Improve error messaging for PowerShell classes that do not implement their declared interface(s) correctly

[michaeltlombardi]

As a PowerShell developer writing and maintaining class-based DSC Resources, I want to understand how my implementation fails to satisfy the declared interface without having to leave my editor to review documentation online so I can keep working without context switching.

Right now, the error message returned from attempting to define a class which does not properly implement its declared interface is not very helpful, as it reports only that the class does not have an implementation of the first missing/incorrect method on the interface(s). It does not identify which interface the method belongs to when there are multiple interfaces or what the signature of the method should be.

class MyComparable : System.Collections.IDictionary, System.Collections.IEnumerable {}

Type        : System.Management.Automation.ParseException
Errors      :
    Extent  : class MyComparable : System.Collections.IDictionary {}
    ErrorId : TypeCreationError
    Message : Error during creation of type "MyComparable". Error message:
              Method 'get_Item' in type 'MyComparable' from assembly 'PowerShell Class Assembly, Version=1.0.0.4,
Culture=neutral, PublicKeyToken=null' does not have an implementation.
Message     : At line:1 char:1
              + class MyComparable : System.Collections.IDictionary {}
              + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
              Error during creation of type "MyComparable". Error message:
              Method 'get_Item' in type 'MyComparable' from assembly 'PowerShell Class Assembly, Version=1.0.0.4,
Culture=neutral, PublicKeyToken=null' does not have an implementation.
ErrorRecord :
    Exception             :
        Type    : System.Management.Automation.ParentContainsErrorRecordException
        Message : At line:1 char:1
                  + class MyComparable : System.Collections.IDictionary {}
                  + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                  Error during creation of type "MyComparable". Error message:
                  Method 'get_Item' in type 'MyComparable' from assembly 'PowerShell Class Assembly, Version=1.0.0.4,
Culture=neutral, PublicKeyToken=null' does not have an implementation.
        HResult : -2146233087
    CategoryInfo          : ParserError: (:) [], ParentContainsErrorRecordException
    FullyQualifiedErrorId : TypeCreationError
    InvocationInfo        :
        ScriptLineNumber : 1
        OffsetInLine     : 1
        HistoryId        : -1
        Line             : class MyComparable : System.Collections.IDictionary {}
        PositionMessage  : At line:1 char:1
                           + class MyComparable : System.Collections.IDictionary {}
                           + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        CommandOrigin    : Internal
TargetSite  :
    Name          : Invoke
    DeclaringType : System.Management.Automation.Runspaces.PipelineBase, System.Management.Automation, Version=7.3.0.3,
Culture=neutral, PublicKeyToken=31bf3856ad364e35
    MemberType    : Method
    Module        : System.Management.Automation.dll
Source      : System.Management.Automation
HResult     : -2146233087
StackTrace  :
   at System.Management.Automation.Runspaces.PipelineBase.Invoke(IEnumerable input)
   at System.Management.Automation.Runspaces.Pipeline.Invoke()
   at Microsoft.PowerShell.Executor.ExecuteCommandHelper(Pipeline tempPipeline, Exception& exceptionThrown,
ExecutionOptions options)

class MyComparable : System.Collections.IEnumerable, System.Collections.IDictionary {}

Type        : System.Management.Automation.ParseException
Errors      :
    Extent  : class MyComparable : System.Collections.IEnumerable, System.Collections.IDictionary {}
    ErrorId : TypeCreationError
    Message : Error during creation of type "MyComparable". Error message:
              Method 'GetEnumerator' in type 'MyComparable' from assembly 'PowerShell Class Assembly, Version=1.0.0.6,
Culture=neutral, PublicKeyToken=null' does not have an implementation.
Message     : At line:1 char:1
              + class MyComparable : System.Collections.IEnumerable, System.Collectio …
              + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
              Error during creation of type "MyComparable". Error message:
              Method 'GetEnumerator' in type 'MyComparable' from assembly 'PowerShell Class Assembly, Version=1.0.0.6,
Culture=neutral, PublicKeyToken=null' does not have an implementation.
ErrorRecord :
    Exception             :
        Type    : System.Management.Automation.ParentContainsErrorRecordException
        Message : At line:1 char:1
                  + class MyComparable : System.Collections.IEnumerable, System.Collectio …
                  + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                  Error during creation of type "MyComparable". Error message:
                  Method 'GetEnumerator' in type 'MyComparable' from assembly 'PowerShell Class Assembly, Version=1.0.0.6,
Culture=neutral, PublicKeyToken=null' does not have an implementation.
        HResult : -2146233087
    CategoryInfo          : ParserError: (:) [], ParentContainsErrorRecordException
    FullyQualifiedErrorId : TypeCreationError
    InvocationInfo        :
        ScriptLineNumber : 1
        OffsetInLine     : 1
        HistoryId        : -1
        Line             : class MyComparable : System.Collections.IEnumerable, System.Collections.IDictionary {}
        PositionMessage  : At line:1 char:1
                           + class MyComparable : System.Collections.IEnumerable, System.Collectio …
                           + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        CommandOrigin    : Internal
TargetSite  :
    Name          : Invoke
    DeclaringType : System.Management.Automation.Runspaces.PipelineBase, System.Management.Automation, Version=7.3.0.3,
Culture=neutral, PublicKeyToken=31bf3856ad364e35
    MemberType    : Method
    Module        : System.Management.Automation.dll
Source      : System.Management.Automation
HResult     : -2146233087
StackTrace  :
   at System.Management.Automation.Runspaces.PipelineBase.Invoke(IEnumerable input)
   at System.Management.Automation.Runspaces.Pipeline.Invoke()
   at Microsoft.PowerShell.Executor.ExecuteCommandHelper(Pipeline tempPipeline, Exception& exceptionThrown,
ExecutionOptions options)

Proposal

Given the proposals in #81, #82, #83, #85, and #86, I propose that the error handling for PowerShell classes that do not implement their declared interfaces be improved to enumerate at least what a valid implementation is, if not specifically how the current implementation fails to meet the requirements of the interface.

While my focus here is on improving the authoring experience for DSC Resource developers, this is more broadly applicable to PowerShell developers in general. This also feeds back into DSC Resource development where the developer needs to create a non-DSC Resource class to use as a property of a DSC Resource (it is convenient, for example, to declare a class that implements IComparable).