if (tb1 == null || cb1 == null || cb2 == null || tb2 == null) return;
IsSelected="True"
((ComboBoxItem)cb1.SelectedItem).Content.ToString()
<Window x:Class="Converter1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:Converter1"
mc:Ignorable="d"
Title="Calculator" Height="120" Width="1000">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width= "*" />
<ColumnDefinition Width="*" />
<ColumnDefinition Width= "*" />
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<TextBox x:Name="tb1" Grid.Column="0" FontSize="75" TextChanged="tb1_TextChanged" />
<ComboBox x:Name="cb1" Grid.Column="1" FontSize="45" SelectionChanged="cb1_SelectionChanged">
<ComboBoxItem Content="Meter" IsSelected="True"/>
<ComboBoxItem Content="Centimeter"/>
<ComboBoxItem Content="Millimeter"/>
</ComboBox>
<ComboBox x:Name="cb2" Grid.Column="2" FontSize="45" SelectionChanged="cb2_SelectionChanged_1">
<ComboBoxItem Content="Meter" IsSelected="True"/>
<ComboBoxItem Content="Centimeter"/>
<ComboBoxItem Content="Millimeter"/>
</ComboBox>
<TextBox x:Name="tb2" Grid.Column="3" FontSize="75" TextChanged="tb2_TextChanged_1" />
</Grid>
</Window>