c# - 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派生自基类的任何对象。我目前的代码如下；

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; }

    }

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; }
}

}

与 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'.

index.razor就是这个；

@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 还很陌生，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)组件的约束？

Answer 1

一种解决方法是在组件用法中显式添加 TypeParam；所以与原始问题中的示例代码相同，但将组件的 index.razor 实现更改如下；

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

这已被标记为功能请求，正如上面的@Vencovsky 所发现的那样，虽然底层 Blazor 类支持类型约束，但如果没有按照上面明确定义的类型，编译器无法强制执行约束。