Question

I’m trying to use ServiceStack for Json serialization/deserialization in MonoDroid project.

I’ve built ServiceStack to run it in MonoDroid environment, but now I have issue with JSON deserialization.

Example code:

public class Track
{
        public string Id { get; set; }
        public string Title { get; set; }
        public string Artist { get; set; }
        public string Hash { get; set; }
        public string Type { get; set; }
    }
}
...
var track = new Track { Id = "1", Artist = "artist name", Hash = "654874", Title = "song title", Type = "mp3", };
var json = ServiceStack.Text.JsonSerializer.SerializeToString(track);
var track1 = ServiceStack.Text.JsonSerializer.DeserializeFromString<Track>(json);

On deserialization I have an exception:
Unhandled Exception:

System.ArgumentNullException: Argument cannot be null.
Parameter name: method

Stacktrace:

[External Code] 
0x5E in ServiceStack.Text.Json.JsonReader.GetParseFn at [...]\Android\ServiceStack.Text\src\ServiceStack.Text\Json\JsonReader.Generic.cs:36 C#
0x2 in ServiceStack.Text.Json.JsonTypeSerializer.GetParseFn at [...]\Android\ServiceStack.Text\src\ServiceStack.Text\Json\JsonTypeSerializer.cs:283 C#
0xF in ServiceStack.Text.Common.TypeAccessor.Create at [...]\Android\ServiceStack.Text\src\ServiceStack.Text\Common\DeserializeType.cs:146  C#
0x142 in ServiceStack.Text.Common.DeserializeType`1[[ServiceStack.Text.Json.JsonTypeSerializer, ServiceStack.Text, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]].GetParseMethod at [...]\Android\ServiceStack.Text\src\ServiceStack.Text\Common\DeserializeType.cs:60 C#
0x43A in ServiceStack.Text.Common.JsReader`1[[ServiceStack.Text.Json.JsonTypeSerializer, ServiceStack.Text, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]].GetParseFn at [...]\Android\ServiceStack.Text\src\ServiceStack.Text\Common\JsReader.cs:100  C#
0x6 in ServiceStack.Text.Json.JsonReader`1[[Database.Models.Track, AssemblyName, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]]..cctor at [...]\Android\ServiceStack.Text\src\ServiceStack.Text\Json\JsonReader.Generic.cs:58  C#
0x1B in ServiceStack.Text.JsonSerializer.DeserializeFromString at [...]\Android\ServiceStack.Text\src\ServiceStack.Text\JsonSerializer.cs:33    C#

As I see there is:

var genericType = typeof(JsonReader<>).MakeGenericType(type);
var mi = genericType.GetMethod("GetParseFn", BindingFlags.Public | BindingFlags.Static);
parseFactoryFn = (ParseFactoryDelegate)Delegate.CreateDelegate(typeof(ParseFactoryDelegate), mi);

genericType is “ServiceStack.Text.Json.JsonReader`1[System.String]”
type is “System.String”

So, there “mi” variable is null

Are any ideas how to fix it, or where to look?

Thanks.

Answer 1

The problem here was that you had linking enabled in your build configuration, and the GetParseFn() method was being linked away, as it wasn’t explicitly being referenced.

The linker is a static analysis tool that is included as part of Mono for Android’s build process. It will scan through your compiled application and actually strip out any pieces of .NET (or in this case, your compiled ServiceStack class library) that aren’t explicitly being referenced. This goes a long way to cut down the size of your application, but you can run into problems, as seen here. Since the GetParseFn() method was never being explicitly referenced (it is being called via reflection), the linker had no idea it was needed and linked it away.

There are several ways you can handle this problem going forward.

1. Disable linking entirely: you can turn off linking in the project’s properties. In general it’s easier to leave it off for debug builds, and turn it on for release builds. This obviously isn’t a full solution if you ever plan on doing a release build, of course.
2. Link only SDK assemblies: this option will tell the linker only to operate on the .NET BCL and Android SDKs, and will leave your libraries alone.
3. Use PreserveAttribute: Using this attribute you can tell the linker to leave in certain pieces that you know are required, even though they’re not explicitly referenced. Since ServiceStack is an external library and not your own, this probably isn’t the best solution in this case.
4. Use Linkskip: This option allows you to specify specific assemblies that shouldn’t be touched by the linker, allowing you to leave the linker on but simply opt out when needed.

Personally, in your situation here I could suggest going the Linkskip route. It would allow you to still get all the benefits of the linker, but have it ignore ServiceStack to avoid problems like this. In your project file, you would end up with something along the lines of this:

<PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
    <AndroidLinkMode>Full</AndroidLinkMode>
    <AndroidLinkSkip>ServiceStack.Text</AndroidLinkSkip>
</PropertyGroup>