嘿，这个问题我之前帮不少开发者捋清楚过——完全可以实现，但得绕个小弯子：WebBrowser调用的是系统自带的「另存为」对话框，这玩意儿是Windows操作系统托管的，不是.NET原生控件，所以没法直接用WebBrowser本身的API去修改它的内容。不过我们可以用Windows API来拦截这个对话框，然后手动给它填路径和文件名。

下面给你一步步拆解具体实现：

核心思路 #

当你的代码触发下载弹出「另存为」对话框时，我们要做这几件事：

• 提前设置一个系统钩子，捕获对话框的创建事件
• 识别出这个对话框是「另存为」窗口
• 找到对话框里的文件名输入框，把我们想要的路径和文件名填进去
• （可选）自动点击「保存」按钮，全程不用用户动手

具体代码实现 #

1. 先引入必要的Windows API函数 #

我们需要几个API来帮忙：FindWindowEx找对话框里的输入框，SendMessage给输入框塞文本，SetWindowsHookEx设置钩子拦截对话框。先在你的窗体类里声明这些：

Imports System.Runtime.InteropServices

Public Class YourForm
    ' 声明需要用到的Windows API
    <DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)>
    Private Shared Function FindWindowEx(ByVal parentHandle As IntPtr, ByVal childAfter As IntPtr, ByVal lclassName As String, ByVal windowTitle As String) As IntPtr
    End Function

    <DllImport("user32.dll", CharSet:=CharSet.Auto, SetLastError:=True)>
    Private Shared Function SendMessage(ByVal hWnd As IntPtr, ByVal Msg As UInteger, ByVal wParam As IntPtr, ByVal lParam As String) As IntPtr
    End Function

    <DllImport("user32.dll", SetLastError:=True)>
    Private Shared Function SetWindowsHookEx(ByVal idHook As Integer, ByVal lpfn As HookProc, ByVal hMod As IntPtr, ByVal dwThreadId As UInteger) As IntPtr
    End Function

    <DllImport("user32.dll", SetLastError:=True)>
    Private Shared Function UnhookWindowsHookEx(ByVal hhk As IntPtr) As Boolean
    End Function

    <DllImport("user32.dll")>
    Private Shared Function CallNextHookEx(ByVal hhk As IntPtr, ByVal nCode As Integer, ByVal wParam As IntPtr, ByVal lParam As IntPtr) As IntPtr
    End Function

    ' 钩子回调的委托定义
    Private Delegate Function HookProc(ByVal nCode As Integer, ByVal wParam As IntPtr, ByVal lParam As IntPtr) As IntPtr

    ' 用到的常量
    Private Const WH_CBT As Integer = 5
    Private Const HCBT_ACTIVATE As Integer = 5
    Private Const WM_SETTEXT As UInteger = &HC
    Private Const BN_CLICKED As UInteger = &HF5&

    ' 钩子句柄和我们要设置的目标路径
    Private _hookHandle As IntPtr = IntPtr.Zero
    Private _targetSavePath As String = "C:\YourCustomFolder\YourCustomFileName.pdf" ' 换成你要的路径+文件名

2. 在触发下载前设置钩子 #

修改你的File_Download方法，先挂上钩子再触发下载，这样对话框弹出来时我们能立刻捕获到：

Private Sub File_Download()
    ' 先挂钩子，准备拦截即将弹出的另存为对话框
    _hookHandle = SetWindowsHookEx(WH_CBT, AddressOf CbtHookProc, Marshal.GetHINSTANCE(System.Reflection.Assembly.GetExecutingAssembly.GetModules()(0)), GetCurrentThreadId())
    
    ' 你的原有下载触发代码
    Dim _Elements As HtmlElementCollection = WebBrowser.Document.GetElementsByTagName("input")
    For Each _Element As HtmlElement In _Elements
        If _Element.GetAttribute("value").Contains("Download") Then
            _Element.InvokeMember("click")
        End If
    Next
End Sub

3. 钩子回调函数处理对话框 #

这个函数是核心，当对话框被激活时，我们识别它并修改内容：

Private Function CbtHookProc(ByVal nCode As Integer, ByVal wParam As IntPtr, ByVal lParam As IntPtr) As IntPtr
    If nCode = HCBT_ACTIVATE Then
        ' 获取对话框标题，确认是「另存为」窗口（注意不同语言系统标题不一样，比如英文是"Save As"）
        Dim windowTitle As String = GetWindowText(wParam)
        If windowTitle.Contains("另存为") Or windowTitle.Contains("Save As") Then
            ' 找到文件名输入框（类名是Edit，大部分Windows版本里ID是1148）
            Dim fileNameBox As IntPtr = FindWindowEx(wParam, IntPtr.Zero, "Edit", Nothing)
            If fileNameBox <> IntPtr.Zero Then
                ' 把我们预设的路径和文件名塞进去
                SendMessage(fileNameBox, WM_SETTEXT, IntPtr.Zero, _targetSavePath)
            End If

            ' 如果你想自动点击保存按钮，取消下面的注释（按钮文本根据系统语言调整）
            ' Dim saveButton As IntPtr = FindWindowEx(wParam, IntPtr.Zero, "Button", "保存(&S)")
            ' If saveButton <> IntPtr.Zero Then
            '     SendMessage(saveButton, BN_CLICKED, IntPtr.Zero, IntPtr.Zero)
            ' End If

            ' 处理完就卸载钩子，别影响其他系统对话框
            UnhookWindowsHookEx(_hookHandle)
            _hookHandle = IntPtr.Zero
        End If
    End If
    Return CallNextHookEx(_hookHandle, nCode, wParam, lParam)
End Function

' 补充获取窗口标题的辅助函数和API
<DllImport("user32.dll", CharSet:=CharSet.Auto, SetLastError:=True)>
Private Shared Function GetWindowText(ByVal hWnd As IntPtr, ByVal lpString As StringBuilder, ByVal nMaxCount As Integer) As Integer
End Function

Private Shared Function GetWindowText(ByVal hWnd As IntPtr) As String
    Dim sb As New StringBuilder(256)
    GetWindowText(hWnd, sb, sb.Capacity)
    Return sb.ToString()
End Function

<DllImport("kernel32.dll")>
Private Shared Function GetCurrentThreadId() As UInteger
End Function

几个要注意的坑 #

• 系统语言适配：「另存为」的标题在不同语言系统里不一样，比如英文是"Save As"，中文是"另存为"，你得根据你的目标用户系统调整判断条件。
• 控件结构变化：不同Windows版本的对话框控件可能略有差异，比如文件名输入框的ID，你可以用Windows自带的Spy++工具（Visual Studio里能找到）去查看具体的控件信息。
• 钩子一定要卸载：处理完对话框后必须卸载钩子，不然会影响其他系统对话框的正常使用。
• 权限问题：确保你设置的保存路径有写入权限，比如不要写到C盘根目录（可能需要UAC权限），不然保存会失败。

内容的提问来源于stack exchange，提问作者Simos Sigma