diff --git a/WGneu/Utils.cs b/WGneu/Utils.cs
new file mode 100644
index 0000000..1827713
--- /dev/null
+++ b/WGneu/Utils.cs
@@ -0,0 +1,29 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Windows.Media;
+using System.Windows;
+
+namespace WGneu
+{
+    class Utils
+    {
+        public static IEnumerable<T> FindVisualChilds<T>(DependencyObject depObj) where T : DependencyObject
+        {
+            if (depObj == null)
+                yield return (T)Enumerable.Empty<T>();
+            for (int i = 0; i < VisualTreeHelper.GetChildrenCount(depObj); i++)
+            {
+                DependencyObject ithChild = VisualTreeHelper.GetChild(depObj, i);
+                if (ithChild == null)
+                    continue;
+                if (ithChild is T t)
+                    yield return t;
+                foreach (T childOfChild in FindVisualChilds<T>(ithChild))
+                    yield return childOfChild;
+            }
+        }
+    }
+}
diff --git a/WGneu/Validator.cs b/WGneu/Validator.cs
new file mode 100644
index 0000000..4906231
--- /dev/null
+++ b/WGneu/Validator.cs
@@ -0,0 +1,72 @@
+using System;
+using System.Collections.Generic;
+using System.Drawing;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Windows.Controls;
+
+namespace WGneu
+{
+    static class Validator
+    {
+        public static ValidationResult CheckNumericInput(TextBox input)
+        {
+            return CheckNumericInput(input, -1);
+        }
+
+        private static ValidationResult CheckNumericInput(TextBox input, int maxLen)
+        {
+            string text = "";
+            int pos = input.CaretIndex;
+            for (int i = 0; i < input.Text.Length; i++)
+            {
+                char ch = input.Text[i];
+                if (Char.IsDigit(ch))
+                    text += ch;
+                if (i == input.CaretIndex - 1)
+                    pos = text.Length;
+            }
+            input.Text = text;
+            input.CaretIndex = pos;
+
+            if (maxLen >= 0 && input.Text.Length > maxLen)
+            {
+                input.Text = input.Text.Substring(0, maxLen);
+                input.CaretIndex = Math.Min(pos, maxLen);
+            }
+
+            return new ValidationResult(true, "");
+        }
+
+        public static ValidationResult CheckLfbisNr(TextBox input)
+        {
+            var res = CheckNumericInput(input, 7);
+            if (!res.IsValid)
+                return res;
+            if (input.Text.Length == 0)
+                return new ValidationResult(true, "");
+            if (input.Text.Length != 7)
+                return new ValidationResult(false, "Betriebsnummer zu kurz");
+
+            // TODO
+
+            return new ValidationResult(true, "Not implemented yet");
+        }
+
+        public static ValidationResult CheckUstIdInput(TextBox input)
+        {
+            return new ValidationResult(false, "Not implemented yet");
+        }
+
+        public static void SetInputInvalid(TextBox input)
+        {
+            input.BorderBrush = System.Windows.Media.Brushes.Red;
+        }
+
+        public static void SetInputValid(TextBox input)
+        {
+            input.ClearValue(TextBox.BorderBrushProperty);
+        }
+    }
+}
diff --git a/WGneu/Windows/MemberListWindow.xaml b/WGneu/Windows/MemberListWindow.xaml
index 3071697..4d64170 100644
--- a/WGneu/Windows/MemberListWindow.xaml
+++ b/WGneu/Windows/MemberListWindow.xaml
@@ -171,6 +171,7 @@
 
                 <Label Content="BetriebsNr.:" HorizontalAlignment="Left" Margin="10,42,0,0" VerticalAlignment="Top" Padding="2"/>
                 <TextBox x:Name="LfbisNrInput" IsReadOnly="True"
+                         TextChanged="LfbisNrInput_TextChanged" LostFocus="LfbisNrInput_LostFocus"
                          Margin="0,40,10,0" VerticalAlignment="Top" FontSize="14" Padding="2" Grid.Column="1" Height="25" Width="64" HorizontalAlignment="Left"/>
 
                 <CheckBox x:Name="BuchführendInput" Content="Buchführend" IsEnabled="False"
diff --git a/WGneu/Windows/MemberListWindow.xaml.cs b/WGneu/Windows/MemberListWindow.xaml.cs
index d70cb02..5ec1060 100644
--- a/WGneu/Windows/MemberListWindow.xaml.cs
+++ b/WGneu/Windows/MemberListWindow.xaml.cs
@@ -294,26 +294,26 @@ namespace WGneu.Windows
 
         private void LockInputs()
         {
-            MgNrInput.IsReadOnly = true;
-            GivenNameInput.IsReadOnly = true;
-            FamilyNameInput.IsReadOnly = true;
-            AddressInput.IsReadOnly = true;
-            PlzInput.IsReadOnly = true;
-            OrtInput.IsEnabled = false;
-            BranchInput.IsEnabled = false;
-            DefaultKgInput.IsEnabled = false;
+            foreach (var tb in Utils.FindVisualChilds<TextBox>(this))
+                if (tb.Name != "SearchInput") tb.IsReadOnly = true;
+            foreach (var cb in Utils.FindVisualChilds<ComboBox>(this))
+                cb.IsEnabled = false;
+            foreach (var cb in Utils.FindVisualChilds<CheckBox>(this))
+                cb.IsEnabled = false;
+            foreach (var rb in Utils.FindVisualChilds<RadioButton>(this))
+                rb.IsEnabled = false;
         }
 
         private void UnlockInputs()
         {
-            MgNrInput.IsReadOnly = false;
-            GivenNameInput.IsReadOnly = false;
-            FamilyNameInput.IsReadOnly = false;
-            AddressInput.IsReadOnly = false;
-            PlzInput.IsReadOnly = false;
-            OrtInput.IsEnabled = true;
-            BranchInput.IsEnabled = true;
-            DefaultKgInput.IsEnabled = true;
+            foreach (var tb in Utils.FindVisualChilds<TextBox>(this))
+                if (tb.Name != "SearchInput") tb.IsReadOnly = false;
+            foreach (var cb in Utils.FindVisualChilds<ComboBox>(this))
+                cb.IsEnabled = true;
+            foreach (var cb in Utils.FindVisualChilds<CheckBox>(this))
+                cb.IsEnabled = true;
+            foreach (var rb in Utils.FindVisualChilds<RadioButton>(this))
+                rb.IsEnabled = true;
         }
 
         private void FillInputs(Member m)
@@ -341,14 +341,24 @@ namespace WGneu.Windows
 
         private void ClearInputs()
         {
-            MgNrInput.Text = "";
-            GivenNameInput.Text = "";
-            FamilyNameInput.Text = "";
-            BranchInput.SelectedItem = null;
-            PlzInput.Text = "";
-            OrtInput.SelectedItem = null;
-            AddressInput.Text = "";
-            DefaultKgInput.SelectedItem = null;
+            foreach (var tb in Utils.FindVisualChilds<TextBox>(this))
+                if (tb.Name != "SearchInput") tb.Text = "";
+            foreach (var cb in Utils.FindVisualChilds<ComboBox>(this))
+                cb.SelectedItem = null;
+        }
+
+        private void LfbisNrInput_TextChanged(object sender, RoutedEventArgs e)
+        {
+            var res = Validator.CheckLfbisNr(LfbisNrInput);
+            if (res.IsValid) Validator.SetInputValid(LfbisNrInput);
+            else Validator.SetInputInvalid(LfbisNrInput);
+        }
+
+        private void LfbisNrInput_LostFocus(object sender, RoutedEventArgs e)
+        {
+            var res = Validator.CheckLfbisNr(LfbisNrInput);
+            if (!res.IsValid)
+                MessageBox.Show(res.ErrorContent.ToString(), "Betriebsnummer ungültig", MessageBoxButton.OK, MessageBoxImage.Warning);
         }
     }
 }