C# Language extern


Example

The extern keyword is used to declare methods that are implemented externally. This can be used in conjunction with the DllImport attribute to call into unmanaged code using Interop services. which in this case it will come with static modifier

For Example:

using System.Runtime.InteropServices;
public class MyClass
{
    [DllImport("User32.dll")]
    private static extern int SetForegroundWindow(IntPtr point);

    public void ActivateProcessWindow(Process p)
    {
        SetForegroundWindow(p.MainWindowHandle);
    }
}

This uses the SetForegroundWindow method imported from the User32.dll library

This can also be used to define an external assembly alias. which let us to reference different versions of same components from single assembly.

To reference two assemblies with the same fully-qualified type names, an alias must be specified at a command prompt, as follows:

/r:GridV1=grid.dll
/r:GridV2=grid20.dll

This creates the external aliases GridV1 and GridV2. To use these aliases from within a program, reference them by using the extern keyword. For example:

extern alias GridV1;
extern alias GridV2;