未找到“MyClass”类型的默认构造函数

Question

I am trying to render a custom view from from xamarin.forms. When I try to render it from with following code

[assembly: ExportRenderer(typeof(SwipeableCard),typeof(CardContainer))] .

But an error throws like

default constructor not found of type 'Com.andtinde.CardContainer' .

I guess it is because the card container does not have a default constructor. And I cannot provide one because it inherits from Adapterview.

What I am doing wrong. Any help would be appriciated.

Answer 1

I think this is a known bug in Xamarin. I had a same problem with a Dependency Service though. I solved it by setting "Link Framework SDKs Only" in Linker behavior of iOS Build Options.

Answer 2

You've already identified what you're doing wrong. The ExportRenderer needs a default constructor to be able to do what it's doing. Why does inheriting from AdapterView prevent you from having a default constructor?

You should be able to do something like:

public class CardContainer
{
    public CardContainer() // This is a default constructor
        : base(...) // If base class requires arguments, pass them here
    {
    }
}

Answer 3

This is not a bug on Xamarin, you must know about how to manage your linked assemblies.all of runtime works in xamarin will get error like this for example you cannot get peroperty of class in runtime or create instance in runtime when that assembly is not linked. You can manage linked assemblies manualy or set those in your application option as Alessio Campanelli answer here , if you don't set it manualy your application will get large size in release package like apk file. How to set it manualy?

1. Create a xml file in your android application name it "linked.xml"
2. Right click and go to peroperties of "linked.xml" and set "Build Action" to "LinkDescription"
3. add your assemblies those are used on runtime like this:

    <?xml version="1.0" encoding="utf-8" ?>
    <linker>
      <assembly fullname="Newtonsoft.Json">
        <type fullname="*" />
      </assembly>
    </linker>

Linker in xamarin

Answer 4

You are probably better off by inheriting from ViewRenderer and providing the CardContainer as the native view type. Then override the ViewRenderer's OnElementChanged method and construct a new CardContainer for the renderer. Below is a short version that would get you started.

Registration:

[assembly: ExportRenderer(typeof(SwipeableCard), typeof(SwipeableCardRenderer ))]

Class definition:

 SwipeableCardRenderer : ViewRenderer<SwipeableCard, CardContainer>

OnElementChanged override:

if (this.Control == null)
{
    this.SetNativeControl(new CardContainer(this.Context));
}

Answer 5

Xamarin Forms PCL: System.MissingMethodException: Default constructor not found for type

This will occur normally whenever you used a class for dependency injection or using device-specific classes. You can try it out by debugging. I had faced the same but solved it by giving the current class for typeof property. should not give the service or any other interface name:

[assembly: Xamarin.Forms.Dependency(typeof(Name Of Class))]

and not:

[assembly: Xamarin.Forms.Dependency(typeof(Name Of Interface))]

Answer 6

in Xamarin after update all nuget packages me also facing this issue.

Finally i use some indirect fix

i called a class by dummy

ex (isLogged == "55" is never reached)

    //added this code to fix default constructor issue for appshell pages
    if(isLogged == "55")
    {
        MainPage = new HomePage();
    }