Convert Android URI to a file path Unity can read

Your original code is fine, but this is a limitation with how JNI finds and invokes methods from C# (well, technically from C++, but Mono is using the same infrastructure).

See the following (related) question on Stack overflow.

The problem is you're trying to find a method that has a signature that is different than the actual File class constructor, and so it fails.

The easiest solution you can do is create a simple plugin (fancy name for a small .jar containing helper methods), then call these Java helper methods from your C# code to achieve what you want.

So found a way to do this without writing a plugin or resorting to reading byte by byte. Seems to run pretty fast. Here's the function I've written which copies the whole of any file given with the intent to another location;

         private void ImportFromIntent(string importPath)
         {
             try
             {
                 // Get the current activity
                 AndroidJavaClass unityPlayerClass = new AndroidJavaClass("com.unity3d.player.UnityPlayer");
                 AndroidJavaObject activityObject = unityPlayerClass.GetStatic<AndroidJavaObject>("currentActivity");
 
                 // Get the current intent
                 AndroidJavaObject intent = activityObject.Call<AndroidJavaObject>("getIntent");
 
                 // Get the intent data using AndroidJNI.CallObjectMethod so we can check for null
                 IntPtr method_getData = AndroidJNIHelper.GetMethodID(intent.GetRawClass(), "getData", "()Ljava/lang/Object;");
                 IntPtr getDataResult = AndroidJNI.CallObjectMethod(intent.GetRawObject(), method_getData, AndroidJNIHelper.CreateJNIArgArray(new object[0]));
                 if (getDataResult.ToInt32() != 0)
                 {
                     // Now actually get the data. We should be able to get it from the result of AndroidJNI.CallObjectMethod, but I don't now how so just call again
                     AndroidJavaObject intentURI = intent.Call<AndroidJavaObject>("getData");
 
                     // Open the URI as an input channel
                     AndroidJavaObject contentResolver = activityObject.Call<AndroidJavaObject>("getContentResolver");
                     AndroidJavaObject inputStream = contentResolver.Call<AndroidJavaObject>("openInputStream", intentURI);
                     AndroidJavaObject inputChannel = inputStream.Call<AndroidJavaObject>("getChannel");
 
                     // Open an output channel
                     AndroidJavaObject outputStream = new AndroidJavaObject("java.io.FileOutputStream", importPath);
                     AndroidJavaObject outputChannel = outputStream.Call<AndroidJavaObject>("getChannel");
 
                     // Copy the file
                     long bytesTransfered = 0;
                     long bytesTotal = inputChannel.Call<long>("size");
                     while (bytesTransfered < bytesTotal)
                     {
                         bytesTransfered += inputChannel.Call<long>("transferTo", bytesTransfered, bytesTotal, outputChannel);
                     }
 
                     // Close the streams
                     inputStream.Call("close");
                     outputStream.Call("close");
                 }
             }
             catch (System.Exception ex)
             {
                 // Handle error
             }
         }