JavaScriptTypeResolver Class

Reference

Definition

Namespace:: System.Web.Script.Serialization

Assembly:: System.Web.Extensions.dll

Important

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Provides the abstract base class for implementing a custom type resolver.

public ref class JavaScriptTypeResolver abstract

public abstract class JavaScriptTypeResolver

type JavaScriptTypeResolver = class

Public MustInherit Class JavaScriptTypeResolver

Inheritance: Object
JavaScriptTypeResolver

Derived: System.Web.Script.Serialization.SimpleTypeResolver

Examples

The following example shows how to create a custom JavaScriptTypeResolver and how to use it to serialize or deserialize an object.

using System;
using System.Linq;
using System.Web.Script.Serialization;

namespace SampleApp
{
    class Program
    {
        static void Main(string[] args)
        {
            // The object array to serialize.
            Person[] people = new Person[]
            {
                new Person()
                {
                    Name = "Kristen Solstad",
                    Age = 15,
                    HomeAddress = new Address()
                    {
                        Street1 = "123 Palm Ave",
                        City = "Some City",
                        StateOrProvince = "ST",
                        Country = "United States",
                        PostalCode = "00000"
                    }
                },
                new Adult()
                {
                    Name = "Alex Johnson",
                    Age = 39,
                    Occupation = "Mechanic",
                    HomeAddress = new Address()
                    {
                        Street1 = "445 Lorry Way",
                        Street2 = "Unit 3A",
                        City = "Some City",
                        Country = "United Kingdom",
                        PostalCode = "AA0 A00"
                    }
                }
            };

            // Serialize the object array, then write it to the console.
            string serializedData = SerializePeopleArray(people);
            Console.WriteLine("Serialized:");
            Console.WriteLine(serializedData);
            Console.WriteLine();

            // Now deserialize the object array.
            Person[] deserializedArray = DeserializePeopleArray(serializedData);
            Console.WriteLine("Deserialized " + deserializedArray.Length + " people.");
            foreach (Person person in deserializedArray)
            {
                Console.WriteLine(person.Name + " (Age " + person.Age + ") [" + person.GetType() + "]");
            }
        }

        static string SerializePeopleArray(Person[] people)
        {
            // The custom type resolver to use.
            // Note: Except for primitives like int and string, *every* type that
            // we might see in the object graph must be listed here.
            CustomTypeResolver resolver = new CustomTypeResolver(
                typeof(Person),
                typeof(Adult),
                typeof(Address));

            // Instantiate the serializer.
            JavaScriptSerializer serializer = new JavaScriptSerializer(resolver);

            // Serialize the object array, then return it.
            string serialized = serializer.Serialize(people);
            return serialized;
        }

        static Person[] DeserializePeopleArray(string serializedData)
        {
            // The custom type resolver to use.
            // Note: This is the same list that was provided to the Serialize routine.
            CustomTypeResolver resolver = new CustomTypeResolver(
                typeof(Person),
                typeof(Adult),
                typeof(Address));

            // Instantiate the serializer.
            JavaScriptSerializer serializer = new JavaScriptSerializer(resolver);

            // Deserialize the object array, then return it.
            Person[] deserialized = serializer.Deserialize<Person[]>(serializedData);
            return deserialized;
        }
    }

    public class Person
    {
        public string Name { get; set; }
        public int Age { get; set; }
        public Address HomeAddress { get; set; }
    }

    public class Adult : Person
    {
        public string Occupation { get; set; }
    }

    public class Address
    {
        public string Street1 { get; set; }
        public string Street2 { get; set; }
        public string City { get; set; }
        public string StateOrProvince { get; set; }
        public string Country { get; set; }
        public string PostalCode { get; set; }
    }

    // A custom JavaScriptTypeResolver that restricts the payload
    // to a set of known good types.
    class CustomTypeResolver : JavaScriptTypeResolver
    {
        private readonly Type[] _allowedTypes;

        public CustomTypeResolver(params Type[] allowedTypes)
        {
            if (allowedTypes == null)
            {
                throw new ArgumentNullException("allowedTypes");
            }

            // Make a copy of the array the caller gave us.
            _allowedTypes = (Type[])allowedTypes.Clone();
        }

        public override Type ResolveType(string id)
        {
            // Iterate over all of the allowed types, looking for a match
            // for the 'id' parameter. Calling Type.GetType(id) is dangerous,
            // so we instead perform a match on the Type.FullName property.
            foreach (Type allowedType in _allowedTypes)
            {
                if (allowedType.FullName == id)
                {
                    return allowedType;
                }
            }

            // The caller provided a type we don't recognize. This could be
            // dangerous, so we'll fail the operation immediately.
            throw new ArgumentException("Unknown type: " + id, "id");
        }

        public override string ResolveTypeId(Type type)
        {
            // Before we serialize data, quickly double-check to make
            // sure we're allowed to deserialize the data. Otherwise it's
            // no good serializing something if we can't deserialize it.
            if (_allowedTypes.Contains(type))
            {
                return type.FullName;
            }

            throw new InvalidOperationException("Cannot serialize an object of type " + type + ". Did you forget to add it to the allow list?");
        }
    }
}

The preceding app outputs the following to the console, formatted for readability.

Serialized:
[
    {
        "__type": "SampleApp.Person",
        "Name": "Kristen Solstad",
        "Age": 15,
        "HomeAddress": {
            "__type": "SampleApp.Address",
            "Street1": "123 Palm Ave",
            "Street2": null,
            "City": "Some City",
            "StateOrProvince": "ST",
            "Country": "United States",
            "PostalCode": "00000"
        }
    },
    {
        "__type": "SampleApp.Adult",
        "Occupation": "Mechanic",
        "Name": "Alex Johnson",
        "Age": 39,
        "HomeAddress": {
            "__type": "SampleApp.Address",
            "Street1": "445 Lorry Way",
            "Street2": "Unit 3A",
            "City": "Some City",
            "StateOrProvince": null,
            "Country": "United Kingdom",
            "PostalCode": "AA0 A00"
        }
    }
]

Deserialized 2 people.
Kristen Solstad (Age 15) [SampleApp.Person]
Alex Johnson (Age 39) [SampleApp.Adult]

In the preceding sample, the Adult type subclasses the Person type. A custom JavaScriptTypeResolver is used to include the type information as part of the generated JSON payload. This allows limited polymorphism when deserializing the JSON payload back into a .NET object graph. The payload can control whether to return a base Person instance or a derived Adult instance back to the caller.

This sample is safe because it uses an allow-list mechanism to control deserialization. The code:

Initializes the CustomTypeResolver with an explicit list of allowed types.
Restricts the deserialization process to only approved list of types. The restriction prevents deserialization attacks, where the remote client specifies a malicious __type in the JSON payload and tricks the server into deserializing a dangerous type.

Even though the app only expects Person and Adult instances to be deserialized as part of the top-level array, it's still necessary to add Address to the allow-list because:

Serializing a Person or Adult also serializes an Address as part of the object graph.
All types that might be present in the object graph need to be accounted for in the allow list. Primitives like int and string do not need to be specified.

Warning

Do not call Type.GetType(id) within the ResolveType method. This could introduce a security vunerability into the app. Instead, iterate through the list of allowed types and compare their Type.FullName property against the incoming id, as shown in the preceding sample.

Remarks

The JavaScriptTypeResolver class provides the services for:

Converting managed type information to a string value through the ResolveTypeId method.
Resolving a string value back to the appropriate managed type through the ResolveType method.

When the JavaScriptSerializer object serializes custom types, it can optionally include in the serialized JavaScript Object Notation (JSON) string a value that contains type information. During deserialization, JavaScriptSerializer can then reference this string value to determine the appropriate managed type to which the JSON string will be converted.

If you provide a type resolver to the JavaScriptSerializer instance, the serializer will use the ResolveTypeId and ResolveType methods to map between the managed type and the string value during the serialization and deserialization process, respectively.

The JavaScriptTypeResolver class is the base class for the SimpleTypeResolver class, which provides an implementation of a type resolver that uses the managed type assembly-qualified name.

Note

When using a JavaScriptTypeResolver, the resulting JSON payload contains a special __type property. This property includes the full type name, including namespace, of the target type. Before using a custom resolver, verify that the full name of the target type does not contain sensitive or privileged information.

Notes to Implementers

When you implement a type resolver, the string that is returned by the ResolveTypeId(Type) method must map back to the same managed type when the string value is passed to the ResolveType(String) method.

Constructors

JavaScriptTypeResolver()

Initializes a new instance of the JavaScriptTypeResolver class.

Methods

Equals(Object)	Determines whether the specified object is equal to the current object. (Inherited from Object)
GetHashCode()	Serves as the default hash function. (Inherited from Object)
GetType()	Gets the Type of the current instance. (Inherited from Object)
MemberwiseClone()	Creates a shallow copy of the current Object. (Inherited from Object)
ResolveType(String)	When overridden in a derived class, returns the Type object that is associated with the specified type name.
ResolveTypeId(Type)	When overridden in a derived class, returns the type name for the specified Type object.
ToString()	Returns a string that represents the current object. (Inherited from Object)

Applies to