How do I parse only part of YAML into an object?

[rcdailey]

Suppose I have the following YAML:

config_one:
  name: foo
  stuff: value

config_two:
  name: bar
  random: value

I want to selectively parse config_one into an object and I want config_two to be ignored:

class ConfigOne
{
  public string Name {get;set;}
  public string Stuff {get;set;}
}

How can I do this? The documentation is pretty lacking, or at least, it uses a lot of terminology that doesn't make much sense to me, and thus I was not able to search for this functionality.

[pensono]

This code should help you. The trick is to use the parser to scan to the point where you would like to deserialize.

https://dotnetfiddle.net/EitE4O

using System;
using System.IO;
using YamlDotNet.Core;
using YamlDotNet.Core.Events;
using YamlDotNet.Serialization;
using YamlDotNet.Serialization.NamingConventions;

var yaml = @"
config_one:
  name: foo
  stuff: value

config_two:
  name: bar
  random: value";

var parser = new Parser(new StringReader(yaml));
var deserializer = new DeserializerBuilder()
	.WithNamingConvention(new CamelCaseNamingConvention())
	.Build();

parser.Consume<StreamStart>();
parser.Consume<DocumentStart>();
parser.Consume<MappingStart>();

while (parser.TryConsume<Scalar>(out var key))
{
	if (key.Value == "config_two")
	{
		var config = deserializer.Deserialize<ConfigTwo>(parser);
		Console.WriteLine(config.Random);
	} 
	else
	{
		parser.SkipThisAndNestedEvents();
		Console.WriteLine($"Skipped {key.Value}");
	}
}

class ConfigTwo
{
  public string Name { get; set; }
  public string Random { get; set ;}
}

[rcdailey]

Minor detail, but I found that because I'm deserializing, I have to use UnderscoredNamingConvention.Instance to get my code to recognize the YAML to C# property mappings. I don't know why this is contrary to many examples I'm seeing. My assumption was that direction (serialize/deserialize) affects the behavior. Deserializing means we have to tell the code to map underscored names config_one to whatever is there in the C# property, which is camel case: ConfigOne. I probably have this wrong but I'm basing this off of what works and what doesn't. If I use camel case enumerator as you have in your example, the YAML does not parse.

[rcdailey]

Thanks again for the solution. I changed my YAML slightly, but I still need to accomplish the same task (only parse config_two). How do you adapt your solution to work with List<ConfigTwo>?

config_one:
  name: foo
  stuff: value

config_two:
  - name: bar1
    random: value1
  - name: bar2
    random: value2

Doing this does not work:

var config = deserializer.Deserialize<List<ConfigTwo>>(parser);

[rcdailey]

Sorry for the false positive. This actually does work, but it breaks when ConfigTwo has a nullable enum member:

enum PossibleNames { Bar1, Bar2 }
class ConfigTwo
{
  public PossibleNames? Name { get; set; }
};

I made it nullable to express that this property in YAML is optional. You don't have to provide name: at all. I'm not sure if this is the appropriate representation of optional YAML properties mapped to enums.

[rcdailey]

Found the solution in #544. Hopefully that gets proper support by the library in the future. I'm closing now since my question is answered. Thanks again!

[pensono]

The problem with nullable enums should be fixed once #600 is merged :)