c# - 泛型类型的显式和隐式转换

Question

我有那些类(class):

public class FilterBackOfficeDiscountFilterType
{
    public FilterBackOfficeDiscountFilterType();
    public FilterBackOfficeDiscountFilterType(string filterValue = null, string filterType = null);

    [JsonProperty(PropertyName = "filterType")]
    public string FilterType { get; set; }
    [JsonProperty(PropertyName = "filterValue")]
    public string FilterValue { get; set; }
}

还有我的:

 public abstract class Filter<T>
    where T : struct
{
    public T Type { get; set; }
    public string Value { get; set; }
}

和:

 public class DiscountFilter : Filter<DiscountFilterType>
{


}

我想以显式或隐式方式将 DiscountFilter 转换为 FilterBackOfficeDiscountFilterType。

所以我添加运算符方法:

 public class DiscountFilter : Filter<DiscountFilterType>
{
    public static implicit operator FilterBackOfficeDiscountFilterType(DiscountFilter filter)
        => new FilterBackOfficeDiscountFilterType(filterType: filter.Type.ToString(), filterValue: filter.Value);
}

但它不编译因为:

Cannot implicitly convert type 'Filter<DiscountFilterType>' to 'FilterBackOfficeDiscountFilterType'

我该怎么做？

DiscountFilterType 是一个枚举。

FilterCreator.Create 是:

public class DiscountFilterCreator : FilterCreator<DiscountFilterType>
{
    public override Filter<DiscountFilterType> Create(DiscountFilterType type, string value)
        => new DiscountFilter {Type = type, Value = string.IsNullOrWhiteSpace(value) ? null : value};
}

它来源于:

public abstract class FilterCreator<T>
    where T : struct
{
    public abstract Filter<T> Create(T type, string value);
}

最后一个 Filter 是:

public abstract class Filter<T>
    where T : struct
{
    public T Type { get; set; }
    public string Value { get; set; }
}

编辑要重现问题，您必须这样做

DiscountFilter filter = new DiscountFilterCreator().Create(DiscountFilterType.normal, "wow"); 

编辑

DiscountFilter x = this.FilterCreator.Create(DiscountFilterType.BrandId, brandId);
        FilterBackOfficeDiscountFilterType y = x;

编辑它有效:

    DiscountFilter x = (DiscountFilter)this.FilterCreator.Create(DiscountFilterType.BrandId, brandId);
        FilterBackOfficeDiscountFilterType y = x;

但我想这样做:

    FilterBackOfficeDiscountFilterType  x = this.FilterCreator.Create(DiscountFilterType.BrandId, brandId);

所以我也需要这个隐式转换:

   public class DiscountFilter : Filter<DiscountFilterType>
{
    public static implicit operator DiscountFilter(Filter<DiscountFilterType> filter)
        => new DiscountFilter {Value = filter.Value};
    public static implicit operator FilterBackOfficeDiscountFilterType(DiscountFilter filter)
        => new FilterBackOfficeDiscountFilterType(filterType: filter.Type.ToString(), filterValue: filter.Value);
}

但是不允许从派生类到基类的隐式转换!

DiscountFilter.implicit operator DiscountFilter(Filter<DiscountFilterType>): user-defined conversions to or from a base class are not allowed   

Answer 1

(隐式)用户定义的从基类到派生类的转换是一个坏主意，因此不受语言支持，如这里已经解释的:User-defined conversion operator from base class

作为您问题的解决方案，我建议按如下方式调整您的基本类型:

public abstract class FilterCreator<TFilter, TFilterType>
    where TFilter : Filter<TFilterType>
    where TFilterType : struct
{
    public abstract TFilter Create(TFilterType type, string value);
}

public abstract class Filter<T>
    where T : struct
{
    public T Type { get; set; }
    public string Value { get; set; }
}

然后您可以将 DiscountFilterCreator 实现为

public class DiscountFilterCreator : FilterCreator<DiscountFilter, DiscountFilterType>
{
    public override DiscountFilter Create(DiscountFilterType type, string value)
        => new DiscountFilter { Type = type, Value = string.IsNullOrWhiteSpace(value) ? null : value };
}

使用它，您将不再需要转换，因为返回的类型是预期的类型。

不确定这正是您想要的。按照 C# 的标准，代码似乎非常抽象，很可能有更好的设计来满足您的需求。

编辑:作为旁注，此实现呈现 FilterCreator几乎没用。 Filter<T>的用处似乎也很值得怀疑——你有没有声明过一个类型为Filter<T>的变量或泛型约束？ ？