C# Code to C world function call

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C

C#

I have small doubt,
I am having function call from C# managed world to C unmanaged code base..

C# Code

Boolean ChsarpFunction(StringBuilder sb) {
int size;

return GetFormattedString(sb,size);
}

[DllImport(MYDLL, CharSet = CharSet.Auto, SetLastError = true, BestFitMapping = false)]
[return: MarshalAs(UnmanagedType.U1)]
... Boolean GetFormattedString( [MarshalAs(UnmanagedType.LPStr)] StringBuilder value, int size);

C Code
bool GetFormattedString (char *str, int* size) {
..
..
strcpy(str,result);
*size = strlen(str);
return (*size > 0); // some result of 0 or 1
}

I don't know or want to set the size for the StringBuilder, What kind of string is optimal to pass by reference.(performance wise)
Any ideas or experience is much appreciated.

Thanks,