c# - 如何在 Blazor 中动态设置 ValidationMessage.For 属性？

Question

本款Docs描述了如何显示验证消息。

<ValidationMessage For="() => Parameters.PropertyA"></ValidationMessage>      

如何动态设置 ValidationMessage.For 属性？
自 For类型为 Expression<Func<TValue>> ，我想传递一个 Func ，但这不会编译:

[Parameter]
public Func<string> PropertyLocator { get; set; }

<ValidationMessage For="PropertyLocator"></ValidationMessage>

这会编译，但不会正确解析验证消息

<ValidationMessage For="() => PropertyLocator"></ValidationMessage>

我还尝试使组件通用，以便它知道 Parameters类型:

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.AspNetCore.Components;

public partial class MyComponent<TParam>
{
    [Parameter]
    public TParam Parameters { get; set; }

    [Parameter]
    public Func<TReportParam, string> PropertyLocator { get; set; }
}


@using System.Linq.Expressions
@typeparam TParam
<ValidationMessage For="@((Expression<Func<string>>)(() => PropertyLocator(this.Parameters)))"></ValidationMessage>


<MyComponent TParam="MyParameters" Parameters="BindToSomeValue" PropertyLocator="(parameters) => parameters.PropertyA" />

但这会导致以下运行时异常:

Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer[100]Unhandled exception rendering component: The provided expression contains a InvocationExpression1 which is not supported.FieldIdentifier only supports simple member accessors (fields,properties) of an object. System.ArgumentException: The providedexpression contains a InvocationExpression1 which is not supported.FieldIdentifier only supports simple member accessors (fields,properties) of an object. atMicrosoft.AspNetCore.Components.Forms.FieldIdentifier.ParseAccessor[String](Expression`1accessor, Object& model, String& fieldName) atMicrosoft.AspNetCore.Components.Forms.FieldIdentifier.Create[String](Expression`1accessor) atMicrosoft.AspNetCore.Components.Forms.ValidationMessage`1[[System.String,System.Private.CoreLib, Version=5.0.0.0, Culture=neutral,PublicKeyToken=7cec85d7bea7798e]].OnParametersSet() atMicrosoft.AspNetCore.Components.ComponentBase.CallOnParametersSetAsync()atMicrosoft.AspNetCore.Components.ComponentBase.RunInitAndSetParametersAsync()

Answer 1

我创建了一个小示例页面。
模型使用DataAnnotations作为验证机制。

    public class DemoInputModel
    {
        [Required]
        public String PropertyOne { get; set; }

        [MinLength(2)]
        public String PropertyTwo { get; set; }

        [MaxLength(5)]
        public String PropertyThree { get; set; }
    }

在页面上，模型被初始化并设置为编辑上下文。我们有三个文本输入和一个选择框。选择框可用于切换验证消息。如果选择框的值被改变，一个新的表达式被分配给 ValidationMessage .

@using System.ComponentModel.DataAnnotations;
@using System.Linq.Expressions;

@page "/test"


<h1>ValidationMessageTest</h1>

<EditForm Model="_model">
    <DataAnnotationsValidator />

    <ValidationMessage For="ValidationResolver"></ValidationMessage>

    <InputText @bind-Value="_model.PropertyOne" />
    <InputText @bind-Value="_model.PropertyTwo" />
    <InputText @bind-Value="_model.PropertyThree" />

    <InputSelect @bind-Value="SelectedValidationProperty">
        <option value="1">1</option>
        <option value="2">2</option>
        <option value="3">3</option>
    </InputSelect>

    @*<ValidationSummary />*@

</EditForm>


@code {

    private DemoInputModel _model = new DemoInputModel
    {
        PropertyOne = "Test",
        PropertyTwo = "42",
        PropertyThree = "Math.PI",
    };

    private String _selectedValidationProperty;

    public String SelectedValidationProperty
    {
        get => _selectedValidationProperty;
        set
        {
            _selectedValidationProperty = value;
            ChangeValidator(value);
        }
    }

    public Expression<Func<String>> ValidationResolver { get; set; }

    protected override void OnInitialized()
    {
        SelectedValidationProperty = "1";
        base.OnInitialized();
    }

    public void ChangeValidator(String value)
    {
        switch (value)
        {
            case "1":
                ValidationResolver = () => _model.PropertyOne;
                break;
            case "2":
                ValidationResolver = () => _model.PropertyTwo;
                break;
            case "3":
                ValidationResolver = () => _model.PropertyThree;
                break;
            default:
                break;
        }
    }
}

你的意思是这样的吗？如果您的模型不只有字符串，就像示例中那样，它会变得稍微复杂一些。 “快速”解决方法可能是使用 Expression对于每种可能的类型。
在幕后，该表达式用于创建 FieldIdentifier . FieldIdentifier然后用于从 EditContext 中获取相应的属性/字段检查验证状态。因此，您在为表达式选择什么方面受到限制。错误消息 FieldIdentifier only supports simple member accessors (fields, properties) of an object 很好地说明了这一限制。