Getting text position under mouse cursor?

To get the Insert position for the TextBox from the X, Y mouse coordinates you have to use several SendMessage API calls.

If you change the TextBox to a RichTextBox though, you can get it for free.  The caret will move as you drag over the RTB.  Try it with the small app below.  Drag the Label over the RTB and drop it anywhere in the Text.

~IM

Public Class Form1
    Inherits System.Windows.Forms.Form

#Region " Windows Form Designer generated code "

    Public Sub New()
        MyBase.New()

        'This call is required by the Windows Form Designer.
        InitializeComponent()

        'Add any initialization after the InitializeComponent() call

    End Sub

    'Form overrides dispose to clean up the component list.
    Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean)
        If disposing Then
            If Not (components Is Nothing) Then
                components.Dispose()
            End If
        End If
        MyBase.Dispose(disposing)
    End Sub

    'Required by the Windows Form Designer
    Private components As System.ComponentModel.IContainer

    'NOTE: The following procedure is required by the Windows Form Designer
    'It can be modified using the Windows Form Designer.  
    'Do not modify it using the code editor.
    Friend WithEvents Label1 As System.Windows.Forms.Label
    Friend WithEvents RichTextBox1 As System.Windows.Forms.RichTextBox
    <System.Diagnostics.DebuggerStepThrough()> Private Sub InitializeComponent()
        Me.Label1 = New System.Windows.Forms.Label
        Me.RichTextBox1 = New System.Windows.Forms.RichTextBox
        Me.SuspendLayout()
        '
        'Label1
        '
        Me.Label1.Location = New System.Drawing.Point(8, 8)
        Me.Label1.Name = "Label1"
        Me.Label1.Size = New System.Drawing.Size(272, 24)
        Me.Label1.TabIndex = 2
        Me.Label1.Text = "Label1"
        '
        'RichTextBox1
        '
        Me.RichTextBox1.AllowDrop = True
        Me.RichTextBox1.Location = New System.Drawing.Point(8, 40)
        Me.RichTextBox1.Name = "RichTextBox1"
        Me.RichTextBox1.Size = New System.Drawing.Size(272, 216)
        Me.RichTextBox1.TabIndex = 3
        Me.RichTextBox1.Text = "RichTextBox1"
        '
        'Form1
        '
        Me.AutoScaleBaseSize = New System.Drawing.Size(5, 13)
        Me.ClientSize = New System.Drawing.Size(288, 266)
        Me.Controls.Add(Me.RichTextBox1)
        Me.Controls.Add(Me.Label1)
        Me.Name = "Form1"
        Me.Text = "Form1"
        Me.ResumeLayout(False)

    End Sub

#End Region

    Private Sub Label1_MouseMove(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles Label1.MouseMove
        If e.Button = MouseButtons.Left Then
            Label1.DoDragDrop(Label1.Text, DragDropEffects.Copy Or DragDropEffects.Move)
        End If
    End Sub

    Private Sub RichTextBox1_DragEnter(ByVal sender As System.Object, ByVal e As System.Windows.Forms.DragEventArgs) Handles RichTextBox1.DragEnter
        If (e.Data.GetDataPresent(DataFormats.Text)) Then
            e.Effect = DragDropEffects.Copy
        Else
            e.Effect = DragDropEffects.None
        End If
    End Sub

    Private Sub RichTextBox1_DragDrop(ByVal sender As Object, ByVal e As System.Windows.Forms.DragEventArgs) Handles RichTextBox1.DragDrop
        If (e.Data.GetDataPresent(DataFormats.Text)) Then
            RichTextBox1.SelectedText = e.Data.GetData(DataFormats.Text)
        End If
    End Sub

End Class