How do I write code to print a text file. C#, XAML, WPF, NOT windows forms

A simple way to do this is to create a FlowDocument and send that to the printer. The following code sample should give you idea about how easy it is to do this:

public void PrintText(string text)
{
  PrintDialog printDialog = new PrintDialog();
  IDocumentPaginatorSource documentPaginator = CreateDocument(text, "SampleTextDocument");
  printDialog.PrintDocument(documentPaginator, "Print sample document");
}

private FlowDocument CreateDocument(string text, string documentName)
{
  FlowDocument flowDocument = new FlowDocument() { Name = documentName };
  Section section = new Section();
  Paragraph paragraph = new Paragraph();
  paragraph.Inlines.Add(new Run(text));
  section.Blocks.Add(paragraph);
  flowDocument.Blocks.Add(section);
  return flowDocument;
}

I leave the part about reading the text file and wiring this to your front end up to you. As you can see, this is a trivial amount of code to write.

Pete,
I cannot thank you enough for your help.
Using Visual Studio 2022 your code would not build due to type conversion errors.
After some fiddling I got a working version which I include below.
For some strange reason, when I pasted to this solution box, some lines were red underlined.
Sincerely,
Tony (email removed)

using System.Windows.Controls;
using System.Windows.Documents;
using System.IO;
using System.Windows;

namespace Streaming
{
    internal class PrintFactory
   {
        public string textfilename;
        public string textitself;
        public string texttitle;

        public PrintFactory(string textfilename, string texttitle) 
        {
            this.textfilename = textfilename;
            this.textitself = File.ReadAllText(textfilename);
            this.texttitle = texttitle;
        }

        public void PrintText()
        {
            PrintDialog printDialog = new PrintDialog();
            if ((bool)printDialog.ShowDialog().GetValueOrDefault())
            {
                FlowDocument flowDocument = new FlowDocument();
                foreach (string line in textitself.Split('\n'))
                {
                    Paragraph myParagraph = new Paragraph();
                    // Without next line, initial character is not printed
                    myParagraph.Margin = new Thickness(5);
                    myParagraph.Inlines.Add(new Run(line));
                    flowDocument.Blocks.Add(myParagraph);
                }
                DocumentPaginator paginator = ((IDocumentPaginatorSource)flowDocument).DocumentPaginator;
                printDialog.PrintDocument(paginator, texttitle );
            }

        }

    }
}