConsoleApplication70.localhost.LineStringSegmentType[]'

Finally, depending in the types we reference, we might get this one as well:

There was an error reflecting type 'ConsoleApplication70.localhost.TopoSurfacePropertyType'.

Gml 3.2.1 - WCF

Proxy is generated but the simplest client throws this exception chain:

There was an error reflecting type 'ConsoleApplication1.ServiceReference1.RingPropertyType'.

"There was an error reflecting type 'ConsoleApplication1.ServiceReference1.RingType'."

...

"Member 'Text' cannot be encoded using the XmlText attribute. You may use the XmlText attribute to encode primitives, enumerations, arrays of strings, or arrays of XmlNode."

Gml 3.2.1 - .Net 2.0

Guess what?

"There was an error reflecting property '_ReferenceSystem'."

"There was an error reflecting type 'ConsoleApplication1.WebReference.AbstractReferenceSystemType'."

...

"Member 'Text' cannot be encoded using the XmlText attribute. You may use the XmlText attribute to encode primitives, enumerations, arrays of strings, or arrays of XmlNode."

Why these errors happen?

The errors appear in run-time when we instantiate the client proxy or web service. At this stage .Net creates the serialization assembly for each type and fails to do it for the proxy. Actually if we have marked the "generate serialization assembly" build option we could already see these errors in compile time.

Now what?

We need to fix the schemas or the proxy in order to make them interoperable with .Net. The fix must be compatible with the original schema, so it may change the schema syntax but must result in an isomorphic schema.

Making Gml 3.2.1 work

There are two reasons for this schema incomparability:

Cyclic schema references

From gml.xsd:

<include schemaLocation="deprecatedTypes.xsd"/>

From deprecatedTypes.xsd:

<include schemaLocation="gml.xsd"/>

One way to solve this is to remove deprecatedTypes.xsd altogether - it is not really required. We'll be nicer and just replace inside it the reference to gml.xsd with these direct references:

<include schemaLocation="dynamicFeature.xsd"/>
<include schemaLocation="topology.xsd"/>
<include schemaLocation="coverage.xsd"/>
<include schemaLocation="coordinateReferenceSystems.xsd"/>
<include schemaLocation="observation.xsd"/>
<include schemaLocation="temporalReferenceSystems.xsd"/>

Non-string lists

.Net does not support the xsd:list data type. When the list is of strings it works anyway since unresolved types are treated as strings. But for other types it is not supported. MSDN contains some more information on xsd:list support in .Net.

basicTypes.xsd contains this:

<simpleType name="doubleList">
   <annotation>
     <documentation>XML List based on XML Schema double type. An element of this type contains a space-separated list of double values</documentation>
   </annotation>
   <list itemType="double" />
</simpleType>

Which becomes this in the .Net proxy:

///
[System.Xml.Serialization.XmlTextAttribute()]
public double[] Value {
  get {
    return this.valueField;
  }
  set {
    this.valueField = value;
  }
}

which causes a run-time error since the XmlTextAttribute is only appropriate on strings.

The solutions is to replace all non-string lists in the schema to strings so the above becomes:

...
<list itemType="string" />
...

And after this Gml 3.2.1 classes are serializable by .Net!


Making Gml 3.1.1 work

This version has two issues:

Same issue with lists as in 3.2.1 - same fix

I've seen this one a couple of times. Basically when there is a multidimensional array of a type which has a single child element in some cases .Net code generation is incorrect. Don't ask, long story... Just replace in geometryPrimitives.xsd:

<complexType name="LineStringSegmentArrayPropertyType">
  <sequence>
    <element ref="gml:LineStringSegment" minOccurs="0" maxOccurs="unbounded"/>
  </sequence>
</complexType>

With:

<complexType name="LineStringSegmentArrayPropertyType">
  <sequence>
    <element ref="gml:LineStringSegment" minOccurs="0" maxOccurs="unbounded"/>
    <any />
  </sequence>
</complexType>

And we have 3.1.1 working as well!

Download fix

I have uploaded the fixed schemas to save your time:



After you extract the zip the fixed schemas are in the below folders:

opengis_net_fixed\gml\3.1.1_fixed
opengis_net_fixed\gml\3.2.1_fixed

@YaronNaveh

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