JsonSerializationException 'Unable to find a constructor' on Xamarin.Android with .NET Standard Library

Question

I have used Xamarin Native UI for Android application and Created different class library for API call and data Deserialize api data using Newtonsoft.Json.

That class library Target Framework is .NET Standar 2.0.

As I have added that reference to the console application so its working fine but same reference i have add in Android project its throw error.

**Error Details** Newtonsoft.Json.JsonSerializationException: Unable to find a constructor to use for type. A class should either have a default constructor, one constructor with arguments or a constructor marked with the JsonConstructor attribute.

As per error message i have used attribute JsonConstructor for default constructor of class.

Example:

public class TestClass
{
    [JsonConstructor]
    public TestClass()
    {

    }
}

What is the Mono Linker setting for the build configuration that you are getting the error with. — SushiHangover
– SushiHangover, Commented Oct 4, 2018 at 5:50
@SushiHangover , As PreserveAttribute required Mono.Android.dll Or 'Xamarin.iOS.dll' reference in that but my class library is Common for both thats why it is not possible. — Manish Sharma
– Manish Sharma, Commented Oct 4, 2018 at 5:59

SushiHangover · Accepted Answer · 2018-10-04 06:05:55Z

4

As PreserveAttribute required Mono.Android.dll Or 'Xamarin.iOS.dll' reference in that but my class library is Common for both thats why it is not possible

Add a PreserveAttribute class to your class library and use that attribute as the Mono Linker only uses the "name" of the attribute and not the namespace/classname...

public sealed class PreserveAttribute : Attribute
{
    public bool AllMembers;
    public bool Conditional;
    public PreserveAttribute (bool allMembers, bool conditional)
    {
        AllMembers = allMembers;
        Conditional = conditional;
    }
    public PreserveAttribute ()
    {
    }
}

And then use that attribute on your JSON model/class:

[Preserve(AllMembers = true)]
public class TestClass
{
  ~~~

answered Oct 4, 2018 at 6:05

SushiHangover

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Comments

Arvind Chourasiya · Accepted Answer · 2018-10-04 06:04:33Z

1

Try using PreserveAttribute at the top of your class

[PreserveAttribute(AllMembers = true)]
public class TestClass
{   
    public TestClass() {}

}

add one more class PreserveAttribute in class library

public sealed class PreserveAttribute : System.Attribute
{
    public bool AllMembers;
    public bool Conditional;
}

Edit Linking- Sdk & user assemblies

Skip linking assemblies- Newtonsoft.Json;

add Newtonsoft.Json in your Skip linking assemblies option

edited Oct 4, 2018 at 6:04

answered Oct 4, 2018 at 5:54

Arvind Chourasiya

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4 Comments

Manish Sharma Over a year ago

This is work with Xamarin forms but with Native ui its not work and I have taken separate class library for Android and iOS for Api call and Business logic.

Arvind Chourasiya Over a year ago

Even I am using PCL with xamarin.android, the same attribute working for me. I got this error before few days,

Manish Sharma Over a year ago

In Same android project you can do this but if I have different class library for common logic, and in that common library i can't do it.

Arvind Chourasiya Over a year ago

@ManishSharma - Add one sealed class with name PreserveAttribute in your class library. I forgot to mention it before. If still wont work try to follow my edit section.

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