Private Declare Auto Function SendMessage Lib "USER32" _
        (ByVal hWnd As IntPtr, _
         ByVal wMsg As Integer, ByVal wParam As Integer, _
         ByVal lParam As Integer) As Integer
 
    Private Const CB_SETDROPPEDWIDTH = &H160
 
    Public Sub SetComboWidth(ByVal cb As ComboBox)
        Dim width As Integer
 
        Dim g As Graphics = ComboBox1.CreateGraphics()
 
        For Each s As String In cb.Items
            Dim cw As Integer = g.MeasureString(s, cb.Font).Width
            If cw > width Then width = cw
        Next
 
        SendMessage(cb.Handle, CB_SETDROPPEDWIDTH, width, 0)
 
    End Sub
 
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles MyBase.Load
        ComboBox1.Items.Add("abc")
        ComboBox1.Items.Add("abcdef")
        ComboBox1.Items.Add("abcdefghijklm")
        ComboBox1.Items.Add("abcdefghijklmnopqrstuv")
        ComboBox1.Items.Add("abcdefghijklmnopqrstuvwxyzabc")
        SetComboWidth(ComboBox1)
    End Sub

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