Methods with return type [object] should return null for an empty result (#7138)

src/System.Management.Automation/engine/lang/scriptblock.cs

@@ -519,7 +519,18 @@ namespace System.Management.Automation
                                        invocationInfo: null,
                                        propagateAllExceptionsToTop: true,
                                        args: args);
-            Diagnostics.Assert(result.Count == 1, "Code generation ensures we return the correct type");
+
+            // This is needed only for the case where the
+            // method returns [object]. If the argument to 'return'
+            // is a pipeline that emits nothing then result.Count will
+            // be zero so we catch that and "convert" it to null. Note that
+            // the return statement is still required in the method, it 
+            // just recieves nothing from it's argument.
+            if (result.Count == 0)
+            {
+                return default(T);
+            }
+
             return (T)result[0];
         }
 

test/powershell/Language/Classes/Scripting.Classes.BasicParsing.Tests.ps1

@@ -116,6 +116,16 @@ Describe 'Positive Parse Properties Tests' -Tags "CI" {
         class C9b { [System.Collections.Generic.List[C9b]] f() { return [C9b]::new() } }
         $c9b = [C9b]::new().f()
         It "Expected a System.Collections.Generic.List[C9b] returned" {  $c9b -is [System.Collections.Generic.List[C9b]] | Should -BeTrue }
+        It 'Methods returning object should return $null if no output was produced' {
+            class Foo {
+                [object] Bar1() { return & {} }
+                static [object] Bar2() { return & {} }
+            }
+            # Test instance method
+            [Foo]::new().Bar1() | Should -BeNullOrEmpty
+            # Test static method
+            [foo]::Bar2() | Should -BeNullOrEmpty
+        }
     }
 
     It 'Positive ParseProperty Attributes Test' {