解决.NET MAUI数字输入框的Format Exception异常问题

场景复现 #

我在.NET MAUI应用中使用了如下配置的Entry控件：

<Entry Text="{Binding EntryInput}" Placeholder="{Binding PlaceholderString}" Keyboard="{Binding KeyboardSetting}" Margin="5, 0, 5, 15" x:Name="entryControl"/>

根据输入类型，我会将KeyboardSetting设为Numeric或默认值。对应的可绑定属性定义如下：

public static readonly BindableProperty EntryInputProperty = BindableProperty.Create(nameof(EntryInput), typeof(string), typeof(MyInputView), default(string), BindingMode.TwoWay);
...
public string EntryInput
{
    get => (string)GetValue(EntryInputProperty);
    set => SetValue(EntryInputProperty, value);
}

我希望视图加载时将Entry文本清空为string.Empty（即使是数字输入框也不设默认值“0”），但使用Numeric键盘时会抛出**System.FormatException: 'The input string '' was not in a correct format.'**异常。用户删除Entry中所有文本时也会触发此异常，还导致UI加载缓慢。

异常堆栈信息 #

at System.Number.ThrowOverflowOrFormatException(ParsingStatus status, ReadOnlySpan`1 value, TypeCode type)
   at System.Number.ParseDouble(ReadOnlySpan`1 value, NumberStyles styles, NumberFormatInfo info)
   at System.Double.Parse(String s, IFormatProvider provider)
   at System.Convert.ToDouble(String value, IFormatProvider provider)
   at System.String.System.IConvertible.ToDouble(IFormatProvider provider)
   at System.Convert.ChangeType(Object value, Type conversionType, IFormatProvider provider)
   at Microsoft.Maui.Controls.BindingExpression.TryConvert(Object& value, BindableProperty targetProperty, Type convertTo, Boolean toTarget) in D:\a\_work\1\s\src\Controls\src\Core\BindingExpression.cs:line 464

解决方案 #

1. 自定义绑定转换器处理空值 #

编写转换器拦截空字符串的转换逻辑，避免触发格式异常：

public class NumericStringConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        // 从ViewModel向Entry转换，空值直接返回string.Empty
        return value?.ToString() ?? string.Empty;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        // 从Entry向ViewModel转换，空值直接返回string.Empty
        var input = value as string;
        return string.IsNullOrEmpty(input) ? string.Empty : input;
    }
}

在XAML中引用并使用转换器：

<ContentPage.Resources>
    <local:NumericStringConverter x:Key="NumericStringConverter"/>
</ContentPage.Resources>

<Entry Text="{Binding EntryInput, Converter={StaticResource NumericStringConverter}}" 
       Placeholder="{Binding PlaceholderString}" 
       Keyboard="{Binding KeyboardSetting}" 
       Margin="5, 0, 5, 15" 
       x:Name="entryControl"/>

2. 调整可绑定属性配置 #

修改可绑定属性的默认值并添加验证逻辑，确保空值合法：

public static readonly BindableProperty EntryInputProperty = BindableProperty.Create(
    nameof(EntryInput), 
    typeof(string), 
    typeof(MyInputView), 
    string.Empty, // 将默认值设为string.Empty而非null
    BindingMode.TwoWay,
    validateValue: (bindable, value) => 
    {
        // 允许空字符串或合法数字格式的字符串
        var strValue = value as string;
        return string.IsNullOrEmpty(strValue) || double.TryParse(strValue, out _);
    });

3. 拦截Entry文本变更事件 #

在代码后台手动处理空值场景，跳过绑定的自动转换逻辑：

entryControl.TextChanged += (sender, e) =>
{
    if (string.IsNullOrEmpty(e.NewTextValue) && entryControl.Keyboard == Keyboard.Numeric)
    {
        var viewModel = BindingContext as MyInputViewModel;
        viewModel?.EntryInput = string.Empty;
        // 锁定UI文本，避免循环触发更新
        entryControl.Text = string.Empty;
    }
};

4. 优化UI加载性能 #

• 视图加载时先设置KeyboardSetting为默认值，再设置EntryInput为string.Empty，最后切换到Numeric键盘，减少绑定触发次数
• 确保ViewModel的INotifyPropertyChanged只在值真正变化时触发，避免不必要的UI更新
• 简化绑定转换逻辑，避免在转换器中执行复杂操作

内容的提问来源于stack exchange，提问作者OXO