You can apply this attribute to classes or structures.

The common language runtime controls the physical layout of the data fields of a class or structure in managed memory. However, if you want to pass the type to unmanaged code, you can use the StructLayoutAttribute attribute to control the unmanaged layout of the type. Use the attribute with LayoutKind.Sequential to force the members to be laid out sequentially in the order they appear. For blittable types, LayoutKind.Sequential controls both the layout in managed memory and the layout in unmanaged memory. For non-blittable types, it controls the layout when the class or structure is marshaled to unmanaged code, but does not control the layout in managed memory. Use the attribute with LayoutKind.Explicit to control the precise position of each data member. This affects both managed and unmanaged layout, for both blittable and non-blittable types. Using LayoutKind.Explicit requires that you use the FieldOffsetAttribute attribute to indicate the position of each field within the type.

C#, Visual Basic, and C++ compilers apply the Sequential layout value to structures by default. For classes, you must apply the LayoutKind.Sequential value explicitly. The Tlbimp.exe (Type Library Importer) also applies the StructLayoutAttribute attribute; it always applies the LayoutKind.Sequential value when it imports a type library.

The following example demonstrates a managed declaration of the GetSystemTime function and defines MySystemTime class with LayoutKind.Explicit layout. GetSystemTime gets the system time and prints to the console.

using System;
using System.Runtime.InteropServices;

namespace InteropSample
{   

   [StructLayout(LayoutKind.Explicit, Size=16, CharSet=CharSet.Ansi)]
   public class MySystemTime 
   {
      [FieldOffset(0)]public ushort wYear; 
      [FieldOffset(2)]public ushort wMonth;
      [FieldOffset(4)]public ushort wDayOfWeek; 
      [FieldOffset(6)]public ushort wDay; 
      [FieldOffset(8)]public ushort wHour; 
      [FieldOffset(10)]public ushort wMinute; 
      [FieldOffset(12)]public ushort wSecond; 
      [FieldOffset(14)]public ushort wMilliseconds; 
   }

   class LibWrapper
   {
      [DllImport("kernel32.dll")]
      public static extern void GetSystemTime([MarshalAs(UnmanagedType.LPStruct)]MySystemTime st);
   };

   class TestApplication
   {      
      public static void Main()
      {
         try
         {
            MySystemTime sysTime = new MySystemTime();
            LibWrapper.GetSystemTime(sysTime);
            Console.WriteLine("The System time is {0}/{1}/{2} {3}:{4}:{5}", sysTime.wDay,
               sysTime.wMonth, sysTime.wYear, sysTime.wHour, sysTime.wMinute, sysTime.wSecond);            
         }         
         catch(TypeLoadException e)
         {
            Console.WriteLine("TypeLoadException : " + e.Message);
         }
         catch(Exception e)
         {
            Console.WriteLine("Exception : " + e.Message);
         }
      }
   }
}

.NET Framework

.NET Framework Client Profile

Portable Class Library

Windows 8 Release Preview, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 SP2, Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

Reference

Other Resources