Pass multiple errors

src/Platform/Microsoft.Testing.Platform/OutputDevice/Terminal/ExceptionFlattener.cs

@@ -0,0 +1,54 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+namespace Microsoft.Testing.Platform.OutputDevice.Terminal;
+
+internal class ExceptionFlattener
+{
+    internal static FlatException[] Flatten(string? errorMessage, Exception? exception)
+    {
+        if (errorMessage is null && exception is null)
+        {
+            return Array.Empty<FlatException>();
+        }
+
+        List<Exception> exceptions = new();
+
+        string? message = !RoslynString.IsNullOrWhiteSpace(errorMessage) ? errorMessage : exception?.Message;
+        string? type = exception?.GetType().FullName;
+        string? stackTrace = exception?.StackTrace;
+        var flatException = new FlatException(message, type, stackTrace);
+
+        List<FlatException> flatExceptions = new()
+        {
+           flatException,
+        };
+
+        // Add all inner exceptions. This will flatten top level AggregateExceptions,
+        // and all AggregateExceptions that are directly in AggregateExceptions, but won't expand
+        // AggregateExceptions that are in non-aggregate exception inner exceptions.
+        IEnumerable<Exception?> aggregateExceptions = exception switch
+        {
+            AggregateException aggregate => aggregate.Flatten().InnerExceptions,
+            _ => [exception?.InnerException],
+        };
+
+        foreach (Exception? aggregate in aggregateExceptions)
+        {
+            Exception? currentException = aggregate;
+            while (currentException is not null)
+            {
+                flatExceptions.Add(new FlatException(
+                    aggregate?.Message,
+                    aggregate?.GetType().FullName,
+                    aggregate?.StackTrace));
+
+                currentException = currentException.InnerException;
+            }
+        }
+
+        return flatExceptions.ToArray();
+    }
+}
+
+internal record FlatException(string? ErrorMessage, string? ErrorType, string? StackTrace);