C# [Out] [MarshalAs(..) 到 VB.NET

Question

我正在将 C# 代码转换为 VB.NET，下面这行让我头疼:

    int GetChannelCount(
        [Out] [MarshalAs(UnmanagedType.U4)] out UInt32 channelCount);

在线转换器不知道如何处理

[Out]

我理解“out”(即“ByRef”)，但我不知道“[Out]”的作用以及如何将其转换为 VB.NET。

谢谢。

ps:如果重要的话，这是完整的代码:

http://netcoreaudio.codeplex.com/SourceControl/latest#trunk/Code/CoreAudio/Interfaces/IAudioEndpointVolume.cs

[Guid("5CDF2C82-841E-4546-9722-0CF74078229A"),InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface IAudioEndpointVolume
{
    [PreserveSig]
    int NotImpl1();

    [PreserveSig]
    int NotImpl2();

    /// <summary>
    /// Gets a count of the channels in the audio stream.
    /// </summary>
    /// <param name="channelCount">The number of channels.</param>
    /// <returns>An HRESULT code indicating whether the operation passed of failed.</returns>
    [PreserveSig]
    int GetChannelCount(
        [Out] [MarshalAs(UnmanagedType.U4)] out UInt32 channelCount);

    /// <summary>
    /// Sets the master volume level of the audio stream, in decibels.
    /// </summary>
    /// <param name="level">The new master volume level in decibels.</param>
    /// <param name="eventContext">A user context value that is passed to the notification callback.</param>
    /// <returns>An HRESULT code indicating whether the operation passed of failed.</returns>
    [PreserveSig]
    int SetMasterVolumeLevel(
        [In] [MarshalAs(UnmanagedType.R4)] float level,
        [In] [MarshalAs(UnmanagedType.LPStruct)] Guid eventContext);

    /// <summary>
    /// Sets the master volume level, expressed as a normalized, audio-tapered value.
    /// </summary>
    /// <param name="level">The new master volume level expressed as a normalized value between 0.0 and 1.0.</param>
    /// <param name="eventContext">A user context value that is passed to the notification callback.</param>
    /// <returns>An HRESULT code indicating whether the operation passed of failed.</returns>
    [PreserveSig]
    int SetMasterVolumeLevelScalar(
        [In] [MarshalAs(UnmanagedType.R4)] float level,
        [In] [MarshalAs(UnmanagedType.LPStruct)] Guid eventContext);

    /// <summary>
    /// Gets the master volume level of the audio stream, in decibels.
    /// </summary>
    /// <param name="level">The volume level in decibels.</param>
    /// <returns>An HRESULT code indicating whether the operation passed of failed.</returns>
    [PreserveSig]
    int GetMasterVolumeLevel(
        [Out] [MarshalAs(UnmanagedType.R4)] out float level);

    /// <summary>
    /// Gets the master volume level, expressed as a normalized, audio-tapered value.
    /// </summary>
    /// <param name="level">The volume level expressed as a normalized value between 0.0 and 1.0.</param>
    /// <returns>An HRESULT code indicating whether the operation passed of failed.</returns>
    [PreserveSig]
    int GetMasterVolumeLevelScalar(
        [Out] [MarshalAs(UnmanagedType.R4)] out float level);

    /// <summary>
    /// Sets the volume level, in decibels, of the specified channel of the audio stream.
    /// </summary>
    /// <param name="channelNumber">The channel number.</param>
    /// <param name="level">The new volume level in decibels.</param>
    /// <param name="eventContext">A user context value that is passed to the notification callback.</param>
    /// <returns>An HRESULT code indicating whether the operation passed of failed.</returns>
    [PreserveSig]
    int SetChannelVolumeLevel(
        [In] [MarshalAs(UnmanagedType.U4)] UInt32 channelNumber,
        [In] [MarshalAs(UnmanagedType.R4)] float level,
        [In] [MarshalAs(UnmanagedType.LPStruct)] Guid eventContext);

    /// <summary>
    /// Sets the normalized, audio-tapered volume level of the specified channel in the audio stream.
    /// </summary>
    /// <param name="channelNumber">The channel number.</param>
    /// <param name="level">The new master volume level expressed as a normalized value between 0.0 and 1.0.</param>
    /// <param name="eventContext">A user context value that is passed to the notification callback.</param>
    /// <returns>An HRESULT code indicating whether the operation passed of failed.</returns>
    [PreserveSig]
    int SetChannelVolumeLevelScalar(
        [In] [MarshalAs(UnmanagedType.U4)] UInt32 channelNumber,
        [In] [MarshalAs(UnmanagedType.R4)] float level,
        [In] [MarshalAs(UnmanagedType.LPStruct)] Guid eventContext);

    /// <summary>
    /// Gets the volume level, in decibels, of the specified channel in the audio stream.
    /// </summary>
    /// <param name="channelNumber">The zero-based channel number.</param>
    /// <param name="level">The volume level in decibels.</param>
    /// <returns>An HRESULT code indicating whether the operation passed of failed.</returns>
    [PreserveSig]
    int GetChannelVolumeLevel(
        [In] [MarshalAs(UnmanagedType.U4)] UInt32 channelNumber,
        [Out] [MarshalAs(UnmanagedType.R4)] out float level);

    /// <summary>
    /// Gets the normalized, audio-tapered volume level of the specified channel of the audio stream.
    /// </summary>
    /// <param name="channelNumber">The zero-based channel number.</param>
    /// <param name="level">The volume level expressed as a normalized value between 0.0 and 1.0.</param>
    /// <returns>An HRESULT code indicating whether the operation passed of failed.</returns>
    [PreserveSig]
    int GetChannelVolumeLevelScalar(
        [In] [MarshalAs(UnmanagedType.U4)] UInt32 channelNumber,
        [Out] [MarshalAs(UnmanagedType.R4)] out float level);

    /// <summary>
    /// Sets the muting state of the audio stream.
    /// </summary>
    /// <param name="isMuted">True to mute the stream, or false to unmute the stream.</param>
    /// <param name="eventContext">A user context value that is passed to the notification callback.</param>
    /// <returns>An HRESULT code indicating whether the operation passed of failed.</returns>
    [PreserveSig]
    int SetMute(
        [In] [MarshalAs(UnmanagedType.Bool)] Boolean isMuted,
        [In] [MarshalAs(UnmanagedType.LPStruct)] Guid eventContext);

    /// <summary>
    /// Gets the muting state of the audio stream.
    /// </summary>
    /// <param name="isMuted">The muting state. True if the stream is muted, false otherwise.</param>
    /// <returns>An HRESULT code indicating whether the operation passed of failed.</returns>
    [PreserveSig]
    int GetMute(
        [Out] [MarshalAs(UnmanagedType.Bool)] out Boolean isMuted);

    /// <summary>
    /// Gets information about the current step in the volume range.
    /// </summary>
    /// <param name="step">The current zero-based step index.</param>
    /// <param name="stepCount">The total number of steps in the volume range.</param>
    /// <returns>An HRESULT code indicating whether the operation passed of failed.</returns>
    [PreserveSig]
    int GetVolumeStepInfo(
        [Out] [MarshalAs(UnmanagedType.U4)] out UInt32 step,
        [Out] [MarshalAs(UnmanagedType.U4)] out UInt32 stepCount);

    /// <summary>
    /// Increases the volume level by one step.
    /// </summary>
    /// <param name="eventContext">A user context value that is passed to the notification callback.</param>
    /// <returns>An HRESULT code indicating whether the operation passed of failed.</returns>
    [PreserveSig]
    int VolumeStepUp(
        [In] [MarshalAs(UnmanagedType.LPStruct)] Guid eventContext);

    /// <summary>
    /// Decreases the volume level by one step.
    /// </summary>
    /// <param name="eventContext">A user context value that is passed to the notification callback.</param>
    /// <returns>An HRESULT code indicating whether the operation passed of failed.</returns>
    [PreserveSig]
    int VolumeStepDown(
        [In] [MarshalAs(UnmanagedType.LPStruct)] Guid eventContext);

    /// <summary>
    /// Queries the audio endpoint device for its hardware-supported functions.
    /// </summary>
    /// <param name="hardwareSupportMask">A hardware support mask that indicates the capabilities of the endpoint.</param>
    /// <returns>An HRESULT code indicating whether the operation passed of failed.</returns>
    [PreserveSig]
    int QueryHardwareSupport(
        [Out] [MarshalAs(UnmanagedType.U4)] out UInt32 hardwareSupportMask);

    /// <summary>
    /// Gets the volume range of the audio stream, in decibels.
    /// </summary>
    /// <param name="volumeMin">The minimum volume level in decibels.</param>
    /// <param name="volumeMax">The maximum volume level in decibels.</param>
    /// <param name="volumeStep">The volume increment level in decibels.</param>
    /// <returns>An HRESULT code indicating whether the operation passed of failed.</returns>
    [PreserveSig]
    int GetVolumeRange(
        [Out] [MarshalAs(UnmanagedType.R4)] out float volumeMin,
        [Out] [MarshalAs(UnmanagedType.R4)] out float volumeMax,
        [Out] [MarshalAs(UnmanagedType.R4)] out float volumeStep);
}

Answer 1

out 是一个修饰符，可以使参数通过 ref 传递，而无需像 C# 中的 ref 那样先进行初始化。它的目的是向传递的参数写入一个值，以便修改方向超出被调用的方法。考虑一些 .NET Framework 类型中的 TryParse() 方法。

string foo = "123";
int value;
if(int.TryParse(foo, out value)) // Returns true if it can be parsed in which case, value now holds the parsed value
{
    // Do something
}

VB.NET 没有与此等效的方法，因为它默认初始化变量，您只需像往常一样调用它 ByRef。在将它与平台调用一起使用的上下文中，就像您发布的代码所做的那样，您只需要调用 Out 属性，该属性对平台调用具有相同的效果，方法是从被调用的数据中编码数据方法调用方法。考虑以下代码:

<PreserveSig>
Function GetChannelCount(<Out, MarshalAs(UnmanagedType.U4)> ByRef channelCount As UInt32) As Integer

End Function

这实质上告诉运行时，您定义的 ByRef 参数应该被视为输出参数，也就是说允许被调用的方法写入它而无需先对其进行初始化。