Pass String in C++ COM dll from C# function

Question

Function Interface of dll;

I think in dll function looks like, with datatype BSTR

CustomConvert(BSTR dataStr)

{........}

dll Interface:

CustomConvert(IntPtr dataStr)    //Returns strings

The data I need to pass is something like this:

string strTemp = "pŒ®í§…Êtf°B²bßZÃQô"; // something like this
obj.CustomConvert(strTemp);

But I am getting the exception "string" cannot convert to "System.IntPtr"; After searching in internet I found something like this.

obj.CustomConvert(System.Runtime.InteropServices.Marshal.StringToBSTR(strTemp));

But System.Runtime.InteropServices.Marshal.StringToBSTR(strTemp) convert strTemp in numerical numbers like 2035295. But I need to pass the actual value in strTemp.

Any help or suggestions?

xanatos · Accepted Answer · 2015-06-26 05:16:45Z

1

To pass a BSTR you can do something like:

public static extern void CustomConvert([MarshalAs(UnmanagedType.BStr)] string dataStr);

and then pass the string directly without doing anything.

Note that in the CustomConvert you mustn't free the BSTR, because it is "owned" by the C#.

answered Jun 26, 2015 at 5:16

xanatos

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6 Comments

xanatos Over a year ago

@NearlyCrazy Ah... Then the obj.CustomConvert(System.Runtime.InteropServices.Marshal.StringToBSTR(strTemp)); is correct. Remember that you'll have to free it with Marshal.FreeBSTR, so it is better if you use a variable, like IntPtr temp = Marshal.StringToBSTR(strTemp); obj.CustomConvert(temp); Marshal.FreeBSTR(temp);

10K35H 5H4KY4 Over a year ago

just curious : what happens if I don't use FreeBSTR ??

xanatos Over a year ago

@NearlyCrazy Someone has to free the memory allocated for the BSTR. Sadly it isn't always clear who has to do it. Normally if you call a method passing a BSTR, the caller (you) maintains the ownership of the BSTR, so has to free it. Clearly if you call a method that returns a BSTR, the caller (you) is given the ownership of the BSTR and has to free it.

10K35H 5H4KY4 Over a year ago

@ xanatos: I have just implement your suggestion. But while I run 'Marshal.FreeBSTR(temp);'. My 'Visual Studio' get stopped but no exception caugth on try catch.

xanatos Over a year ago

@xanatos Sadly it isn't always clear who has to do it and Normally are the keywords here :-) Clearly it is already freed by the called method. Remove the Marshal.FreeBSTR(temp) and annotate somewhere that the method seems to free the passed BSTR

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