Improve JavaProxyThrowable by translating managed stack trace

src/Mono.Android/Android.Runtime/AndroidRuntime.cs

@@ -79,7 +79,7 @@ public override void RaisePendingException (Exception pendingException)
 		{
 			var je  = pendingException as JavaProxyThrowable;
 			if (je == null) {
-				je  = new JavaProxyThrowable (pendingException);
+				je  = JavaProxyThrowable.Create (pendingException);
 			}
 			var r = new JniObjectReference (je.Handle);
 			JniEnvironment.Exceptions.Throw (r);

src/Mono.Android/Android.Runtime/JavaProxyThrowable.cs

@@ -1,23 +1,83 @@
 using System;
+using System.Diagnostics;
+using System.Reflection;
+
+using StackTraceElement = Java.Lang.StackTraceElement;
 
 namespace Android.Runtime {
 
-	class JavaProxyThrowable : Java.Lang.Error {
+	sealed class JavaProxyThrowable : Java.Lang.Error {
 
 		public  readonly Exception InnerException;
 
-		public JavaProxyThrowable (Exception innerException)
-			: base (GetDetailMessage (innerException))
+		JavaProxyThrowable (string message, Exception innerException)
+			: base (message)
 		{
 			InnerException  = innerException;
 		}
 
-		static string GetDetailMessage (Exception innerException)
+		public static JavaProxyThrowable Create (Exception innerException)
+		{
+			if (innerException == null) {
+				throw new ArgumentNullException (nameof (innerException));
+			}
+
+			// We prepend managed exception type to message since Java will see `JavaProxyThrowable` instead.
+			var proxy = new JavaProxyThrowable ($"[{innerException.GetType ()}]: {innerException.Message}", innerException);
+
+			try {
+				proxy.TranslateStackTrace ();
+			} catch (Exception ex) {
+				// We shouldn't throw here, just try to do the best we can do
+				Console.WriteLine ($"JavaProxyThrowable: translation threw an exception: {ex}");
+				proxy = new JavaProxyThrowable (innerException.ToString (), innerException);
+			}
+
+			return proxy;
+		}
+
+		void TranslateStackTrace ()
 		{
-			if (innerException == null)
-				throw new ArgumentNullException ("innerException");
+			var trace = new StackTrace (InnerException, fNeedFileInfo: true);
+			if (trace.FrameCount <= 0) {
+				return;
+			}
+
+			StackTraceElement[]? javaTrace = null;
+			try {
+				javaTrace = GetStackTrace ();
+			} catch (Exception ex) {
+				// Report...
+				Console.WriteLine ($"JavaProxyThrowable: obtaining Java stack trace threw an exception: {ex}");
+				// ..but ignore
+			}
+
+
+			StackFrame[] frames = trace.GetFrames ();
+			int nElements = frames.Length + (javaTrace?.Length ?? 0);
+			StackTraceElement[] elements = new StackTraceElement[nElements];
+
+			for (int i = 0; i < frames.Length; i++) {
+				StackFrame managedFrame = frames[i];
+				MethodBase? managedMethod = managedFrame.GetMethod ();
+
+				var throwableFrame = new StackTraceElement (
+					declaringClass: managedMethod?.DeclaringType?.FullName,
+					methodName: managedMethod?.Name,
+					fileName: managedFrame?.GetFileName (),
+					lineNumber: managedFrame?.GetFileLineNumber () ?? -1
+				);
+
+				elements[i] = throwableFrame;
+			}
+
+			if (javaTrace != null) {
+				for (int i = frames.Length; i < nElements; i++) {
+					elements[i] = javaTrace[i - frames.Length];
+				}
+			}
 
-			return innerException.ToString ();
+			SetStackTrace (elements);
 		}
 	}
 }

src/Mono.Android/Java.Lang/Throwable.cs

@@ -253,7 +253,7 @@ public static Throwable FromException (System.Exception e)
 			if (e is Throwable)
 				return (Throwable) e;
 
-			return new Android.Runtime.JavaProxyThrowable (e);
+			return Android.Runtime.JavaProxyThrowable.Create (e);
 		}
 
 		public static System.Exception ToException (Throwable e)

tests/Mono.Android-Tests/System/ExceptionTest.cs

@@ -1,5 +1,7 @@
 using System;
+using System.Diagnostics;
 using System.Globalization;
+using System.Reflection;
 
 using Android.App;
 using Android.Content;
@@ -17,18 +19,53 @@ static Java.Lang.Throwable CreateJavaProxyThrowable (Exception e)
 			var JavaProxyThrowable_type = typeof (Java.Lang.Object)
 				.Assembly
 				.GetType ("Android.Runtime.JavaProxyThrowable");
-			return (Java.Lang.Throwable) Activator.CreateInstance (JavaProxyThrowable_type, e);
+			MethodInfo? create = JavaProxyThrowable_type.GetMethod (
+				"Create",
+				BindingFlags.Static | BindingFlags.Public,
+				new Type[] { typeof (Exception) }
+			);
+
+			Assert.AreNotEqual (null, create, "Unable to find the Android.Runtime.JavaProxyThrowable.Create(Exception) method");
+			return (Java.Lang.Throwable)create.Invoke (null, new object[] { e }); // Don't append Java stack trace
 		}
 
 		[Test]
 		public void InnerExceptionIsSet ()
 		{
-			var ex  = new InvalidOperationException ("boo!");
-			using (var source = new Java.Lang.Throwable ("detailMessage", CreateJavaProxyThrowable (ex)))
+			Exception ex;
+			try {
+				throw new InvalidOperationException ("boo!");
+			} catch (Exception e) {
+				ex = e;
+			}
+
+			using (Java.Lang.Throwable proxy = CreateJavaProxyThrowable (ex))
+			using (var source = new Java.Lang.Throwable ("detailMessage", proxy))
 			using (var alias  = new Java.Lang.Throwable (source.Handle, JniHandleOwnership.DoNotTransfer)) {
+				CompareStackTraces (ex, proxy);
 				Assert.AreEqual ("detailMessage", alias.Message);
 				Assert.AreSame (ex, alias.InnerException);
 			}
 		}
+
+		void CompareStackTraces (Exception ex, Java.Lang.Throwable throwable)
+		{
+			var managedTrace = new StackTrace (ex);
+			StackFrame[] managedFrames = managedTrace.GetFrames ();
+			Java.Lang.StackTraceElement[] javaFrames = throwable.GetStackTrace ();
+
+			// Java
+			Assert.IsTrue (javaFrames.Length >= managedFrames.Length,
+					$"Java should have at least as many frames as .NET does; java({javaFrames.Length}) < managed({managedFrames.Length})");
+			for (int i = 0; i < managedFrames.Length; i++) {
+				var mf = managedFrames[i];
+				var jf = javaFrames[i];
+
+				Assert.AreEqual (mf.GetMethod ()?.Name,                   jf.MethodName, $"Frame {i}: method names differ");
+				Assert.AreEqual (mf.GetMethod ()?.DeclaringType.FullName, jf.ClassName,  $"Frame {i}: class names differ");
+				Assert.AreEqual (mf.GetFileName (),                       jf.FileName,   $"Frame {i}: file names differ");
+				Assert.AreEqual (mf.GetFileLineNumber (),                 jf.LineNumber, $"Frame {i}: line numbers differ");
+			}
+		}
 	}
 }