C# Definition:

       /// <summary>
    /// contains information about the current state of both physical and virtual memory, including extended memory
     /// <summary>
    /// The MEMORYSTATUS structure contains information about the current state of both physical and virtual memory.
    /// </summary>
    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
    public class MEMORYSTATUSEX
    public struct MEMORYSTATUS
    {
        /// <summary>
        /// Size of the structure, in bytes. You must set this member before calling GlobalMemoryStatusEx.
        /// Size of the MEMORYSTATUS data structure, in bytes. You do not need to set this member before calling the GlobalMemoryStatus function; the function sets it.
        /// </summary>
        public uint dwLength;

        /// <summary>
        /// Number between 0 and 100 that specifies the approximate percentage of physical memory that is in use (0 indicates no memory use and 100 indicates full memory use).
        /// Windows NT:  Percentage of approximately the last 1000 pages of physical memory that is in use.
        /// </summary>
        public uint dwMemoryLoad;

        /// <summary>
        /// Total size of physical memory, in bytes.
        /// Total size of physical memory, in bytes.
        /// </summary>
        public ulong ullTotalPhys;
        public uint dwTotalPhys;

        /// <summary>
        /// Size of physical memory available, in bytes.
        /// Size of physical memory available, in bytes
        /// </summary>
        public ulong ullAvailPhys;
        public uint dwAvailPhys;

        /// <summary>
        /// Size of the committed memory limit, in bytes. This is physical memory plus the size of the page file, minus a small overhead.
        /// Size of the committed memory limit, in bytes.
        /// </summary>
        public ulong ullTotalPageFile;
        public uint dwTotalPageFile;

        /// <summary>
        /// Size of available memory to commit, in bytes. The limit is ullTotalPageFile.
        /// Size of available memory to commit, in bytes.
        /// </summary>
        public ulong ullAvailPageFile;
        public uint dwAvailPageFile;

        /// <summary>
        /// Total size of the user mode portion of the virtual address space of the calling process, in bytes.
        /// </summary>
        public ulong ullTotalVirtual;
        public uint dwTotalVirtual;

        /// <summary>
        /// Size of unreserved and uncommitted memory in the user mode portion of the virtual address space of the calling process, in bytes.
        /// </summary>
        public ulong ullAvailVirtual;
        public uint dwAvailVirtual;

        /// <summary>
        /// Size of unreserved and uncommitted memory in the extended portion of the virtual address space of the calling process, in bytes.
        /// </summary>
        public ulong ullAvailExtendedVirtual;
    } // class MEMORYSTATUS

VB Definition:

      ''' <summary>
      ''' The MEMORYSTATUS structure contains information about the current state of both physical and virtual memory.
      ''' </summary>
      <StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Auto)> _
      Public Class MEMORYSTATUS
        ''' <summary>
        ''' Size of available memory to commit, in bytes.
        ''' </summary>
        Public dwAvailPageFile As UInt32
        ''' <summary>
        ''' Size of physical memory available, in bytes
        ''' </summary>
        Public dwAvailPhys As UInt32
        ''' <summary>
        ''' Size of unreserved and uncommitted memory in the user mode portion of the virtual address space of the calling process, in bytes.
        ''' </summary>
        Public dwAvailVirtual As UInt32
        ''' <summary>
        ''' Size of the MEMORYSTATUS data structure, in bytes. You do not need to set this member before calling the GlobalMemoryStatus function; the function sets it.
        ''' </summary>
        Public dwLength As UInt32
        ''' <summary>
        ''' Number between 0 and 100 that specifies the approximate percentage of physical memory that is in use (0 indicates no memory use and 100 indicates full memory use).
        ''' Windows NT:  Percentage of approximately the last 1000 pages of physical memory that is in use.
        ''' </summary>
        Public dwMemoryLoad As UInt32
        ''' <summary>
        ''' Size of the committed memory limit, in bytes.
        ''' </summary>
        Public dwTotalPageFile As UInt32
        ''' <summary>
        ''' Total size of physical memory, in bytes.
        ''' </summary>
        Public dwTotalPhys As UInt32
        ''' <summary>
        ''' Total size of the user mode portion of the virtual address space of the calling process, in bytes.
        ''' </summary>
        Public dwTotalVirtual As UInt32
      End Class

        /// <summary>
        /// Initializes a new instance of the <see cref="T:MEMORYSTATUSEX"/> class.
        /// </summary>
        public MEMORYSTATUSEX()
        {
        this.dwLength = (uint)Marshal.SizeOf(typeof(NativeMethods.MEMORYSTATUSEX));
        }
       }

User-Defined Field Types:

None.

       //
       // Alternate, Structure Version.  This One Allows Correct Marshalling As A "ref" Parameter,
       // But You Have To Ensure The Value Of dwLength Gets Set Correctly Via A Wrapper Method.
       //
    /// <summary>Contains information about the current state of both physical and virtual
    /// memory, including extended memory. The GlobalMemoryStatusEx function stores
    /// information in this structure.</summary>
    /// <remarks>MEMORYSTATUSEX reflects the state of memory at the time of the call. It also
    /// reflects the size of the paging file at that time. The operating system can enlarge
    /// the paging file up to the maximum size set by the administrator.</remarks>
    [StructLayout( LayoutKind.Sequential )]
    public struct MEMORYSTATUSEX
    {
        /// <summary>The size of the structure, in bytes. You must set this member before
        /// calling GlobalMemoryStatusEx.</summary>
        public uint dwLength;
        /// <summary>A number between 0 and 100 that specifies the approximate percentage
        /// of physical memory that is in use (0 indicates no memory use and 100
        /// indicates full memory use).</summary>
        public uint dwMemoryLoad;
        /// <summary>The amount of actual physical memory, in bytes.</summary>
        public ulong ullTotalPhys;
        /// <summary>The amount of physical memory currently available, in bytes. This is the
        /// amount of physical memory that can be immediately reused without having to write
        /// its contents to disk first. It is the sum of the size of the standby, free, and
        /// zero lists.</summary>
        public ulong ullAvailPhys;
        /// <summary>The current committed memory limit for the system or the current process,
        /// whichever is smaller, in bytes. To get the system-wide committed memory limit,
        /// call GetPerformanceInfo.</summary>
        public ulong ullTotalPageFile;
        /// <summary>The maximum amount of memory the current process can commit, in bytes.
        /// This value is equal to or smaller than the system-wide available commit value.
        /// To calculate the system-wide available commit value, call GetPerformanceInfo
        /// and subtract the value of CommitTotal from the value of CommitLimit.</summary>
        public ulong ullAvailPageFile;
        /// <summary>The size of the user-mode portion of the virtual address space of the
        /// calling process, in bytes. This value depends on the type of process, the type
        /// of processor, and the configuration of the operating system. For example, this
        /// value is approximately 2 GB for most 32-bit processes on an x86 processor and
        /// approximately 3 GB for 32-bit processes that are large address aware running
        /// on a system with 4-gigabyte tuning enabled.</summary>
        public ulong ullTotalVirtual;
        /// <summary>The amount of unreserved and uncommitted memory currently in the user-mode
        /// portion of the virtual address space of the calling process, in bytes.</summary>
        public ulong ullAvailVirtual;
        /// <summary>Reserved. This value is always 0.</summary>
        public ulong ullAvailExtendedVirtual;
    }

Notes:

Also see MEMORYSTATUSEX, GlobalMemoryStatus, GlobalMemoryStatusEx.

VB Definition:

    <StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Auto)> _
    Public Class MEMORYSTATUSEX

        ''' <summary>
        ''' Initializes a new instance of the <see cref="T:MEMORYSTATUSEX" /> class.
        ''' </summary>
        Public Sub New()
        Me.dwLength = CType(Marshal.SizeOf(GetType(MEMORYSTATUSEX)), UInt32)
        End Sub
        ' Fields
        ''' <summary>
        ''' Size of the structure, in bytes. You must set this member before calling GlobalMemoryStatusEx.
        ''' </summary>
        Public dwLength As UInt32
        ''' <summary>
        ''' Number between 0 and 100 that specifies the approximate percentage of physical memory that is in use (0 indicates no memory use and 100 indicates full memory use).
        ''' </summary>
        Public dwMemoryLoad As UInt32
        ''' <summary>
        ''' Total size of physical memory, in bytes.
        ''' </summary>
        Public ullTotalPhys As UInt64
        ''' <summary>
        ''' Size of physical memory available, in bytes.
        ''' </summary>
        Public ullAvailPhys As UInt64
        ''' <summary>
        ''' Size of the committed memory limit, in bytes. This is physical memory plus the size of the page file, minus a small overhead.
        ''' </summary>
        Public ullTotalPageFile As UInt64
        ''' <summary>
        ''' Size of available memory to commit, in bytes. The limit is ullTotalPageFile.
        ''' </summary>
        Public ullAvailPageFile As UInt64
        ''' <summary>
        ''' Total size of the user mode portion of the virtual address space of the calling process, in bytes.
        ''' </summary>
        Public ullTotalVirtual As UInt64
        ''' <summary>
        ''' Size of unreserved and uncommitted memory in the user mode portion of the virtual address space of the calling process, in bytes.
        ''' </summary>
        Public ullAvailVirtual As UInt64
        ''' <summary>
        ''' Size of unreserved and uncommitted memory in the extended portion of the virtual address space of the calling process, in bytes.
        ''' </summary>
        Public ullAvailExtendedVirtual As UInt64
    End Class