■No34644に追記(魔界の仮面弁士の記事)
> ただし、標準出力された内容の末尾が改行で無いと
> OutputDataReceived イベントが発生しない点にご注意ください。

Process.OutputDataReceived イベントの代わりに
StandardOutput.ReadAsync メソッドを使うようにしてみました。

これなら「PAUSE」や「TIME」にも対応できるかと思います。


Public Class Form1
    Private proc As Process
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        If Me.components Is Nothing Then
            Me.components = New System.ComponentModel.Container()
        End If
        Button1.Text = "開始"
        Button2.Text = "標準入力を渡す"
        Button2.Enabled = False
        'コマンド
        TextBox1.Text = Environment.GetEnvironmentVariable("ComSpec")
        '引数
        TextBox2.Text = ""
        '標準入力の内容
        TextBox3.Text = "DIR /D ""C:\Program Files (x86)\"""
        '標準出力 & 標準エラー出力
        RichTextBox1.ReadOnly = True
        RichTextBox1.BackColor = Color.Black
        RichTextBox1.ForeColor = Color.White
        RichTextBox1.ScrollBars = RichTextBoxScrollBars.Both
        RichTextBox1.WordWrap = False
        RichTextBox1.HideSelection = False
    End Sub
    Private Async Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Button1.Enabled = False
        TextBox1.ReadOnly = True
        TextBox2.ReadOnly = True
        RichTextBox1.Clear()
        Button2.Enabled = True
        proc = Process.Start(New ProcessStartInfo(TextBox1.Text, TextBox2.Text) With
        {
            .UseShellExecute = False,
            .CreateNoWindow = True,
            .RedirectStandardInput = True,
            .RedirectStandardOutput = True,
            .RedirectStandardError = True
        })
        Me.components.Add(proc)
        Await Task.WhenAll(
            ConsumeOutput(proc.StandardOutput, False),
            ConsumeOutput(proc.StandardError, True),
            Task.Run(AddressOf proc.WaitForExit)
        )
        proc.Close()
        Me.components.Remove(proc)
        TextBox1.ReadOnly = False
        TextBox2.ReadOnly = False
        Button2.Enabled = False
        Button1.Enabled = True
    End Sub
    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
        If proc IsNot Nothing Then
            proc.StandardInput.Write(TextBox3.Text & vbLf)
        Else
            Debug.WriteLine("既に閉じられている")
        End If
    End Sub
    Private Async Function ConsumeOutput(reader As System.IO.TextReader, isError As Boolean) As Task
        Dim buffer(2047) As Char
        Dim length As Integer = Await reader.ReadAsync(buffer, 0, buffer.Length)
        Dim foreColor = If(isError, Color.Yellow, Color.White)
        Dim backColor = If(isError, Color.Red, Color.Empty)
        While length > 0
            Dim message As New String(buffer, 0, length)
            Invoke(
                New MethodInvoker(
                   Sub()
                       Dim pos = RichTextBox1.TextLength
                       RichTextBox1.Select(pos, 0)
                       RichTextBox1.SelectedText = If(message, "")
                       RichTextBox1.Select(pos, length)
                       RichTextBox1.SelectionColor = foreColor
                       RichTextBox1.SelectionBackColor = backColor
                       RichTextBox1.SelectionStart = RichTextBox1.TextLength
                   End Sub
                )
            )
            length = Await reader.ReadAsync(buffer, 0, buffer.Length)
        End While
    End Function
End Class