How to transform XML as a string w/o using files in .NET?

Let's say I have two strings:

• one is XML data
• and the other is XSL data.

The xml and xsl data are stored in database columns, if you must know.

How can I transform the XML in C# w/o saving the xml and xsl as files first? I would like the output to be a string, too (HTML from the transformation).

It seems C# prefers to transform via files. I couldn't find a string-input overload for Load() in XslCompiledTransform. So, that's why I'm asking.

Here's what I went with. It's a combination of your answers. I voted up the answers that inspired this:

string output = String.Empty;
using (StringReader srt = new StringReader(xslInput)) // xslInput is a string that contains xsl
using (StringReader sri = new StringReader(xmlInput)) // xmlInput is a string that contains xml
{
    using (XmlReader xrt = XmlReader.Create(srt))
    using (XmlReader xri = XmlReader.Create(sri))
    {
        XslCompiledTransform xslt = new XslCompiledTransform();
        xslt.Load(xrt);
        using (StringWriter sw = new StringWriter())
        using (XmlWriter xwo = XmlWriter.Create(sw, xslt.OutputSettings)) // use OutputSettings of xsl, so it can be output as HTML
        {
            xslt.Transform(xri, xwo);
            output = sw.ToString();
        }
    }
}

Note: this statement is required in the xsl, in order to output as HTML:

<xsl:output method="html" omit-xml-declaration="yes" />

You can XmlReader.Create() from a StringReader or a MemoryStream . XslCompileTransfrom can Load() from an XmlReader.

It took me a long time (literally years) to work out how concise code using Stream and/or TextWriter can be if you use the proper idioms.

Assuming transform and input are strings:

StringWriter sw = new StringWriter();
using (XmlReader xrt = XmlReader.Create(new StringReader(transform))
using (XmlReader xri = XmlReader.Create(new StringReader(input))
using (XmlWriter xwo = XmlWriter.Create(sw))
{
   XslCompiledTransform xslt = new XslCompiledTransform();
   xslt.Load(xrt);
   xslt.Transform(xri, xwo);
}
string output = sw.ToString();

I send the xml content and then load the XSLT document, apply the transformation and then return the new xml.

public static string Transform(string xmlString)
{
    string output = String.Empty;
    try
    {
        // Load an XML string into the XPathDocument.
        StringReader rdr = new StringReader(xmlString);
        XPathDocument myXPathDoc = new XPathDocument(rdr);

        var myXslTrans = new XslTransform();
        //load the Xsl 
        myXslTrans.Load("XSLTFile.xslt");
        //create the output stream
        StringWriter sw = new StringWriter();
        XmlWriter xwo = XmlWriter.Create(sw);
        //do the actual transform of Xml
        myXslTrans.Transform(myXPathDoc, null, xwo);
        output = sw.ToString();
        xwo.Close();
        return output;
    }
    catch (Exception e)
    {
        Console.WriteLine("Exception: {0}", e.ToString());
        throw;
    }
}

NOTE: "XSLTFile.xslt" It is added to the solution and set the property "Copy to Output Directory" to "Copy always".

edit: using-blocks added

// input-xml
string xmlinput = String.Empty;
// xslt
string xsltinput = String.Empty;
// output-xml
string xmloutput = String.Empty;

// Prepare input-xml
XPathDocument doc = new XPathDocument(new StringReader(xmlinput));

// Prepare XSLT
XslTransform xslt = new XslTransform();
// Creates a XmlReader from your xsl string
using (XmlReader xmlreader = XmlReader.Create(new StringReader(xsltinput)))
{
    //Load the stylesheet.
    xslt.Load(xmlreader);

    // transform
    using (StringWriter sw = new StringWriter())
    {
        xslt.Transform(doc, null, sw);

        // save to string
        xmloutput = sw.ToString();
    }
}

A VB.Net version inspired by Robert Rossney's answer:

Private Function TransformXML(XMLPath As String, XSLPath As String) As String
    Dim XSLT As XslCompiledTransform = New XslCompiledTransform()
    Dim sWriter As StringWriter = New StringWriter
    Dim xReader As XmlReader = XmlReader.Create(XMLPath)

    Using xWriter As XmlWriter = XmlWriter.Create(sWriter)
        XSLT.Load(XSLPath)
        XSLT.Transform(xReader, xWriter)
    End Using
    Return sWriter.ToString
End Function

I would use the XmlReader.Create(DatabaseBlobStream) and XmlWriter.Create(StringBuilder) overloads. Using the following DatabaseBlobStream object

DatabaseBlobStream.cs

internal class DatabaseBlobStream : Stream
{
    private readonly IDataReader reader;
    private readonly int columnIndex;
    private long streamPosition;

    internal DatabaseBlobStream(IDataReader reader, int columnIndex)
    {
        this.reader = reader;
        this.columnIndex = columnIndex;
    }

    public override bool CanRead
    {
        get
        {
            return reader.GetFieldType(columnIndex) == typeof (byte[])
                   && !reader.IsDBNull(columnIndex);
        }
    }

    public override bool CanSeek
    {
        get { return false; }
    }

    public override bool CanWrite
    {
        get { return false; }
    }

    public override void Flush()
    {
        throw new Exception("This stream does not support writing.");
    }

    public override long Length
    {
        get { throw new Exception("This stream does not support the Length property."); }
    }

    public override long Position
    {
        get
        {
            return streamPosition;
        }
        set
        {
            streamPosition = value;
        }
    }

    public override int Read(byte[] buffer, int offset, int count)
    {
        if (reader.IsDBNull(columnIndex))
            return 0;

        int bytesRead = (int)reader.GetBytes(columnIndex, streamPosition + offset, buffer, 0, count);
        streamPosition += bytesRead;
        return bytesRead;
    }

    public override long Seek(long offset, SeekOrigin origin)
    {
        throw new Exception("This stream does not support seeking.");
    }

    public override void SetLength(long value)
    {
        throw new Exception("This stream does not support setting the Length.");
    }

    public override void Write(byte[] buffer, int offset, int count)
    {
        throw new Exception("This stream does not support writing.");
    }

    public override void Close()
    {
        try
        {
            reader.Close();
        }
        finally
        {
            base.Close();
        }
    }

    protected override void Dispose(bool disposing)
    {
        try
        {
            reader.Dispose();
        }
        finally
        {
            base.Dispose(disposing);
        }
    }
}