using System;

using System.Collections.Generic;

using System.ComponentModel.DataAnnotations;

using System.Linq;

using Newtonsoft.Json;

using Newtonsoft.Json.Serialization;

public class Program

{

    public static void Main()

    {

        string json = @"

        {

            ""a"": ""123456789|123456789|123456789|123456789|123456789|"",

            ""b"": ""123456789|123456789|123456789|123456789|123456789|"",

            ""c"": ""123456789|123456789|1234"",

            ""d"": ""123456789|123456789|123456789|123456789|12345"",

            ""e"": null,

            ""f"": 386

        }";

​

        var settings = new JsonSerializerSettings

        {

            ContractResolver = new StringTruncatingPropertyResolver(30)

        };

        var foo = JsonConvert.DeserializeObject<Foo>(json, settings);

​

        Console.WriteLine("a: " + foo.A);  // should be truncated to 40 chars due to the applied MaxLength attribute on Foo.A

        Console.WriteLine("b: " + foo.B);  // should be truncated to 30 chars due to the default max length passed to the resolver

        Console.WriteLine("c: " + foo.C);  // no truncation needed because the length is already less than the default max length

        Console.WriteLine("d: " + foo.D);  // should be truncated to 30 chars due to the default max length passed to the resolver

        Console.WriteLine("e: " + foo.E);  // no truncation needed because the value is null

        Console.WriteLine("f: " + foo.F);  // truncation does not apply because the value is an int, not a string

    }

}

​

public class Foo

{

    [MaxLength(40)]

    public string A { get; set; }

    public string B { get; set; }

    public string C { get; set; }

    public string D { get; set; }

    public string E { get; set; }

    public int F { get; set; }

}

​

public class StringTruncatingPropertyResolver : DefaultContractResolver

{

    public int DefaultMaxLength { get; private set; }

​

    public StringTruncatingPropertyResolver(int defaultMaxLength)

    {

        DefaultMaxLength = defaultMaxLength;

    }

​

    protected override IList<JsonProperty> CreateProperties(Type type, MemberSerialization memberSerialization)

    {

        IList<JsonProperty> props = base.CreateProperties(type, memberSerialization);

​

        // Apply a StringTruncatingValueProvider to all string properties

        foreach (JsonProperty prop in props.Where(p => p.PropertyType == typeof(string)))

        {

            var attr = prop.AttributeProvider

                           .GetAttributes(true)

                           .OfType<MaxLengthAttribute>()

                           .FirstOrDefault();

            int maxLength = attr != null ? attr.Length : DefaultMaxLength;

            prop.ValueProvider = new StringTruncatingValueProvider(prop.ValueProvider, maxLength);

        }

​

        return props;

    }

​

    class StringTruncatingValueProvider : IValueProvider

    {

        private IValueProvider InnerValueProvider { get; set; }

        private int MaxLength { get; set; }

​

        public StringTruncatingValueProvider(IValueProvider innerValueProvider, int maxLength)

        {

            InnerValueProvider = innerValueProvider;

            MaxLength = maxLength;

        }

​

        // GetValue is called by Json.Net during serialization.

        // The target parameter has the object from which to read the string;

        // the return value is a string that gets written to the JSON.

        public object GetValue(object target)

        {

            return InnerValueProvider.GetValue(target);

        }

​

        // SetValue gets called by Json.Net during deserialization.

        // The value parameter has the string value read from the JSON;

        // target is the object on which to set the (possibly truncated) value.

        public void SetValue(object target, object value)

        {

            string s = (string)value;

            if (s != null && s.Length > MaxLength)

            {

                s = s.Substring(0, MaxLength);

            }

            InnerValueProvider.SetValue(target, s);

        }

    }

}

​