For Each System.Windows.Forms.TextBox 
            If Not IsNumeric(TextBox1.Text) Then
                MessageBox.Show("Ungültiger Wert")
            End If
        Next

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Imports System.Runtime.CompilerServices
<Extension()> _
    Public Sub OnlyNumeric(ByVal T As TextBox, ByVal allow As Boolean)
        If T IsNot Nothing Then
            If allow Then
                AddHandler T.KeyDown, AddressOf m_DoubleKeyDown
            Else
                RemoveHandler T.KeyDown, AddressOf m_DoubleKeyDown
            End If
        End If
    End Sub
 
Private Sub m_DoubleKeyDown(ByVal sender As Object, ByVal e As _
  System.Windows.Forms.KeyEventArgs)
        'Navigation immer durchlassen...
        If IsNaviKeyCode(e.KeyCode) OrElse IsNumericKeyCode(e.KeyCode) Then _
          Exit Sub
 
        Beep()
        e.SuppressKeyPress = True
        e.Handled = True
    End Sub
 
Friend Function IsNaviKeyCode(ByVal KeyCode As System.Windows.Forms.Keys) As _
  Boolean
        Select Case KeyCode
            Case Keys.Back, Keys.Delete, Keys.Escape, Keys.Enter, Keys.Return
                Return True
            Case Keys.Up, Keys.Down, Keys.Left, Keys.Right, Keys.Home, Keys.End
                Return True
            Case Else
                Return False
        End Select
    End Function
 
    Friend Function IsNumericKeyCode(ByVal KeyCode As _
      System.Windows.Forms.Keys) As Boolean
        Select Case KeyCode
            Case Keys.D0 To Keys.D9
                Return True
            Case Keys.NumPad0 To Keys.NumPad9
                Return True
            Case Else
                Return False
        End Select
    End Function

 Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) _
   Handles Me.Load
        TextBox1.OnlyNumeric(True)
    End Sub

    Private Sub TextBox_Validating(ByVal sender As Object, ByVal e As _
      System.ComponentModel.CancelEventArgs) Handles TextBox1.Validating, _
      TextBox2.Validating, TextBox3.Validating
 
        Dim tb As TextBox = TryCast(sender, TextBox)
        If Not IsNumeric(tb.Text) Then
            MessageBox.Show("Ungültiger Wert")
        End If
 
    End Sub

Dim t() As TextBox = {txt_A, txt_B,txt_C}
 
        For Each box As TextBox In t
 
            box.BackColor = Color.Blue
 
            'Oder bei dir:
            If Not IsNumeric(box.Text) Then
 
                MsgBox("Dem geht nett")
 
            End If
 
        Next

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