Form:-

Imports System.IO

Public Class slip20

    Dim path As String

    Private Sub FontStyleToolStripMenuItem_Click(sender As Object, e As EventArgs) Handles FontStyleToolStripMenuItem.Click

        FontDialog1.ShowDialog()

        RichTextBox1.Font = FontDialog1.Font

    End Sub

    Private Sub FontColorToolStripMenuItem_Click(sender As Object, e As EventArgs) Handles FontColorToolStripMenuItem.Click

        ColorDialog1.ShowDialog()

        RichTextBox1.ForeColor = ColorDialog1.Color

    End Sub

    Private Sub SaveToolStripMenuItem_Click(sender As Object, e As EventArgs) Handles SaveToolStripMenuItem.Click

        If DialogResult.OK = SaveFileDialog1.ShowDialog() Then   ' To check file name is valid or not

            path = SaveFileDialog1.FileName

            RichTextBox1.SaveFile(path, RichTextBoxStreamType.PlainText)

            MsgBox("File save successfully.........")

        End If

       End Sub

    Private Sub OpenToolStripMenuItem_Click(sender As Object, e As EventArgs) Handles OpenToolStripMenuItem.Click

        If DialogResult.OK = OpenFileDialog1.ShowDialog() Then   ' To check file name is valid or not

            path = OpenFileDialog1.FileName

            RichTextBox1.Text = (File.ReadAllText(path)).ToString

            MsgBox("File Open successfully.........")

        End If

    End Sub

End Class

Output:-

1) Take RichTextBox, MenuStrip, FontDialog, ColorDialog, OpenFileDialog, SaveFileDialog on         design form.

2) Set Menu on MenuStrip.

Theory:-

Imports System.IO - This class contains a File method which is used to read content from a file.

FontDialog1.ShowDialog() - It is use to Display fontDialog. Which helps to select particular font from dialogbox.

FontDialog1.Font - It returns selected Font.

 ColorDialog1.ShowDialog() - It is used to Display color Palette. Which helps to select a particular color from Palette.

ColorDialog1.Color - It returns selected color.

SaveFileDialog1.FileName - It returns filename with path.

RichTextBox1.SaveFile(path, RichTextBoxStreamType.PlainText) - This method save the file at given path and write a text into a file from RichTextBox.

(File.ReadAllText(path)).ToString - This File method is used to read text from a file in a byte. It needs to convert into the string.