Xamarin.Forms plugin with multi-targeting project

Question

Hello,

I'm having trouble making a plugin for xamarin.forms.

To make the plugin I am following the following example:

https://learn.microsoft.com/es-es/nuget/guides/create-packages-for-xamarin

The problem is that when in my example xamarin.forms project, I try to access the Log method from the 'Core' project of my example, the dependency is not resolved well.

In the CrossLoggingLibrary.shared.cs class, the CreateMultiTargeting () method throws an exception:

static IMultiTargeting CreateMultiTargeting ()  
         {  
#if NETSTANDARD1_0 || NETSTANDARD2_0  
             return null;  
#else  
             return new MultiTargetingImplementation ();  
#endif  
         }  

It goes through the if netstandard ....

To call it, I do it as I have seen in this other example:

https://github.com/jsuarezruiz/Xamarin.Forms-Samples/tree/master/MultiTargeting

CrossMultiTargeting.Current.Sample ();

Any ideas? Thank you.

Answer 1

Hi @JarvanZhang , thanks for answering.

When you use multi targing project template in Visual Studio, its create automatically one class as this:

public static class CrossMultiTargeting  
    {  
        static Lazy<IMultiTargeting> implementation =   
			new Lazy<IMultiTargeting>(() => CreateMultiTargeting(), System.Threading.LazyThreadSafetyMode.PublicationOnly);  
  
        public static bool IsSupported => implementation.Value == null ? false : true;     
        public static IMultiTargeting Current  
        {  
            get  
            {  
                IMultiTargeting ret = implementation.Value;  
                if (ret == null)  
                {  
                    throw NotImplementedInReferenceAssembly();  
                }  
                return ret;  
            }  
        }  
        static IMultiTargeting CreateMultiTargeting()  
        {  
#if NETSTANDARD1_0 || NETSTANDARD2_0  
            return null;  
#else  
            return new MultiTargetingImplementation();  
#endif  
        }  
 
        internal static Exception NotImplementedInReferenceAssembly() =>  
            new NotImplementedException("...");  
    }  

The exception is throwed because the method CreateMultiTargeting return null.

I need know how can I use this kind of project for make plugins.

Answer 2

Hello,​

Welcome to our Microsoft Q&A platform!

The exception is throwed because the method CreateMultiTargeting return null.

This should be the correct behavior. By default, the TargetFramework of the shared project is .Net Standard 2.0 now. So it will pass the #if NETSTANDARD1_0 || NETSTANDARD2_0 statement and return null. You could right-click the shared project->Properties to check the version of TargetFramework.

You can also change the version to 'NETSTANDARD2_1' for test, it will retrun new MultiTargetingImplementation(); in this case.

   static IMultiTargeting CreateMultiTargeting()  
   {  
       #if NETSTANDARD2_1  
           return null;  
       #else  
               return new MultiTargetingImplementation();  
       #endif  
   }  

Best Regards,

Jarvan Zhang

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