Read and write models

Applies To: yes OData Core Lib V7

Models built with EdmLib APIs are in object representation, while CSDL documents are in XML representation. The conversion from models to CSDL is accomplished by the CsdlWriter APIs which are mostly used by OData services to expose metadata documents (CSDL). In contrast, the conversion from CSDL to models is done by the CsdlReader APIs which are usually used by OData clients to read metadata documents from services.

This section shows how to read and write entity data models using EdmLib APIs. We will use and extend the sample from the previous section.

Using the CsdlWriter APIs

We have already used one of the CsdlWriter APIs to write the model to a CSDL document in the previous section.

namespace EdmLibSample
{
    class Program
    {
        ...
        private static void WriteModelToCsdl(IEdmModel model, string fileName)
        {
            using (var writer = XmlWriter.Create(fileName))
            {
                IEnumerable<EdmError> errors;
                CsdlWriter.TryWriteCsdl(model, writer, CsdlTarget.OData, out errors);
            }
        }
    }
}

The CsdlWriter.TryWriteCsdl() method is prototyped as:

namespace Microsoft.OData.Edm.Csdl
{
    public class CsdlWriter
    {
        ...
        public static bool TryWriteCsdl(
            IEdmModel model,
            XmlWriter writer,
            CsdlTarget target,
            out IEnumerable<EdmError> errors);
        ...
    }
}

The second parameter writer requires an XmlWriter which can be created through the overloaded XmlWriter.Create() methods. Remember to either apply a using statement on an XmlWriter instance or explicitly call XmlWriter.Flush() (or XmlWriter.Close()) to flush the buffer to its underlying stream. The third parameter target specifies the target implementation of the CSDL being generated, which can be either CsdlTarget.EntityFramework or CsdlTarget.OData. The 4th parameter errors is used to pass out the errors encountered in writing the model. If the method returns true (indicate success), the errors should be an empty Enumerable; otherwise it contains all the errors encountered.

Using the CsdlReader APIs

The simplest CsdlReader API is prototyped as:

namespace Microsoft.OData.Edm.Csdl
{
    public class CsdlReader
    {
        public static bool TryParse(XmlReader reader, out IEdmModel model, out IEnumerable<EdmError> errors);
    }
}

The first parameter reader takes an XmlReader that reads a CSDL document. The second parameter model passes out the parsed model. The third parameter errors passes out the errors encountered in parsing the CSDL document. If the return value of this method is true (indicate success), the errors should be an empty Enumerable; otherwise it will contain all the errors encountered.

Roundtrip the model

In the Program.cs file, insert the following code to the Program class:

namespace EdmLibSample
{
    class Program
    {
        public static void Main(string[] args)
        {
            ...
            WriteModelToCsdl(model, "csdl.xml");
#region     !!!INSERT THE CODE BELOW!!!
            var model1 = ReadModel("csdl.xml");
            WriteModelToCsdl(model1, "csdl1.xml");
#endregion
        }
        ...
        private static IEdmModel ReadModel(string fileName)
        {
            using (var reader = XmlReader.Create(fileName))
            {
                IEdmModel model;
                IEnumerable<EdmError> errors;
                if (CsdlReader.TryParse(reader, out model, out errors))
                {
                    return model;
                }
                return null;
            }
        }
    }
}

This code first reads the model from the CSDL document csdl.xml, and then writes the model to another CSDL document csdl1.xml.

Run the sample

Build and run the sample. Then open the file csdl.xml and the file csdl1.xml under the output directory. The content of csdl1.xml should look like the following:

csdl

You can see that the contents of csdl.xml and csdl1.xml are exactly the same except for the order of the elements. This is because EdmLib will reorder the elements when parsing a CSDL document.

References

[Tutorial & Sample] Refering when Constructing EDM Model.