Add C# binding using SWIG, and support for Xamarin.

[ming]

Support C# binding using SWIG. The resulting C# binding can then be used for C# apps, as well as for Xamarin projects.

Requirements:

1. Install swig-csharp
2. (For C#/Xamarin development): Download Visual Studio from ​xamarin.com.

How to create C# binding:

1. Go to pjsip-apps/src/swig.
2. Run make.

To create Xamarin sample app:

1. From Visual Studio, create new solution "Forms". Name it pjsua2xamarin in directory pjsip-apps/src/swig/csharp (we will refer this directory as [csharp_dir]).
2. In the multiplatform section (pjsua2xamarin.pjsua2xamarin):
   • Add pjsua2 folder ([csharp_dir]/pjsua2xamarin/pjsua2xamarin/pjsua2).
   • Add sample.cs file ([csharp_dir]/pjsua2xamarin/pjsua2xamarin/sample.cs).
3. Add code to run sample (such as in pjsua2xamarin.Droid MainActivity.cs or pjsua2xamarin.iOS AppDelegate.cs):

      pjsua2xamarin.sample test = new sample();
      test.test1();
   

4. Add PJSIP libraries.
   • For Android: Add lib folder ([csharp_dir]/Droid/lib) and change the Build Action of libpjsua2.so to Android Native Library. For more details, please refer to the ​official doc.
   • For iOS: Add the fat static library file (each architecture's resulting libpjsua2.a is located in [csharp_dir]/iOS/lib/[arch]). For more info, please refer to the ​official doc.
5. Set application permissions.
   See our sample apps (ipjsua for iOS and pjsua2 sample for Android) to see the typical basic permissions required.

Issues and solutions:

• The type or namespace name 'HandleRef' does not exist.
  SWIG requires .NET 2 or later by default and uses HandleRef. Make sure you are using the supported .NET framework version.
  ​https://github.com/swig/swig/issues/423
  ​https://github.com/swig/swig/issues/455
• TypeInitializationException, dll not found, unable to find pjsua2.
  This issue is likely caused by unsuccessful addition of the PJSIP libraries (see step 4 above), invalid path, or incorrect architecture of the libraries.
• Crashes when calling Endpoint.libInit(), or during initialization.
  This is likely caused by unauthorized permission (see step 5 above). App must list the required permissions and in some cases, specifically request for the permissions to the user, and user must grant those permissions.

[ming]

In 5735:

Fixed #2086: Add C# binding using SWIG, and support for Xamarin

[ming]

In 5736:

Re #2086: Add more info for better understanding of the lifetime of PJSUA2 objects. And add an example to explicitly delete PJSUA2 objects.