How to add to xmldocument schema from xmlschema object

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XML

.NET

API

C#6.0

I have problem with add schema to xml document. I added a schema to the document and returning ActionResult I get an xml document without a schema. I defined the schema and added individual elements which I later added to the Xml document How can I fix this problem?

What I have tried:

<pre>[HttpGet("ConvertJsonToXML")]
            [Produces("application/xml")]
            public ActionResult ConvertJsonToXML(int? idProjectTlcStairsHistory)
            {
                //Find ID
                var result = _project.FindProjectResult(idProjectTlcStairsHistory);
                // Find name project
                var nameProject = _project.ProjectName(idProjectTlcStairsHistory);
                // Change format on improve format
                var fileName = _project.ReplaceInvalidChars(nameProject);
                JObject o = JObject.Parse(result);
                XmlDocument doc = new XmlDocument();              
                string value = o.ToString();
                doc = JsonConvert.DeserializeXmlNode(value, "StairsCalculationsData");
                XmlSerializerNamespaces nameSpace = new XmlSerializerNamespaces();
                nameSpace.Add("xsd", "http://www.w3.org/2001/XMLSchema");
          
                //Schema
                XmlSchema schema = new XmlSchema();
                schema.ElementFormDefault = XmlSchemaForm.Qualified;
                schema.AttributeFormDefault = XmlSchemaForm.Unqualified;
                schema.Version = "1.0";
                schema.Namespaces = nameSpace;
                XmlSchemaSet schemaSet = new XmlSchemaSet();
                schemaSet.Add(schema);
                XmlSchemaElement element = new XmlSchemaElement();
                element.Name = "SpiralStairs";
                element.SchemaTypeName = new XmlQualifiedName("string", 
                "http://www.w3.org/2001/XMLSchema");
                XmlAttribute attr = doc.CreateAttribute("myns:blah", "http://myns.com");
                attr.Value = "my own attribute";
                element.UnhandledAttributes = new XmlAttribute[] { attr };
                schema.Items.Add(element);
               
                doc.Schemas.Add(schema);
                doc.Schemas.Add(schemaSet);
             
                var file = XMLConvert.AddXmlDeclaration(doc);
                doc.LoadXml(file);
            
                doc.Save(@"C:\Projekty XML\" + fileName + ".xml");
    
                return Ok(doc);      
    
            }

Result - the result he gets without a schematic

<?xml version="1.0" encoding="utf-8"?>
<StairsCalculationsData>
<centralPipeDiameter>150</centralPipeDiameter>
<leftTurn>false</leftTurn>
<rightTurn>true</rightTurn>
<stairsTurn>-1</stairsTurn>
<centralPipeHandrail>false</centralPipeHandrail>
<centralPipeHandrailRadius>120</centralPipeHandrailRadius>
<radius>1000</radius>
<walkingWidth>1185</walkingWidth>
<walkingLineRadius>1000</walkingLineRadius>
<secondWalkingLineRadius>0</secondWalkingLineRadius>
<isStandardRadius>true</isStandardRadius>
<isTlcStandard>true</isTlcStandard>
<maxStepHeight>240</maxStepHeight>
<selectedRadius>1300</selectedRadius>
<selectedStep />
<levels>
<levelNumber>0</levelNumber>
<landing>
<initialDirection>4</initialDirection>
<landingShape>0</landingShape>
<startEdgeHeight>100</startEdgeHeight>
<endEdgeHeight>100</endEdgeHeight>
<startLandingAngle>90</startLandingAngle>
<endLandingAngle>0</endLandingAngle>
<isLandingAngleProperly>true</isLandingAngleProperly>
<landingWidth>1570.7963267948965</landingWidth>
<landingAngle>90</landingAngle>
</landing>
<isLastLevel>false</isLastLevel>
<angleNextLevel>20</angleNextLevel>
<startFlightAngleFromNextLevel>0</startFlightAngleFromNextLevel>
<levelMessages />
<isStepWidthProperly>true</isStepWidthProperly>
<stepDeepInWalkingLine>347.3</stepDeepInWalkingLine>
<heightLevel>2750</heightLevel>
<isStepHeightProperly>false</isStepHeightProperly>
<stepHeight>114.58333333333333</stepHeight>
<rotationRange>1.2777777777777777</rotationRange>
<startFlightAngle>190</startFlightAngle>
<flightRotationRangeAngle>460</flightRotationRangeAngle>
<stepAngle>20</stepAngle>
<isStepsAmountProperly>true</isStepsAmountProperly>
<stepsAmount>24</stepsAmount>
<stepsPerTurn>18</stepsPerTurn>
<isAngleDiffProperly>true</isAngleDiffProperly>
<angleDiff>0</angleDiff>
<isConvinienceProperly>false</isConvinienceProperly>
<convinience>576.47</convinience>
<isFreeSpaceByHeliceProperly>false</isFreeSpaceByHeliceProperly>
<endFreeSpaceByHelice>1440.2</endFreeSpaceByHelice>
<isFreeSpaceProperly>false</isFreeSpaceProperly>
<endFreeSpace>1504.2</endFreeSpace>
<isFlightFreeSpaceProperly>false</isFlightFreeSpaceProperly>
<flightFreeSpace>1847.9</flightFreeSpace>
<isFreeSpaceAboveLandingsProperly>true</isFreeSpaceAboveLandingsProperly>
<freeSpaceAboveLandings>2800</freeSpaceAboveLandings>
</levels>

</StairsCalculationsData>