Question

我有一个界面：

public interface IReminder<T> where T : class, IIdentifiableEntity
{
    IEnumerable<T> GetRemindersToBeSent(IRepository<T> repository);

}

和实现此接口的类TimesheetReminder：

public class TimesheetReminder : IReminder<InvoiceSummary>
{

    public IEnumerable<InvoiceSummary> GetRemindersToBeSent(IRepository<InvoiceSummary> invoiceSummaryRepository)
    {
        var today = DateTime.Today;
        return invoiceSummaryRepository.List.Where(inv =>
            inv.InvoiceSummaryStatus.CKAStatusName == "Draft" &&
            inv.InsertDateUTC <= today.AddDays(-3)            &&
            inv.InsertDateUTC >= today.AddDays(-6)            &&
            inv.EndDate       <= today.AddDays(-3)
        );
    }

InvoiceSummary实施IIdentifyableEntity，但

public static class ReminderFactory<T> where T : class, IIdentifiableEntity
{
    public static IReminder<T> GetReminder(string applicationType)
    {
        IReminder<T> reminder;

        switch (applicationType)
        {
            case "Invoicing":
                reminder = (IReminder<T>)new TimesheetReminder();
                break;
            default:
                reminder = null;
                break;
        }

        return reminder;
    }
}

开票案例返回null。

如果TimesheetReminder未实现IReminder的{{1}}我会理解，但确实如此。

我做错了什么？

Answer 1

什么是T？ TimeSheetReminder为IReminder<InvoiceSummary>，因此如果T不是InvoiceSummary，则无法进行参考转换：

class Foo: IIdentifiableEntity { ... }
var reminder = new TimesheetReminder() as IReminder<Foo>; //returns null

Answer 2

尝试以下....

IReminder<InvoiceSummary> reminder = new TimesheetReminder() as IReminder<InvoiceSummary>;

Answer 3

我认为您通过传递 IIdentifiableEntity 界面来创建工厂实例，如下所示：

var factory = ReminderFactory<IIdentifiableEntity>.GetReminder("Invoicing");

即使我们输入强制转换

，也会始终返回 null

reminder = new TimesheetReminder() as IReminder<T>;

<强>解决方案：

在创建工厂实例时，我们需要传递 InvoiceSummary

的具体类类型

var factory = ReminderFactory<InvoiceSummary>.GetReminder("Invoicing");

即使我们可以通过删除硬编码字符串来修改创建工厂实例＆＃34; Invoicing＆＃34;

var factory = ReminderFactory.Create<InvoiceSummary>();

示例代码如下所示：

public class Program
{
    static void Main(string[] args)
    {
        //Hp --> Logic: Create factory instance by passing concrete class type.
        var factory = ReminderFactory.Create<InvoiceSummary>();
        var x = factory.GetRemindersToBeSent(new InvoiceRepository());
        x.ToList().ForEach(item =>
        Console.WriteLine($"{item.EntityName}:{item.TotalAmount}"));

        Console.ReadKey();
    }
}

public interface IIdentifiableEntity
{
    string EntityName { get; set; }
}

public static class ReminderFactory
{
    public static IReminder<T> Create<T>() where T : IIdentifiableEntity
    {
        IReminder<T> reminder = null;
        if (typeof(InvoiceSummary) == typeof(T))
        {
            reminder = new TimesheetReminder() as IReminder<T>;
        }

        return reminder;
    }
}

public interface IReminder<T> where T : IIdentifiableEntity
{
    IEnumerable<T> GetRemindersToBeSent(IRepository<T> repository);
}

public interface IRepository<T>
{
    IEnumerable<T> List { get; }
}

public class InvoiceRepository : IRepository<InvoiceSummary>
{
    public IEnumerable<InvoiceSummary> List => new List<InvoiceSummary> {
        new InvoiceSummary { EntityName = "Invoice", TotalAmount = 100.00M } };
}

public class InvoiceSummary : IIdentifiableEntity
{
    public string EntityName { get; set; }
    public decimal TotalAmount { get; set; }
}

public class TimesheetReminder : IReminder<InvoiceSummary>
{
    public IEnumerable<InvoiceSummary> GetRemindersToBeSent(
        IRepository<InvoiceSummary> repository) =>
        repository.List.Where(I => IsEqual("Invoice", I.EntityName));

    private bool IsEqual(string source, string target) =>
       string.Equals(source, target, StringComparison.CurrentCultureIgnoreCase);
}

Answer 4

出于任何其他原因，如果您仍想在创建工厂实例时传递界面（ IIdentifiableEntity ）

var factory = ReminderFactory<IIdentifiableEntity>.GetReminder("Invoicing");

解决方案（概念证明）：我不喜欢，因为它设计不好。

我们需要将接口 （IReminder，IRepository） 更改为covaraint。
在阅读IIdentifiableEntity数据时，我们需要转换为相应的concreate类类型。

示例代码如下所示：请遵循代码注释

public class Program
{
    static void Main(string[] args)
    {
        //Hp --> Note: While creating factory instance we are passing interface type.
        var factory = ReminderFactory<IIdentifiableEntity>.GetReminder("Invoicing");
        var x = factory.GetRemindersToBeSent(new InvoiceRepository());

        //Hp --> Note: While reading data we need to cast to corrsponding concreate class type.
        x.Cast<InvoiceSummary>().ToList().ForEach(item =>
        Console.WriteLine($"{item.EntityName}:{item.TotalAmount}"));
        Console.ReadKey();
    }
}

public interface IRepository<out T>
{
    // Hp --> Note: You can't use setter since T it is out parameter (covariant)
    IEnumerable<T> List { get; }
}

public interface IReminder<out T> where T : class, IIdentifiableEntity
{
    //Hp --> Note: You can't use IRepository<T> here since T is out parameter (covariant)
    //Instead of T use interface IIdentifiableEntity
    IEnumerable<T> GetRemindersToBeSent(IRepository<IIdentifiableEntity> repository);
}

public class TimesheetReminder : IReminder<InvoiceSummary>
{
    public IEnumerable<InvoiceSummary> GetRemindersToBeSent(
        //Hp --> Note: We need to cast IIdentifiableEntity to corrsponding concreate class type.
        IRepository<IIdentifiableEntity> repository) =>
        repository.List.Where(I => IsEqual("Invoice", I.EntityName)).Cast<InvoiceSummary>();

    private bool IsEqual(string source, string target) =>
       string.Equals(source, target, StringComparison.CurrentCultureIgnoreCase);
}

public interface IIdentifiableEntity
{
    string EntityName { get; set; }
}

public static class ReminderFactory<T> where T : class, IIdentifiableEntity
{
    public static IReminder<T> GetReminder(string applicationType)
    {
        IReminder<T> reminder;
        switch (applicationType)
        {
            case "Invoicing":
                reminder = new TimesheetReminder() as IReminder<T>;
                break;
            default:
                reminder = null;
                break;
        }
        return reminder;
    }
}

public class InvoiceRepository : IRepository<InvoiceSummary>
{
    public IEnumerable<InvoiceSummary> List => new List<InvoiceSummary> {
        new InvoiceSummary { EntityName = "Invoice", TotalAmount = 100.00M } };
}

public class InvoiceSummary : IIdentifiableEntity
{
    public string EntityName { get; set; }
    public decimal TotalAmount { get; set; }
}

cast作为泛型接口返回null

4 个答案: