Error Using - DLLIMPORT

[M Riaz Bashir]

Hi, I am facing error while using following:

    \[DllImport("user32.dll")\]
    static extern IntPtr SendMessage(IntPtr hWnd, uint Msg, long wParam, \[MarshalAs(UnmanagedType.LPStr)\] StringBuilder lParam);

error is : Error 1 The type or namespace name 'DllImport' could not be found (are you missing a using directive or an assembly reference?) D:\CS\READVALUES\READVALUES\READVALUES\Form1.cs 16 10 READVALUES