在 Blazor 组件中使用具有继承的泛型类型

Question

我正在尝试在 Blazor 中创建一个“通用列表”组件，并希望该组件能够接受从基类派生的任何对象。我的代码目前如下；

基类：

    public class Model
    {

        // PK for the record
        [Key]
        public int Id { get; set; }

        // holds the front-end name for the record
        [Required(ErrorMessage = "Name is required")]
        public string Name { get; set; } = "";

        // holds the date and time the record was created
        [Required(ErrorMessage = "Created Timestamp is required")]
        public DateTime Created { get; set; } = DateTime.Now;

        // holds the username of the person who created the record
        [Required(ErrorMessage = "Creating user is required")]
        public string CreatedBy { get; set; } = "";


        // holds the date and time the record was updated
        public DateTime Updated { get; set; } = new DateTime(0);

        // holds the username of the person who last updated the record
        public string UpdatedBy { get; set; } = "";

    }

派生类：

public class ModelDesc: Model
    {

        // holds the description of the stakeholder
        public string Description { get; set; }

    }

该组件定义如下； DisplayGenericList.razor：

@typeparam T 


<h3>DisplayGenericList</h3>

@foreach (T lpObj in ListItems)
{
    <span>@lpObj.Name, @lpObj.Id</span>
}

@code {


    [Parameter]
    /// <summary>
    /// Contains the list of items to be shown in the list
    /// </summary>
    public List<T> ListItems { get; set; }
}

}

使用DisplayGenericList.razor.cs如下；

    public partial class DisplayGenericList<T> where T:Model
    {
    }

仅此一项就可以编译，但是，当我尝试在页面上使用该组件时，出现以下错误；CS0314 The type 'T' cannot be used as type parameter 'T' in the generic type or method 'DisplayGenericList<T>'. There is no boxing conversion or type parameter conversion from 'T' to 'ProjectName.Data.Interfaces.Model'.

该index.razor仅仅是这一点;

@page "/"

@using ProjectName.Data.Models

@using ProjectName.Shared.Components.Generics

<h1>Hello, world!</h1>

Welcome to your new app.



<hr />

<DisplayGenericList ListItems="lItems" />


@code
{

    private List<ModelDesc> lItems = new List<ModelDesc>()
    {
        new ModelDesc() {Name = "Item 1", Description = "Description 1", Created = DateTime.Now, CreatedBy = "User 1"},
        new ModelDesc() {Name = "Item 2", Description = "Description 2", Created = DateTime.Now, CreatedBy = "User 1"},
        new ModelDesc() {Name = "Item 3", Description = "Description 3", Created = DateTime.Now, CreatedBy = "User 2"},
        new ModelDesc() {Name = "Item 4", Description = "Description 4", Created = DateTime.Now, CreatedBy = "User 2"},
        new ModelDesc() {Name = "Item 5", Description = "Description 5", Created = DateTime.Now, CreatedBy = "User 3"}
    };

}

我对 Blazor 相当陌生，所以我怀疑我做错了什么，但是谁能建议我在这里应该做什么来强制（和使用）对组件的约束？

Answer 1

解决方法是在组件用法中显式添加 TypeParam；所以与原始问题示例中的代码相同，但更改组件的 index.razor 实现如下；

<DisplayGenericList ListItems="lItems" T="ModelDesc" />

这已被标记为功能请求，正如上面的@Vencovsky 所发现的，虽然底层 Blazor 类支持类型约束，但编译器无法在没有按照上述方式明确定义类型的情况下强制执行约束。