上次的Select能用，但有缺陷，当值改变时，没有引发一个属于EditForm的值改变事件，就是说还没有连接到表单，功能不完善。另外，像较为低层的input、select控件，其值只能接受string或其更低层的object，其他类型的结构如数字（如int、short）、日期等都需要进行转换为string赋给它，值改变了再转换后传回来，这个可以看看InputBase源码就明白了，地址：aspnetcore/src/Components/Web/src/Forms/InputBase.cs at main · dotnet/aspnetcore · GitHub

 所以我这个Select只接受Value为string类型的值，把转换工作移到外面来，因为放到内部转换不是很可行。这个看看原生的InputSelect就明白了，它根本只给了个ChildContent，像option这些东西需要自己另外弄，遭到了很多人的吐槽。暴露在外边，就没有转换这个任务了，但写代码的任务更多了。编写基础组件是为了更简便、清晰、实用，只是使用起来需要更多的知识技能，只要掌握了就好。

代码：

@implements IDisposable
<div class="d-md-flex"><label class="control-label mt-2" style="width:@LabelWidth ;text-align-last:justify;"><b>@Label</b></label><div style="width:@SelectWidth"><select value="@Value" @onchange="SelectedChanaged" class="form-select @cssDanger" disabled="@Disabled">@if (HasAll){<option value="@string.Empty">全部</option>}@foreach (var item in Source){<option value="@item.Value">@item.DisplayName</option>}</select></div>
</div>@code {[CascadingParameter]private EditContext? CascadedEditContext { get; set; }[Parameter]public required string Label { get; set; }[Parameter]public required string Value { get; set; }[Parameter]public string LabelWidth { get; set; } = "80";//Label 四个字的居多，80px刚好，2个字的为50px,三个字与四个字对齐，使用的是字符平均分布[Parameter]public string SelectWidth { get; set; } = "200";[Parameter]public required IEnumerable<ISelectItem> Source { get; set; }[Parameter]public EventCallback<string> ValueChanged { get; set; }[Parameter]public Expression<Func<string>>? ValueExpression { get; set; }[Parameter]public bool Disabled { get; set; }[Parameter]public bool HasAll { get; set; }public SelectBox(){_validationStateChangedHandler = OnValidateStateChanged;}private readonly EventHandler<ValidationStateChangedEventArgs> _validationStateChangedHandler;private bool _hasInitializedParameters;private void SelectedChanaged(ChangeEventArgs e){if (e.Value != null){var value = (string)e.Value;_ = ValueChanged.InvokeAsync(value);EditContext?.NotifyFieldChanged(FieldIdentifier);}}private string cssDanger = "";protected EditContext EditContext { get; set; } = default!;protected internal FieldIdentifier FieldIdentifier { get; set; }protected override void OnParametersSet(){base.OnParametersSet();if (!_hasInitializedParameters){if (ValueExpression == null){throw new InvalidOperationException($"SelectBox requires a value for the 'ValueExpression'" +$"parameter. Normally this is provided automatically when using 'bind-Value'.");}FieldIdentifier = FieldIdentifier.Create(ValueExpression);if (CascadedEditContext != null){EditContext = CascadedEditContext;EditContext.OnValidationStateChanged += _validationStateChangedHandler;}_hasInitializedParameters = true;}LabelWidth = LabelWidth.Contains("px") ? LabelWidth : LabelWidth + "px";SelectWidth = SelectWidth.Contains("px") ? SelectWidth : SelectWidth + "px";Label = GetLabel(Label);}private void OnValidateStateChanged(object? sender, ValidationStateChangedEventArgs eventArgs){if (EditContext is null){return;}if (EditContext.GetValidationMessages(FieldIdentifier).Any()){cssDanger = "border-danger";//border-danger 红色浅些，可以改为使用项目原生site.css 中的invalid。}else{cssDanger = "";}StateHasChanged();}private static string GetLabel(string label){if (string.IsNullOrWhiteSpace(label)){return string.Empty;}if (label.Contains('：')){return label;}return label + "：";}public void Dispose(){if (EditContext is not null){EditContext.OnValidationStateChanged -= _validationStateChangedHandler;}GC.SuppressFinalize(this);}
}

ISelectItem.cs

public interface ISelectItem
{string Value { get; set; }string DisplayName { get; }
}

SelectItem.cs（也可以使用其他继承了ISelectItem接口的类）

[Serializable]
public class SelectItem : ISelectItem
{private string? name;private IList<SelectItem>? childItems;public required string Value { get; set; }public string DisplayName{get => name == null ? Value : name;//如果DisplayName值未设置，显示Value值。set => name = value ?? throw new NullReferenceException("显示值不能为空！"); }//这个是为Select级联使用的public IList<SelectItem> ChildItems{get => childItems ??= new List<SelectItem>();set => childItems = value;}
}

外部转换工作示例：

public int Id { get; set; }
public string ID //使用这个属性绑定
{get => Id.ToString();set => Id = value == "" ? 0 : int.Parse(value);
}