Question

I’m trying to use JsonPath for .NET (http://code.google.com/p/jsonpath/downloads/list) and I’m having trouble finding an example of how to parse a Json string and a JsonPath string and get a result.

Has anyone used this?

Answer 1

The problem you are experiencing is that the C# version of JsonPath does not include a Json parser so you have to use it with another Json framework that handles serialization and deserialization.

The way JsonPath works is to use an interface called IJsonPathValueSystem to traverse parsed Json objects. JsonPath comes with a built-in BasicValueSystem that uses the IDictionary interface to represent Json objects and the IList interface to represent Json arrays.

You can create your own BasicValueSystem-compatible Json objects by constructing them using C# collection initializers but this is not of much use when your Json is coming in in the form of strings from a remote server, for example.

So if only you could take a Json string and parse it into a nested structure of IDictionary objects, IList arrays, and primitive values, you could then use JsonPath to filter it! As luck would have it, we can use Json.NET which has good serialization and deserialization capabilities to do that part of the job.

Unfortunately, Json.NET does not deserialize Json strings into a format compatible with the BasicValueSystem. So the first task for using JsonPath with Json.NET is to write a JsonNetValueSystem that implements IJsonPathValueSystem and that understands the JObject objects, JArray arrays, and JValue values that JObject.Parse produces.

So download both JsonPath and Json.NET and put them into a C# project. Then add this class to that project:

public sealed class JsonNetValueSystem : IJsonPathValueSystem
{
    public bool HasMember(object value, string member)
    {
        if (value is JObject)
                return (value as JObject).Properties().Any(property => property.Name == member);
        if (value is JArray)
        {
            int index = ParseInt(member, -1);
            return index >= 0 && index < (value as JArray).Count;
        }
        return false;
    }

    public object GetMemberValue(object value, string member)
    {
        if (value is JObject)
        {
            var memberValue = (value as JObject)[member];
            return memberValue;
        }
        if (value is JArray)
        {
            int index = ParseInt(member, -1);
            return (value as JArray)[index];
        }
        return null;
    }

    public IEnumerable GetMembers(object value)
    {
        var jobject = value as JObject;
        return jobject.Properties().Select(property => property.Name);
    }

    public bool IsObject(object value)
    {
        return value is JObject;
    }

    public bool IsArray(object value)
    {
        return value is JArray;
    }

    public bool IsPrimitive(object value)
    {
        if (value == null)
            throw new ArgumentNullException("value");

        return value is JObject || value is JArray ? false : true;
    }

    private int ParseInt(string s, int defaultValue)
    {
        int result;
        return int.TryParse(s, out result) ? result : defaultValue;
    }
}

Now with all three of these pieces we can write a sample JsonPath program:

class Program
{
    static void Main(string[] args)
    {
        var input = @"
              { ""store"": {
                    ""book"": [ 
                      { ""category"": ""reference"",
                            ""author"": ""Nigel Rees"",
                            ""title"": ""Sayings of the Century"",
                            ""price"": 8.95
                      },
                      { ""category"": ""fiction"",
                            ""author"": ""Evelyn Waugh"",
                            ""title"": ""Sword of Honour"",
                            ""price"": 12.99
                      },
                      { ""category"": ""fiction"",
                            ""author"": ""Herman Melville"",
                            ""title"": ""Moby Dick"",
                            ""isbn"": ""0-553-21311-3"",
                            ""price"": 8.99
                      },
                      { ""category"": ""fiction"",
                            ""author"": ""J. R. R. Tolkien"",
                            ""title"": ""The Lord of the Rings"",
                            ""isbn"": ""0-395-19395-8"",
                            ""price"": 22.99
                      }
                    ],
                    ""bicycle"": {
                      ""color"": ""red"",
                      ""price"": 19.95
                    }
              }
            }
        ";
        var json = JObject.Parse(input);
        var context = new JsonPathContext { ValueSystem = new JsonNetValueSystem() };
        var values = context.SelectNodes(json, "$.store.book[*].author").Select(node => node.Value);
        Console.WriteLine(JsonConvert.SerializeObject(values));
        Console.ReadKey();
    }
}

which produces this output:

["Nigel Rees","Evelyn Waugh","Herman Melville","J. R. R. Tolkien"]

This example is based on the Javascript sample at the JsonPath site:

• Javascript Usage and Example