C# Definition:

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 4)]
public struct MIXERLINECONTROLS
{
     private UInt32 cbStruct;       /* size in bytes of MIXERLINECONTROLS */
     private UInt32 dwLineID;       /* line id (from MIXERLINE.dwLineID) */
     private UInt32 dwControlID;    /* MIXER_GETLINECONTROLSF_ONEBYID */
//    private UInt32 dwControlType;  //UNIONED with dwControlID /* MIXER_GETLINECONTROLSF_ONEBYTYPE */
     private UInt32 cControls;      /* count of controls pmxctrl points to */
     private UInt32 cbmxctrl;       /* size in bytes of _one_ MIXERCONTROL */
     private IntPtr pamxctrl;       /* pointer to first MIXERCONTROL array */

     #region Properties

     /// <summary>size in bytes of MIXERLINECONTROLS</summary>
     public UInt32 StructSize
     {
        get { return this.cbStruct; }
        set { this.cbStruct = value; }
     }
     /// <summary>line id (from MIXERLINE.dwLineID)</summary>
     public UInt32 LineID
     {
        get { return this.dwLineID; }
        set { this.dwLineID = value; }
     }
     /// <summary>MIXER_GETLINECONTROLSF_ONEBYID</summary>
     public UInt32 ControlID
     {
        get { return this.dwControlID; }
        set { this.dwControlID = value; }
     }
     /// <summary>MIXER_GETLINECONTROLSF_ONEBYTYPE</summary>
     public MIXERCONTROL_CONTROLTYPE ControlType
     {
        get { return (MIXERCONTROL_CONTROLTYPE)this.dwControlID; }
        set { this.dwControlID = (uint)value; }
     }
     /// <summary>count of controls pmxctrl points to</summary>
     public UInt32 Controls
     {
        get { return this.cControls; }
        set { this.cControls = value; }
     }
     /// <summary>size in bytes of _one_ MIXERCONTROL</summary>
     public UInt32 MixerControlItemSize
     {
        get { return this.cbmxctrl; }
        set { this.cbmxctrl = value; }
     }
     /// <summary>pointer to first MIXERCONTROL array</summary>
     public IntPtr MixerControlArray
     {
        get { return this.pamxctrl; }
        set { this.pamxctrl = value; }
     }

     #endregion
}

VB Definition:

Structure MIXERLINECONTROLS
Public TODO
End Structure

User-Defined Field Types:

None.

THIS IS MY CONVERSION FROM THE C# ABOVE. I have used an UInt32 instead of the MIXERCONTROL_CONTROLTYPE enum as I didn't bother converting it. Also I set dwLineID as an Integer as I was getting a negative number come back from the APIs.

Notes:

The winmm dll may not execute properly on 64-bit systems. Consequently, the StructLayout must be Sequential, with CharSet = Ansi, and Pack = 4. This particular type has some special layout considerations (the c union keyword) which typically means developers would use the explicit layout. However, the explicit layout breaks down on 64-bit systems for any type with an IntPtr in the type. Consequently, the C# types have been redesigned to use Sequential layout so that they will automatically adjust to 64-bit systems. This means private fields with public properties which perform the gunky work of making it look like there is a union.

    <StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Ansi, Pack:=4)> _
    Public Structure MIXERLINECONTROLS
    Private cbStruct As UInt32       '/* size in bytes of MIXERLINECONTROLS */
    Private dwLineID As Integer       '/* line id (from MIXERLINE.dwLineID) */
    Private dwControlID As UInt32     '/* MIXER_GETLINECONTROLSF_ONEBYID */
    'private UInt32 dwControlType;  //UNIONED with dwControlID /* MIXER_GETLINECONTROLSF_ONEBYTYPE */
    Private cControls As UInt32      '/* count of controls pmxctrl points to */
    Private cbmxctrl As UInt32       '/* size in bytes of _one_ MIXERCONTROL */
    Private pamxctrl As IntPtr       '/* pointer to first MIXERCONTROL array */

Sample Code:

There is more code that is necessary to use the C# types listed above.

private const uint MIXERCONTROL_CT_CLASS_MASK   = 0xF0000000u;
private const uint MIXERCONTROL_CT_CLASS_CUSTOM = 0x00000000u;
private const uint MIXERCONTROL_CT_CLASS_METER  = 0x10000000u;
private const uint MIXERCONTROL_CT_CLASS_SWITCH = 0x20000000u;
private const uint MIXERCONTROL_CT_CLASS_NUMBER = 0x30000000u;
private const uint MIXERCONTROL_CT_CLASS_SLIDER = 0x40000000u;
private const uint MIXERCONTROL_CT_CLASS_FADER  = 0x50000000u;
private const uint MIXERCONTROL_CT_CLASS_TIME   = 0x60000000u;
private const uint MIXERCONTROL_CT_CLASS_LIST   = 0x70000000u;

private const uint MIXERCONTROL_CT_SUBCLASS_MASK = 0x0F000000u;

private const uint MIXERCONTROL_CT_SC_SWITCH_BOOLEAN = 0x00000000u;
private const uint MIXERCONTROL_CT_SC_SWITCH_BUTTON  = 0x01000000u;

private const uint MIXERCONTROL_CT_SC_METER_POLLED = 0x00000000u;

private const uint MIXERCONTROL_CT_SC_TIME_MICROSECS = 0x00000000u;
private const uint MIXERCONTROL_CT_SC_TIME_MILLISECS = 0x01000000u;

private const uint MIXERCONTROL_CT_SC_LIST_SINGLE   = 0x00000000u;
private const uint MIXERCONTROL_CT_SC_LIST_MULTIPLE = 0x01000000u;

private const uint MIXERCONTROL_CT_UNITS_MASK     = 0x00FF0000u;
private const uint MIXERCONTROL_CT_UNITS_CUSTOM   = 0x00000000u;
private const uint MIXERCONTROL_CT_UNITS_BOOLEAN  = 0x00010000u;
private const uint MIXERCONTROL_CT_UNITS_SIGNED   = 0x00020000u;
private const uint MIXERCONTROL_CT_UNITS_UNSIGNED = 0x00030000u;
private const uint MIXERCONTROL_CT_UNITS_DECIBELS = 0x00040000u; /* in 10ths */
private const uint MIXERCONTROL_CT_UNITS_PERCENT  = 0x00050000u; /* in 10ths */

/// <summary>Commonly used control types for specifying MIXERCONTROL.dwControlType</summary>
public enum MIXERCONTROL_CONTROLTYPE : uint
{
     CUSTOM     =(MIXERCONTROL_CT_CLASS_CUSTOM | MIXERCONTROL_CT_UNITS_CUSTOM),
     BOOLEANMETER   =(MIXERCONTROL_CT_CLASS_METER | MIXERCONTROL_CT_SC_METER_POLLED | MIXERCONTROL_CT_UNITS_BOOLEAN),
     SIGNEDMETER    =(MIXERCONTROL_CT_CLASS_METER | MIXERCONTROL_CT_SC_METER_POLLED | MIXERCONTROL_CT_UNITS_SIGNED),
     PEAKMETER      =(SIGNEDMETER + 1),
     UNSIGNEDMETER  =(MIXERCONTROL_CT_CLASS_METER | MIXERCONTROL_CT_SC_METER_POLLED | MIXERCONTROL_CT_UNITS_UNSIGNED),
     BOOLEAN    =(MIXERCONTROL_CT_CLASS_SWITCH | MIXERCONTROL_CT_SC_SWITCH_BOOLEAN | MIXERCONTROL_CT_UNITS_BOOLEAN),
     ONOFF      =(BOOLEAN + 1),
     MUTE       =(BOOLEAN + 2),
     MONO       =(BOOLEAN + 3),
     LOUDNESS       =(BOOLEAN + 4),
     STEREOENH      =(BOOLEAN + 5),
     BASS_BOOST     =(BOOLEAN + 0x00002277),
     BUTTON     =(MIXERCONTROL_CT_CLASS_SWITCH | MIXERCONTROL_CT_SC_SWITCH_BUTTON | MIXERCONTROL_CT_UNITS_BOOLEAN),
     DECIBELS       =(MIXERCONTROL_CT_CLASS_NUMBER | MIXERCONTROL_CT_UNITS_DECIBELS),
     SIGNED     =(MIXERCONTROL_CT_CLASS_NUMBER | MIXERCONTROL_CT_UNITS_SIGNED),
     UNSIGNED       =(MIXERCONTROL_CT_CLASS_NUMBER | MIXERCONTROL_CT_UNITS_UNSIGNED),
     PERCENT    =(MIXERCONTROL_CT_CLASS_NUMBER | MIXERCONTROL_CT_UNITS_PERCENT),
     SLIDER     =(MIXERCONTROL_CT_CLASS_SLIDER | MIXERCONTROL_CT_UNITS_SIGNED),
     PAN        =(SLIDER + 1),
     QSOUNDPAN      =(SLIDER + 2),
     FADER      =(MIXERCONTROL_CT_CLASS_FADER | MIXERCONTROL_CT_UNITS_UNSIGNED),
     VOLUME     =(FADER + 1),
     BASS       =(FADER + 2),
     TREBLE     =(FADER + 3),
     EQUALIZER      =(FADER + 4),
     SINGLESELECT   =(MIXERCONTROL_CT_CLASS_LIST | MIXERCONTROL_CT_SC_LIST_SINGLE | MIXERCONTROL_CT_UNITS_BOOLEAN),
     MUX        =(SINGLESELECT + 1),
     MULTIPLESELECT =(MIXERCONTROL_CT_CLASS_LIST | MIXERCONTROL_CT_SC_LIST_MULTIPLE | MIXERCONTROL_CT_UNITS_BOOLEAN),
     MIXER      =(MULTIPLESELECT + 1),
     MICROTIME      =(MIXERCONTROL_CT_CLASS_TIME | MIXERCONTROL_CT_SC_TIME_MICROSECS | MIXERCONTROL_CT_UNITS_UNSIGNED),
     MILLITIME      =(MIXERCONTROL_CT_CLASS_TIME | MIXERCONTROL_CT_SC_TIME_MILLISECS | MIXERCONTROL_CT_UNITS_UNSIGNED),
}

    '/// <summary>size in bytes of MIXERLINECONTROLS</summary>
    Public Property StructSize As UInt32
        Get
        Return cbStruct
        End Get
        Set(ByVal value As UInt32)
        cbStruct = value
        End Set
    End Property

    '/// <summary>line id (from MIXERLINE.dwLineID)</summary>
    Public Property LineID As Integer
        Get
        Return dwLineID
        End Get
        Set(ByVal value As Integer)
        dwLineID = value
        End Set
    End Property

    '/// <summary>MIXER_GETLINECONTROLSF_ONEBYID</summary>
    Public Property ControlID As UInt32
        Get
        Return dwControlID
        End Get
        Set(ByVal value As UInt32)
        dwControlID = value
        End Set
    End Property

    '/// <summary>MIXER_GETLINECONTROLSF_ONEBYTYPE</summary>
    Public Property ControlType As UInt32
        Get
        Return dwControlID
        End Get
        Set(ByVal value As UInt32)
        dwControlID = value
        End Set
    End Property

    '/// <summary>count of controls pmxctrl points to</summary>
    Public Property Controls As UInt32
        Get
        Return cControls
        End Get
        Set(ByVal value As UInt32)
        cControls = value
        End Set
    End Property

    '/// <summary>size in bytes of _one_ MIXERCONTROL</summary>
    Public Property MixerControlItemSize As UInt32
        Get
        Return cbmxctrl
        End Get
        Set(ByVal value As UInt32)
        cbmxctrl = value
        End Set
    End Property

    '/// <summary>pointer to first MIXERCONTROL array</summary>
    Public Property MixerControlArray As IntPtr
        Get
        Return pamxctrl
        End Get
        Set(ByVal value As IntPtr)
        pamxctrl = value
        End Set
    End Property

    End Structure