Implementing APIs in C# is a tough job for beginners. Before implementing API you should know how to implement structure in C#, type conversion, safe/unsafe code, managed/unmanaged code and lots more.

Before implementing complex APIs we will start with simple MessageBox API. To implement code for the MessageBoxAPI open a new C# project and add one button. When button gets clicked the code will display a Message Box.

Since we are using external library, add a namespace:

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using System.Runtime.InteropServices;

Add the following lines to declare the API

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[DllImport("User32.dll")]
public static extern int MessageBox(int h, string m, string c, int type);

Here the DllImport attribute is used for calling the method from unmanaged code. "User32.dll" indicates the library name. The DllImport attribute specifies the dll location that contains the implementation of an extern method. Thestatic modifier is used to declare a static member, which belongs to the type itself rather than to a specific object,extern is used to indicate that the method is implemented externally. A method that is decorated with the DllImportattribute must have the extern modifier.

MessageBox is the function name, which returns int and takes 4 parameters as shown in declaration.

Many APIs use structures to pass and retrieve values, as it is less expensive. It also uses constant data type for passing constant data and simple data types for passing built-in data types as seen in the previous declaration of the MessageBox function.

Add following code for button click event:

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protected void button1_Click(object sender, System.EventArgs e)
{
	MessageBox (0,"API Message Box","API Demo",0);
}

Compile and run project, after clicking on the button you will see a MessageBox, which you called using API the function!!!

Using Structures

Working with APIs, which use complex structures, or structures inside structures, is somewhat more complex than using simple APIs. But once you understand the implementation then the whole API world is yours.

In next example we will use GetSystemInfo API which returns information about the current system.

The first step is open a new C# form and add one button on it. Go to the code window of the form and add a namespace:

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using System.Runtime.InteropServices;

Declare the structure, which is the parameter of GetSystemInfo.

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[StructLayout(LayoutKind.Sequential)]
public struct SYSTEM_INFO {
	public uint dwOemId;
	public uint dwPageSize;
	public uint lpMinimumApplicationAddress;
	public uint lpMaximumApplicationAddress;
	public uint dwActiveProcessorMask;
	public uint dwNumberOfProcessors;
	public uint dwProcessorType;
	public uint dwAllocationGranularity;
	public uint dwProcessorLevel;
	public uint dwProcessorRevision;
}

Declare the API function:

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[DllImport("kernel32")]
static extern void GetSystemInfo(ref SYSTEM_INFO pSI); 

Where ref is next to the method parameter keyword it causes a method to refer to the same variable that was passed into the method.

Add the following code in the button click event in which we first create a struct object and then pass it to function.

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protected void button1_Click (object sender, System.EventArgs e)
{
	try
	{
		SYSTEM_INFO pSI = new SYSTEM_INFO();
		GetSystemInfo(ref pSI);
		//

		//

		//

Once you retrieve the structure, perform operations on the required parameter

e.g.listBox1.InsertItem (0,pSI.dwActiveProcessorMask.ToString());:

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		//

		//

		//

	}
	catch(Exception er)
	{
		MessageBox.Show (er.Message);
	}
}