Type Mismatch when mocking Command with Result type return value.

[Gaby-Torres]

When mocking a command with Result return value, a type mismatch error is thrown:

The following _TypeError was thrown while running async test code:
type 'MockCommand<double, ResultDart<void, Exception>>' is not a subtype of type 'Command<double,
ResultDart<void, Exception>>' in type cast

Code:

//This Mocks Properly
Command<double, void> setFoo = MockCommand<double, void>(
  initialValue: null,
  noReturnValue: true,
);

//This does not
Command<double, Result<void>> updateFoo =
    MockCommand<double, Result<void>>(
      initialValue: Success(()),
      noReturnValue: false,
    );

[escamoteur]

Thank you for reporting this issue! I've identified and fixed the problem.

Root Cause

The issue was in the MockCommand class definition in lib/mock_command.dart:

// Before (line 5) - BROKEN
class MockCommand<TParam, TResult> extends Command<TParam, TResult?> {

Notice the ? making TResult nullable. This caused a type mismatch because:

• Your variable type: Command<double, Result<void>>
• Actual MockCommand type: MockCommand<double, Result<void>> which extends Command<double, Result<void>?>

Since Result<void>? is not a subtype of Result<void>, the assignment failed.

This was inconsistent with CommandSync and CommandAsync, which both extend Command<TParam, TResult> without the ?.

Fix

I've removed the unnecessary ? from the MockCommand class declaration:

// After - FIXED ✅
class MockCommand<TParam, TResult> extends Command<TParam, TResult> {

All 67 tests pass with this change.

Workaround for Current Version

If you need a workaround before the fix is published, you can make your variable type nullable:

Command<double, Result<void>?> updateFoo =
    MockCommand<double, Result<void>>(
      initialValue: Success(()),
      noReturnValue: false,
    );

This fix will be included in the next release. Thanks again for the detailed bug report!