Get Specific Element on Different XMLs

Question

I need to get a specific element from an xml string to know its corrensponding concrete type to deserialize it. Let's call Function Code as the specific element and getting this element is a little challenging for me.

Each function code's corresponds to a specific schema design, and it looks like this:

1    <?xml version="1.0" encoding="utf-8"?>
2    <Document xmlns="some.namespace.of.schema.design.1">
3      <SchemaDesign1>
4        <Header>
5          <FunctionCode>FunctionCode1</FunctionCode>
6          <OtherElement1>...</OtherElement1>
7          <OtherElement2>...</OtherElement2>

I need the value of the Function Code element on line 5 which is FunctionCode1. But notice that on line 3 that the element name is specific to its concrete type also.

So for another Function Code, e.g. FunctionCode2, the element on line 3 will not be the same:

1    <?xml version="1.0" encoding="utf-8"?>
2    <Document xmlns="some.namespace.of.schema.design.2">
3      <SchemaDesign2>
4        <Header>
5          <FunctionCode>FunctionCode2</FunctionCode>
6          <OtherElement1>...</OtherElement1>
7          <OtherElement2>...</OtherElement2>

I can only think of using string.IndexOf("<FunctionCode>") and get the function code's value until it finds the respective closing tag. Is there a better approach for this without reading the whole string?

Here is a sample XML I got:

<?xml version="1.0" encoding="UTF-8"?>
<Document xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="urn:iso:std:iso:20022:tech:xsd:caaa.013.001.07">
    <AccptrDgnstcReq>
        <Hdr>
            <MsgFctn>DGNP</MsgFctn>
            <PrtcolVrsn>6.0</PrtcolVrsn>
            <XchgId>378</XchgId>
            <CreDtTm>2013-08-08T22:18:35.07+02:00</CreDtTm>
            <InitgPty>
                <Id>66000001</Id>
                <Tp>OPOI</Tp>
                <Issr>ACQR</Issr>
            </InitgPty>
            <RcptPty>
                <Id>epas-acquirer-1</Id>
                <Tp>ACQR</Tp>
            </RcptPty>
        </Hdr>
        <DgnstcReq>
            <Envt>
                <Acqrr>
                    <Id>
                    <Id>9287351</Id>
                    </Id>
                    <ParamsVrsn>2013-08-07 08:00:00</ParamsVrsn>
                </Acqrr>
                <MrchntId>
                    <Id>EPASMER001</Id>
                </MrchntId>
                <POIId>
                    <Id>66000001</Id>
                    <Tp>OPOI</Tp>
                    <Issr>ACQR</Issr>
                </POIId>
            </Envt>
        </DgnstcReq>
        <SctyTrlr>
            <CnttTp>AUTH</CnttTp>
            <AuthntcdData>
                <Rcpt>
                    <KEK>
                        <KEKId>
                            <KeyId>SpecV1TestKey</KeyId>
                            <KeyVrsn>2010060715</KeyVrsn>
                            <DerivtnId>OYclpQE=</DerivtnId>
                        </KEKId>
                        <KeyNcrptnAlgo>
                            <Algo>DKP9</Algo>
                        </KeyNcrptnAlgo>
                        <NcrptdKey>4pAgABc=</NcrptdKey>
                    </KEK>
                </Rcpt>
                <MACAlgo>
                    <Algo>MCCS</Algo>
                </MACAlgo>
                <NcpsltdCntt>
                    <CnttTp>DATA</CnttTp>
                </NcpsltdCntt>
                <MAC>3Dahc1K96Pc=</MAC>
            </AuthntcdData>
        </SctyTrlr>
    </AccptrDgnstcReq>
</Document>

Answer 1

So given you have two XDocument, for each sample XML, called doc1 and doc2 respectively, then this code:

var ns1 = doc1.Root.GetDefaultNamespace();
var ns2 = doc2.Root.GetDefaultNamespace();

var functionCode1 = doc1.Root.Descendants(ns1 + "FunctionCode").First().Value;
var functionCode2 = doc2.Root.Descendants(ns2 + "FunctionCode").First().Value;

Console.WriteLine(functionCode1);
Console.WriteLine(functionCode2);

...produces:

FunctionCode1
FunctionCode2

So, the general case for this, given you have an unknown XML document in this format, is:

var ns = doc.Root.GetDefaultNamespace();

var functionCode = doc.Root.Descendants(ns + "FunctionCode").First().Value;