C# marshal string PTCHAR TCHAR* help needed



Hi, i am trying to call A dll FUNCTION from a C# application.
i am using this function

int hhpGetErrorMessage(PTCHAR ptcErrorMsg,int nMaxChars)
ptcErrMsg: TCHAR buffer to hold error message string.
nMaxChars: Maximum number of characters that can fit in ptcErrorMsg
including NULL.

i defined this function in C# as
// Miscellaneous Error Code Message function.
[DllImport("hhpImgrSdk.dll", CharSet=CharSet.Auto)]
public static extern int hhpGetErrorMessage(string ptcErrorMsg,int
nMaxChars );

i call this function as

string s();

i get an error message " Use of unassigned local variable 's'"

when i declare s as string s =new string (' ',128);

it seems to pass the error msg, but the result i get from s is junk
a junk value such as "œ—ž¾'Ì•Š"¬⁤ œäûr"

i know ptcErrorMsg will hold the error msg returned by
when i tried to change my function to accept a ref to a string such as
[DllImport("hhpImgrSdk.dll", CharSet=CharSet.Auto)]
public static extern int hhpGetErrorMessage(ref string ptcErrorMsg,int
nMaxChars );

i got an Exception "An unhandled exception of type

i dont know for sure if its a Marshaling problem?
i know that PTCHAR,TCHAR* can be substituted in C# by string..
i cannot change the DLL !!
how do i declare a string of a certain size without an initial value

any idea?

thanks all
(e-mail address removed)



Nicholas Paldino [.NET/C# MVP]


You want to make the declaration like this:

[DllImport("hhpImgrSdk.dll", CharSet=CharSet.Auto)]
public static extern int hhpGetErrorMessage(StringBuilder ptcErrorMsg, int

And then call it like this:

// Allocate a StringBuilder.
StringBuilder pobjErrorMsg = new StringBuilder(128);

// Now make the call.
hhpGetErrorMessage(pobjErrorMsg, pobjErrorMsg.Capacity);

Hope this helps.



mo A

Hi Nicholas,,

That worked great,, but i had to change CharSet to CharSet.Ansi, cause

gave me some junk..

i really appreciate it.

take care


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