Here, you will learn the steps for using a simple C++ DLL in C# in .NET Framework 4.

Open Visual C++ Win 32 Project.

Unmanaged C++ Dll call From Managed C# Application
In Application Settings, chose "DLL" and select "Empty Project".

Unmanaged C++ Dll call From Managed C# Application
The solution will look like the following.

Unmanaged C++ Dll call From Managed C# Application
Add New Item (*.cpp) in the Source folder.

Unmanaged C++ Dll call From Managed C# Application
Select the C++ (.cpp) File type.

Unmanaged C++ Dll call From Managed C# Application
Write down the following simple code in this .cpp file.
  1. #include < stdio.h >
  3. extern "C" {
  4. __declspec(dllexport) int add(int a, int b) {
  5. return a + b;
  6. }
  7. __declspec(dllexport) int subtract(int a, int b) {
  8. return a - b;
  9. }
  10. }

Now, export *.res file in the "Resource" folder for version information.

Unmanaged C++ Dll call From Managed C# Application
Now, the Solution Explorer will look like the following.

Unmanaged C++ Dll call From Managed C# Application
After compiling, you will see the Debug folder as below.

Unmanaged C++ Dll call From Managed C# Application
Now, our Unmanaged DLL is ready. Let us build a Managed C# App.

Unmanaged C++ Dll call From Managed C# Application
The design is like this.

Unmanaged C++ Dll call From Managed C# Application
The code is as follows.
  1. [DllImport("OurDll.dll", CallingConvention = CallingConvention.Cdecl)]
  2. publicstaticexternint subtract(int a, int b);
  4. privatevoid button2_Click(object sender, EventArgs e) {
  5. int x = Convert.ToInt32(textBox1.Text);
  6. int y = Convert.ToInt32(textBox2.Text);
  7. int z = subtract(x, y);
  8. MessageBox.Show("Required Answer is " + Convert.ToString(z), "Answer", MessageBoxButtons.OK, MessageBoxIcon.Information);
  9. }
The output will be like this.
Unmanaged C++ Dll call From Managed C# Application
In case of the previous version of .NET (3.5), the code will be like below.

  1. [DllImport("OurDll.dll")]
  2. publicstaticexternint subtract(int a, int b);
  4. privatevoid button2_Click(object sender, EventArgs e) {
  5. int x = Convert.ToInt32(textBox1.Text);
  6. int y = Convert.ToInt32(textBox2.Text);
  7. int z = subtract(x, y);
  8. MessageBox.Show("Required Answer is " + Convert.ToString(z), "Answer", MessageBoxButtons.OK, MessageBoxIcon.Information);
  9. }

The output will be the same for both.


Explanation

extern "C" helps to show all code within brackets from outside.

__declspec(dllexport) int add(int a,int b) is a prefix which makes DLL functions available from your external application.

In .NET Framework 3.5 or the previous version, the code is like -

  1. [DllImport("OurDll.dll")]
  2. publicstaticexternint add(int a, int b);
For ..NET 4 and above, the code will be liek this.
  1. [DllImport("OurDll.dll", CallingConvention = CallingConvention.Cdecl)]
  2. publicstaticexternint add(int a,int b);
If you only write framework 3.5 [DllImport("OurDll.dll")], you get an exception -

"… the managed PInvoke signature does not match the unmanaged signature …"


Now, you can call a C++ dll from your C# application (.NET Framework 4)

Thank You!