Libcurl error in a Windows Forms Application in Visual C++ 2010 express edition - libcurl

Question

I have used libcurl in win32 console application and it is working fine. I have followed instruction from here. http://theetrain.ca/tech/installing-curl-using-visual-studio-2010-beginners-guide/. Now I want to use Lib curl in Windows Forms Application. But it is showing some errors.
1>Read website app.obj : error LNK2031: unable to generate p/invoke for "extern "C" void * __clrcall curl_easy_init(void)" (?curl_easy_init##$$J0YMPAXXZ); calling convention missing in metadata
1>Read website app.obj : error LNK2028: unresolved token (0A000011) "extern "C" void * __clrcall curl_easy_init(void)" (?curl_easy_init##$$J0YMPAXXZ) referenced in function "private: void __clrcall Readwebsiteapp::Form1::button1_Click(class System::Object ^,class System::EventArgs ^)" (?button1_Click#Form1#Readwebsiteapp##$$FA$AAMXP$AAVObject#System##P$AAVEventArgs#4##Z)
1>Read website app.obj : error LNK2019: unresolved external symbol "extern "C" void * __clrcall curl_easy_init(void)" (?curl_easy_init##$$J0YMPAXXZ) referenced in function "private: void __clrcall Readwebsiteapp::Form1::button1_Click(class System::Object ^,class System::EventArgs ^)" (?button1_Click#Form1#Readwebsiteapp##$$FA$AAMXP$AAVObject#System##P$AAVEventArgs#4##Z)
1>C:\Documents and Settings\nemo1\my documents\visual studio 2010\Projects\Readwebsiteapp\Debug\Read website app.exe : fatal error LNK1120: 2 unresolved externals
Please give me some advise to solve this problem. Thank you. My code is:
#define CURL_STATICLIB
#include <stdio.h>
#include <curl/curl.h>
#include <curl/types.h>
#include <curl/easy.h>
#include <string.h>
using namespace std;
#pragma once
namespace Readwebsiteapp {
using namespace System;
using namespace System::ComponentModel;
using namespace System::Collections;
using namespace System::Windows::Forms;
using namespace System::Data;
using namespace System::Drawing;
/// <summary>
/// Summary for Form1
/// </summary>
public ref class Form1 : public System::Windows::Forms::Form
{
public:
Form1(void)
{
InitializeComponent();
//
//TODO: Add the constructor code here
//
}
protected:
/// <summary>
/// Clean up any resources being used.
/// </summary>
~Form1()
{
if (components)
{
delete components;
}
}
private: System::Windows::Forms::Button^ button1;
protected:
private: System::Windows::Forms::TextBox^ textBox1;
private:
/// <summary>
/// Required designer variable.
/// </summary>
System::ComponentModel::Container ^components;
#pragma region Windows Form Designer generated code
/// <summary>
/// Required method for Designer support - do not modify
/// the contents of this method with the code editor.
/// </summary>
void InitializeComponent(void)
{
this->button1 = (gcnew System::Windows::Forms::Button());
this->textBox1 = (gcnew System::Windows::Forms::TextBox());
this->SuspendLayout();
//
// button1
//
this->button1->Location = System::Drawing::Point(120, 70);
this->button1->Name = L"button1";
this->button1->Size = System::Drawing::Size(75, 23);
this->button1->TabIndex = 0;
this->button1->Text = L"button1";
this->button1->UseVisualStyleBackColor = true;
this->button1->Click += gcnew System::EventHandler(this, &Form1::button1_Click);
//
// textBox1
//
this->textBox1->Location = System::Drawing::Point(120, 153);
this->textBox1->Name = L"textBox1";
this->textBox1->Size = System::Drawing::Size(100, 20);
this->textBox1->TabIndex = 1;
//
// Form1
//
this->AutoScaleDimensions = System::Drawing::SizeF(6, 13);
this->AutoScaleMode = System::Windows::Forms::AutoScaleMode::Font;
this->ClientSize = System::Drawing::Size(420, 337);
this->Controls->Add(this->textBox1);
this->Controls->Add(this->button1);
this->Name = L"Form1";
this->Text = L"Form1";
this->ResumeLayout(false);
this->PerformLayout();
}
#pragma endregion
private: System::Void button1_Click(System::Object^ sender, System::EventArgs^ e)
{
textBox1->Text = "hello";
CURL *curl;
CURLcode result;
curl = curl_easy_init();
}
};
}

Answer 1

Since you're compiling managed C++, your native curl calls need to be marshaled through p/Invoke. It might be easiest to wrap these in flat "C" calls to make the marshaling simpler. In any case, I'd use the P/Invoke Interop Assistant tool (http://clrinterop.codeplex.com/releases/view/14120) to generate the necessary signatures.