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I'm using YamlDotNet with an Azure Function v2 to serialise YAML from a markdown file (hosted on GitHub) to a .net object. I'm struggling with this error when attempting to deserialize the YAML string

Expected 'StreamEnd', got 'DocumentStart

I'm getting the markdown file content using HttpClient with a GET request to https://github.com/martinkearn/Content/raw/fd83bf8218b7c5e01f8b498e8a831bcd3fc3c961/Blogs/Test.md which returns a raw markdown file in the response body.

My Model is

public class Article

 public string Title get; set; 
 public string Author get; set; 
 public List<string> Categories get; set; 

My YAML is

---
title: Test File 
author: Martin Kearn 
categories:
 - Test
 - GitHubCMS 
 - One More Tag
 - another tag
---

Here is my code

// get url from request body
var url = "https://raw.githubusercontent.com/martinkearn/Content/fd83bf8218b7c5e01f8b498e8a831bcd3fc3c961/Blogs/Test.md";

// get raw file and extract YAML
using (var client = new HttpClient())

 //setup HttpClient
 client.BaseAddress = new Uri(url);
 client.DefaultRequestHeaders.Add("User-Agent", "ExtractYAML Function");

 //setup httpContent object
 var response = await client.GetAsync(url);

 string rawFile = await response.Content.ReadAsStringAsync();
 using (var r = new StringReader(rawFile))
 
 var deserializer = new DeserializerBuilder()
 .WithNamingConvention(new CamelCaseNamingConvention())
 .Build();

 //This line is causing Expected 'StreamEnd', got 'DocumentStart'
 var article = deserializer.Deserialize<Article>(r);
 

Your actual downloaded file contains:

---
title: Test File 
author: Martin Kearn 
categories:
 - Test
 - GitHubCMS 
 - One More Tag
 - another tag
---

# Test file

The --- is the end-of-directives marker, which is optional if you don't have any directives ( %YAML 1.2, %TAG .... ).

Since you have an empty line after the second directive, this is counted as if your second document contained

---
null
# Test file

You should at least get rid of that empty line and possible remove the second end-of-directives marker, putting the comment at the end of the first document

The end-of-document indicator in YAML is ... at the beginning of a line.

Make your file read:

title: Test File 
author: Martin Kearn 
categories:
 - Test
 - GitHubCMS 
 - One More Tag
 - another tag
# Test file

or at most:

---
title: Test File 
author: Martin Kearn 
categories:
 - Test
 - GitHubCMS 
 - One More Tag
 - another tag
# Test file