Use JSON.NET to generate JSON schema with extra attributes

I am using JSON.NET to generate JSON Schema from c# object class. But I was unable to add any other json schema attributes e.g. maxLength, pattern(regex to validate email), etc

Below is my working code, I can only generate json schema with required attribute. It would be great if anyone can post some code example about how to add those extra attribute for json schema.

Thanks,

my code example

public class Customer
{
    [JsonProperty(Required = Required.Always)]
    public int CustomerID { get; set; }

    [JsonProperty(Required = Required.Always)]
    public string FirstName { get; set; }

    [JsonProperty(Required = Required.Always)]
    public string LastName { get; set; }

    [JsonProperty(Required = Required.Always)]
    public string Email { get; set; }

    [JsonProperty(Required = Required.AllowNull)]
    public string Phone { get; set; }
}

{
    "title" : "Customer",
    "type" : "object",
    "properties" : {
        "CustomerID" : {
            "required" : true,
            "type" : "integer"
        },
        "FirstName" : {
            "required" : true,
            "type" : "string"
        },
        "LastName" : {
            "required" : true,
            "type" : "string"
        },
        "Email" : {
            "required" : true,
            "type" : "string"
        },
        "Phone" : {
            "required" : true,
            "type" : [
                "string",
                "null"
            ]
        }
    }
}

Paweł Bulwan

James Newton-King is right in his answer, I'll just expand it with a code example so people stumbling onto this page don't need to study the whole documentation.

So you can use the attributes provided with .NET to specify those addidional options, like maximum length of the string or allowed regex pattern. Here are some examples:

public class MyDataModel
{
    public enum SampleEnum { EnumPosition1, EnumPosition2, EnumPosition3 }

    [JsonProperty(Required = Required.Always)]
    [RegularExpression(@"^[0-9]+$")]
    public string PatternTest { get; set; }

    [JsonProperty(Required = Required.Always)]
    [MaxLength(3)]
    public string MaxLength3 { get; set; }

    [JsonProperty(Required = Required.AllowNull)]
    [EnumDataType(typeof(SampleEnum))]
    public string EnumProperty { get; set; }
}

The annotations above come from System.ComponentModel.DataAnnotations namespace.

To make those additional attributes affect resulting json schema, you need to use JSchemaGenerator class distributed with Json.NET Schema package. If you use older JsonSchemaGenerator, then some upgrade is needed, as it's now deprecated and does not contain new functions like the aforementioned.

Here's a sample function that generates Json Schema for the class above:

    /// <summary>
    /// Generates JSON schema for a given C# class using Newtonsoft.Json.Schema library.
    /// </summary>
    /// <param name="myType">class type</param>
    /// <returns>a string containing JSON schema for a given class type</returns>
    internal static string GenerateSchemaForClass(Type myType)
    {
        JSchemaGenerator jsonSchemaGenerator = new JSchemaGenerator();
        JSchema schema = jsonSchemaGenerator.Generate(myType);
        schema.Title = myType.Name;

        return schema.ToString();
    }

and you can use it just like this:

 string schema = GenerateSchemaForClass(typeof(MyDataModel));

Json.NET Schema now has much improved schema generation support.

You can annotate properties with .NET's Data Annotation attributes to specify information like minimum, maximum, minLength, maxLength and more on a schema.

There is also JSchemaGenerationProvider that lets you take complete control when generating a schema for a type.

More details here: http://www.newtonsoft.com/jsonschema/help/html/GeneratingSchemas.htm

You can create custom JsonConverter something like this. I used reflection to fill out properties.

  public class UserConverter : JsonConverter
{
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        var user = (User)value;
        var result = new StringBuilder("{");

        result.Append("title : " + user.GetType().Name + ", ");
        result.Append("properties : {");

        foreach (var prop in user.GetType().GetProperties())
        {
            result.Append(prop.Name + ": {");
            result.Append("value : " + Convert.ToString(prop.GetValue(user, null)) + ", ");

            var attribute = (JsonPropertyAttribute)Attribute.GetCustomAttributes(prop)[0];
            if (attribute.Required == Required.Always)
                result.Append("required : true, ");

            result.Append("type : " + prop.PropertyType.Name.ToLower());
            result.Append(" }");
        }
        writer.WriteValue(result.ToString());
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var user = new User { UserName = (string)reader.Value };

        return user;
    }

    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(User);
    }

}

[JsonConverter(typeof(UserConverter))]
public class User
{
    [JsonProperty(Required = Required.Always)]
    public string UserName { get; set; }
}

//Run  
string json = JsonConvert.SerializeObject(manager, Formatting.Indented);

Console.WriteLine(json);

You could use the JavaScriptSerializer class.Like:

namespace ExtensionMethods
{
    public static class JSONHelper
    {
        public static string ToJSON(this object obj)
        {
            JavaScriptSerializer serializer = new JavaScriptSerializer();
            return serializer.Serialize(obj);
        }

        public static string ToJSON(this object obj, int recursionDepth)
        {
            JavaScriptSerializer serializer = new JavaScriptSerializer();
            serializer.RecursionLimit = recursionDepth;
            return serializer.Serialize(obj);
        }
    }
}

Use it like this:

using ExtensionMethods;

...

List<Person> people = new List<Person>{
                   new Person{ID = 1, FirstName = "Scott", LastName = "Gurthie"},
                   new Person{ID = 2, FirstName = "Bill", LastName = "Gates"}
                   };


string jsonString = people.ToJSON();