How to convertan LPTSTR (C++) variable into a C# string.

Question

2.00/5 (1 vote)

See more:

Hi everyone,
I have an issue with this conversion problem.
From a C# component I need to call a C++ Dll method:

C++

C#

extern "C"
    {
        __declspec(dllexport) bool getVal(LPTSTR& ab);
    }

In c# I define a string 'cd.

C#

C#

[DllImport("...dll")]


public static extern bool getVal(???);

private void Testbutton_Click(object sender, EventArgs e)
{
    string cd????


    getVal(??cd???)
);

Question:

How do I call, from C#, the method so that the string defined in the C++ LPTSTR variable is copied into the C# string variable?
I don't know how big the returned string can be.

** UPDATE**

This is the C++ method:

C++

C#

extern "C"
{
    __declspec(dllexport) bool getVal(LPTSTR& retVal);
}

this is how retVal is set:

C#

extern bool getVal(LPTSTR& retVal)
 {
.
.
.
    String^ val = sr->ReadLine();

    retVal = (LPTSTR)(Marshal::StringToHGlobalAnsi(val)).ToPointer();
.
.
.
    return true;
 }

I need help calling this method from C#.
Currently I have:

C#

C#

[DllImport("myLibrary.dll", CallingConvention = CallingConvention.Cdecl)]

public static extern bool getVal([MarshalAsAttribute(UnmanagedType.LPTStr)] out String retVal);

private void Testbutton_Click(object sender, EventArgs e)
{
    string myVal;


    getVal(out myVal)
);

what I get in myVal is garbage, like chinese chars (ANSI/Unicode issue??), not the value I can print out and read if, debugging the dll, I access the val C++ variable.
Maybe the conversion from String^ to LPTSTR is not correct?

Posted 20-Aug-15 6:48am

Pietro_In_Milan

Updated 22-Aug-15 9:08am

v3

Add a Solution

Comments

Philippe Mori 20-Aug-15 21:33pm

Follow the same pattern as Win32 API calls...

1 solution

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Sergey Alexandrovich Kryukov · Accepted Answer · 2015-08-20T07:12:00

By default, C string is marshaled as .NET string in P/Invoke. To be certain, you can use the attribute System.Runtime.InteropServices.MarshalAsAttribute:
https://msdn.microsoft.com/en-us/library/system.runtime.interopservices.marshalasattribute%28v=vs.110%29.aspx[^].

The attribute should be applied to separate method parameter, as the target AttributeTargets.Parameter is supported (please see the attribute declaration above).

Say, for LPTSRT, you need to use https://msdn.microsoft.com/en-us/library/6e0wtfh2%28v=vs.110%29.aspx[^];
System.Runtime.InteropServices.UnmanagedType.LPTStr:
https://msdn.microsoft.com/en-us/library/system.runtime.interopservices.unmanagedtype%28v=vs.110%29.aspx[^].

By the way, you don't have to use extern "C" just to have undercorated function name. Why not using default decorated (mangled) names directly (see https://en.wikipedia.org/wiki/Name_mangling[^])? You can find out exact decorated name using some binary dump utility, such as "dumpbin.exe" (https://msdn.microsoft.com/en-us/library/c1h23y6c.aspx[^], can be executed under the "Visual Studio Command Prompt") and use it directly in the EntryPoint named parameter (property) of System.Runtime.InteropServices.DllImportAttribute:
https://msdn.microsoft.com/en-us/library/Aa984739%28v=VS.71%29.aspx[^],
https://msdn.microsoft.com/en-us/library/system.runtime.interopservices.dllimportattribute%28v=vs.110%29.aspx[^],
https://msdn.microsoft.com/en-us/library/system.runtime.interopservices.dllimportattribute.entrypoint(v=vs.110).aspx[^].

—SA