Instead of walking the string twice, move the call to IndexOf to before the if and test offset >= 0 instead.

good point, will fix

In the common case, you are creating the same string twice with arg.Substring(offiset + 1), once here, and once below.

So this can be made much simpler, something like:

string argValue = arg.Substring(offset + 1);
bool? boolValue = GetBoolValue(argValue);
_collectedArgs.Add(new CommandParameter(arg.Substring(0, offset), boolValue ?? argValue);

will fix

null-coalescing operator requires same type on both sides, I can use ternary operator to simplify it instead

Nevermind, even ternary expects same type

We can use out parameter.

Again, you can simply a bunch like above with ??.

will fix

same as above, has to be same types so can't simplify with null-coalescing or ternary operator

You can still simplify though, something like:

object argValue = arg.Substring(offset + 1);
 _collectedArgs.Add(new CommandParameter(arg.Substring(0, offset), GetArgPossiblyAsBool(argValue)));

And GetArgPossiblyAsBool returns object.

Will change

Doesn't work, can't implicitly convert object to bool/string

I don't think these tests reflect the real user scenario.

Passing $BoolString is completely different than passing $true.

$BoolString is getting replaced with $true, but $true gets replaced with True.

See comment above regarding usage from different shells.

Updated tests for both $true and true

We should have a test that accepts a string parameter and you pass true and false - we want to make sure that when we convert to bool, the bool gets properly converted back to the correct string.

And come to think about it, there is a problem - we'll lose the case that the user specified.

Yeah, that's a problem. I've changed it so it only works with switches and added tests for other cases

I think this needs to be True, not $true.

Or maybe it needs to be both $true and True - because it depends on the shell how $true is expanded.

Isn't the use case:

powershell -file foo.ps1 -myswitch:$true

Not clear to me when we need True as I don't think we want to support:

powershell -file foo.ps1 -myswitch:true

Ok, I think I understand what you're getting it by your comment below. From a non-powershell shell like bash (which I thought would be the primary use case), I would think we want to support PowerShell syntax for passing $true and $false. From within powershell, I suppose we shouldn't expect the user to escape $true to be just a string. I can make this change.

Sorry again -
Can we use LanguagePrimitives.TryConvertTo or LanguagePrimitives.TryConvertArg?
Can we use TryGetBoolValue pattern?

                               string argValue1 = arg.Substring(offset + 1); 
                               string argValue0 = arg.Substring(0, offset);
                               if (TryGetBoolValue(argValue1, out bool boolValue))  
                               {  
                                       _collectedArgs.Add(new CommandParameter(argValue0, boolValue));  
                              }  
                               else  
                               {  
                                       _collectedArgs.Add(new CommandParameter(argValue0, argValue1));  
                               }  

Sure

Please add a protection comment like "Don't change case!". And below too.

sure