How to use Interop Services in C# (Focus on external window)

Hello everyone, in this article we are going to talk about interoperability through interop services and how to use unmanaged code in C#. We are going to reach the code that does not CLR in C# and visual Studio.

Let's begin.

First I want to talk about what is interoperability.
We can use the libraries and the developed API's which developed with unmanaged code. So we can gain some time for our projects and we can take the advantages of the existing APIs. For example windows has own APIs. But most of them are not managed code or we can use another api with C or C++ programming languages or some ActiveX and COM components. In these cases we are going to need the Interop services to use them in our C# CLR programs.

Common Language Runtime has it own security security systems. When we try to access the external APIs with interop services it will bypass these security mechanism. So we need to be carefull when we use these external APIs.

In our example we are going to reach the some windows APIs to get the some values easly. We may write so many codes to do same things in our C# program.

We are going to find a window with its specified Title and focus it. After it we are going to send some keys and pressings in it.

First we need to import our library into namespace.


using System.Runtime.InteropServices;

And the we are define the prototype methods of the C language API that we are going to use. We are importing the library with DllImport method and the we defined function prototype from the imported API like below.

Be careful when you import the API's. Do not import them inside methods. Import them inside directly CLASSES.


[DllImport("user32.dll")]
private static extern IntPtr FindWindow(string lpClassName, string lpWindowName);

[DllImport("user32.dll")]
private static extern bool SetForegroundWindow(IntPtr hWnd);

Let's talk a little bit about above methods:

FindWindow It will find a window which has specified window title.
SetForegroundWindow It will focus on the specified window.

Now take a look at below code block...


public static  void StartAndWrite()
{
       IntPtr windowPointer = IntPtr.Zero;
       int processCount = 100;
       for (int i = 0; (i < processCount) && (windowPointer == IntPtr.Zero); i++)
       {
           windowPointer = FindWindow(null, "exampleNotepad.txt - Not Defteri");
       }
       if (windowPointer != IntPtr.Zero)
       {
           SetForegroundWindow(windowPointer);
           Thread.Sleep(500);
           SendKeys.SendWait("BurakHamdiTUFAN");
           SendKeys.SendWait(Environment.NewLine);
           SendKeys.SendWait("Thecodeprogram");
           SendKeys.SendWait("{ENTER}");
           SendKeys.Flush();
       }
}

Above code block FindWindow(null, "exampleNotepad.txt - Not Defteri"); will check the windows and assign the the specified window to the defined Pointer above. Then later we check the pointer and if it is not null with SetForegroundWindow(windowPointer); we will be focused on assigned window. Then we are going to send the texts into focused window.

Below image you can see output of the example:

Just make sure the application with specified title has been started. Because this method is looking for the running processes to be focused.

You can rach the example application on Github : https://github.com/thecodeprogram/InteropServices_Example

That is all in this article.

Have a good Interop Servicing.

Burak Hamdi TUFAN

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Author

Burak Hamdi TUFAN

I am a software developer experienced 15 years and here to share all my programming experiences. I have worked on so many platforms and programming languages especially C, C#, C++ and Java. I am studing PhD at Kocaeli University on Aviation Technologies. I am building softwares and technologies on aviation.

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