External methods

When a method declaration includes an extern modifier, that method is said to be an external method. External methods are implemented externally, typically using a language other than C#. Because an external method declaration provides no actual implementation, the method-body of an external method simply consists of a semicolon. An external method may not be generic.

The extern modifier is typically used in conjunction with a DllImport attribute (§17.5.1), allowing external methods to be implemented by DLLs (Dynamic Link Libraries). The execution environment may support other mechanisms whereby implementations of external methods can be provided.

When an external method includes a DllImport attribute, the method declaration must also include a static modifier. This example demonstrates the use of the extern modifier and the DllImport attribute:

using System.Text;
using System.Security.Permissions;
using System.Runtime.InteropServices;

class Path
{
[DllImport("kernel32", SetLastError=true)]
static extern bool CreateDirectory(string name, SecurityAttribute sa);

[DllImport("kernel32", SetLastError=true)]
static extern bool RemoveDirectory(string name);

[DllImport("kernel32", SetLastError=true)]
static extern int GetCurrentDirectory(int bufSize, StringBuilder buf);

[DllImport("kernel32", SetLastError=true)]
static extern bool SetCurrentDirectory(string name);
}