Passing array from JScript to COM object: Error converting argument 1 to type class StringArray

[FrankLeinbach]

Discussed in #630

^{Originally posted by FrankLeinbach January 24, 2025}
I have run into a breaking issue in moving from an implementation I made of IActiveScript to ClearScript.

I have JScript that creates a COM object like this:

var thirdPartyObj = new ActiveXObject("ThirdPartyObj.API");

within the script, I also create an array of strings to pass to a method on this object:

var stringData = [];
stringData.push("String1");
stringData.push("String2");
stringData.push("String3");

In the previous iteration, I had a problem directly calling the third party object's method with this array, like this:

thirdPartyObj.ProcessStrings(stringData)

This gave me an error. To circumvent this, I created a global host object function that simply took the JavaScript array and returned an array of String (string[]).

thirdPartyObj.ProcessStrings(JSArrayToStringArray(stringData));

This worked in the original IActiveScript implementation.

Now, with ClearScript, every time I pass this to the ProcessStrings function, regardless of passing it directly or wrapping the array in the function, I get the error "Error converting argument 1 to type class StringArray".

I have tried a number of engine properties, but none have worked:

AllowReflection
DisableListIndexTypeRestriction
DisableTypeRestriction
EnableAutoHostVariables
EnforceAnonymousTypeAccess
UseReflectionBindFallback

Any insight would be appreciated.

[ClearScriptLib]

Hi @FrankLeinbach,

First, a general comment: Because ClearScript's primary goal is to provide full access to .NET from script code, its default behavior is to marshal most .NET objects, including arrays, by proxy. So, instead of an array type that's native to the script language, script code gets a special object – a proxy – that acts as a reference to the managed array and provides full access to it. The script engine has no way of determining that the proxy refers to an array and no way of converting it to another array type.

Now, with ClearScript, every time I pass this to the ProcessStrings function, regardless of passing it directly or wrapping the array in the function, I get the error "Error converting argument 1 to type class StringArray".

It's difficult to answer without knowing what ProcessStrings expects for an argument – that is, what exactly the definition of StringArray is. However, if we assume that, in your original, working scenario, you passed a managed string array (the return value from JSArrayToStringArray) directly to JScript via COM Interop, then ClearScript's MarshalArraysByValue feature might help. Could you give it a try?

Thanks!

[FrankLeinbach]

Thank you for the quick response. I will try this on Monday when I'm back at my computer and post whether or not it worked.

[FrankLeinbach]

This appears to work. Thank you for the help.

Are there any issues that are brought up by MarshalArraysByValue?

[ClearScriptLib]

Hi @FrankLeinbach,

This appears to work. Thank you for the help.

That's great to hear. Thank you!

Are there any issues that are brought up by MarshalArraysByValue?

Yes, there are caveats:

1. The flag affects all managed arrays that the host passes or returns to the script engine. The feature cannot be used selectively.
2. A managed array can reference itself – either directly or indirectly. COM Interop cannot marshal such an array correctly, so ClearScript leaves array elements that would create circular references unassigned – i.e., null.
3. The conversion of a managed array to a script array is lossy. The script array should contain all the data in the correct order (except as noted above), but .NET-specific array functionality will not be accessible – e.g., methods such as Clear and Reverse, interfaces such as ICloneable and IEnumerable, etc.

Good luck!