<StackPanel>
    <TextBox x:Name="textBox" TextChanged="TextBox_TextChanged" />
    <CheckBox x:Name="checkBox" IsChecked="{Binding ElementName=textBox, Path=Text.Length, Converter={StaticResource TextLengthToBooleanConverter}}" Content="选中复选框" />
</StackPanel>

public class TextLengthToBooleanConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        int textLength = (int)value;
        return textLength > 0;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

private void TextBox_TextChanged(object sender, TextChangedEventArgs e)
{
    // 强制更新复选框的绑定
    BindingExpression bindingExpression = checkBox.GetBindingExpression(CheckBox.IsCheckedProperty);
    bindingExpression.UpdateTarget();
}