使用 C# 9 的records作为强类型ID – 路由和查询参数

使用 C# 9 的records作为强类型ID - 路由和查询参数

上一篇文章,我介绍了使用 C# 9 的record类型作为强类型id,非常简洁

public record ProductId(int Value);

但是在强类型id真正可用之前,还有一些问题需要解决,比如,ASP.NET Core并不知道如何在路由参数或查询字符串参数中正确的处理它们,在这篇文章中,我将展示如何解决这个问题。

路由和查询字符串参数的模型绑定

假设我们有一个这样的实体:

public record ProductId(int Value);

public class Product
{
    public ProductId Id { get; set; }
    public string Name { get; set; }
    public decimal UnitPrice { get; set; }
}

和这样的API接口:

[ApiController]
[Route("api/[controller]")]
public class ProductController : ControllerBase
{
    ...

    [HttpGet("{id}")]
    public ActionResult<Product> GetProduct(ProductId id)
    {
         return Ok(new Product { 
                Id = id,
                Name = "Apple",
                UnitPrice = 0.8M  
			 });
    }
}

现在,我们尝试用Get方式访问这个接口 /api/product/1

{
    "type": "https://tools.ietf.org/html/rfc7231#section-6.5.13",
    "title": "Unsupported Media Type",
    "status": 415,
    "traceId": "00-3600640f4e053b43b5ccefabe7eebd5a-159f5ca18d189142-00"
}

现在问题就来了,返回了415,.NET Core 不知道怎么把URL的参数转换为ProductId,由于它不是int,是我们定义的强类型ID,并且没有关联的类型转换器。

实现类型转换器

这里的解决方案是为实现一个类型转换器ProductId,很简单:

public class ProductIdConverter : TypeConverter
{
    public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType) =>
        sourceType == typeof(string);
    public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType) =>
        destinationType == typeof(string);

    public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
    {
        return value switch
        {
            string s => new ProductId(int.Parse(s)),
            null => null,
            _ => throw new ArgumentException($"Cannot convert from {value} to ProductId", nameof(value))
        };
    }

    public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
    {
        if (destinationType == typeof(string))
        {
            return value switch
            {
                ProductId id => id.Value.ToString(),
                null => null,
                _ => throw new ArgumentException($"Cannot convert {value} to string", nameof(value))
            };
        }

        throw new ArgumentException($"Cannot convert {value ?? "(null)"} to {destinationType}", nameof(destinationType));
    }
}

(请注意,为简洁起见,我只处理并转换string,在实际情况下,我们可能还希望支持转换int)

我们的ProductId使用TypeConverter特性将该转换器与记录相关联:

[TypeConverter(typeof(ProductIdConverter))]
public record ProductId(int Value);

现在,让我们尝试再次访问这个接口:

{
    "id": {
        "value": 1
    },
    "name": "Apple",
    "unitPrice": 0.8
}

现在是返回了,但是还有点问题,id 在json中显示了一个对象,如何在json中处理,是我们下一篇文章给大家介绍的,现在还有一点是,我上面写了一个ProductId的转换器,但是如果我们的类型足够多,那也有很多工作量,所以需要一个公共的通用转换器。

通用强类型id转换器

首先,让我们创建一个Helper

  • 检查类型是否为强类型ID,并获取值的类型
  • 获取值得类型,创建并缓存一个委托
public static class StronglyTypedIdHelper
{
    private static readonly ConcurrentDictionary<Type, Delegate> StronglyTypedIdFactories = new();

    public static Func<TValue, object> GetFactory<TValue>(Type stronglyTypedIdType)
        where TValue : notnull
    {
        return (Func<TValue, object>)StronglyTypedIdFactories.GetOrAdd(
            stronglyTypedIdType,
            CreateFactory<TValue>);
    }

    private static Func<TValue, object> CreateFactory<TValue>(Type stronglyTypedIdType)
        where TValue : notnull
    {
        if (!IsStronglyTypedId(stronglyTypedIdType))
            throw new ArgumentException($"Type '{stronglyTypedIdType}' is not a strongly-typed id type", nameof(stronglyTypedIdType));

        var ctor = stronglyTypedIdType.GetConstructor(new[] { typeof(TValue) });
        if (ctor is null)
            throw new ArgumentException($"Type '{stronglyTypedIdType}' doesn't have a constructor with one parameter of type '{typeof(TValue)}'", nameof(stronglyTypedIdType));

        var param = Expression.Parameter(typeof(TValue), "value");
        var body = Expression.New(ctor, param);
        var lambda = Expression.Lambda<Func<TValue, object>>(body, param);
        return lambda.Compile();
    }

    public static bool IsStronglyTypedId(Type type) => IsStronglyTypedId(type, out _);

    public static bool IsStronglyTypedId(Type type, [NotNullWhen(true)] out Type idType)
    {
        if (type is null)
            throw new ArgumentNullException(nameof(type));

        if (type.BaseType is Type baseType &&
            baseType.IsGenericType &&
            baseType.GetGenericTypeDefinition() == typeof(StronglyTypedId<>))
        {
            idType = baseType.GetGenericArguments()[0];
            return true;
        }

        idType = null;
        return false;
    }
}

这个 Helper 帮助我们编写类型转换器,现在,我们可以编写通用转换器了。

public class StronglyTypedIdConverter<TValue> : TypeConverter
    where TValue : notnull
{
    private static readonly TypeConverter IdValueConverter = GetIdValueConverter();

    private static TypeConverter GetIdValueConverter()
    {
        var converter = TypeDescriptor.GetConverter(typeof(TValue));
        if (!converter.CanConvertFrom(typeof(string)))
            throw new InvalidOperationException(
                $"Type '{typeof(TValue)}' doesn't have a converter that can convert from string");
        return converter;
    }

    private readonly Type _type;
    public StronglyTypedIdConverter(Type type)
    {
        _type = type;
    }

    public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
    {
        return sourceType == typeof(string)
            || sourceType == typeof(TValue)
            || base.CanConvertFrom(context, sourceType);
    }

    public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType)
    {
        return destinationType == typeof(string)
            || destinationType == typeof(TValue)
            || base.CanConvertTo(context, destinationType);
    }

    public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
    {
        if (value is string s)
        {
            value = IdValueConverter.ConvertFrom(s);
        }

        if (value is TValue idValue)
        {
            var factory = StronglyTypedIdHelper.GetFactory<TValue>(_type);
            return factory(idValue);
        }

        return base.ConvertFrom(context, culture, value);
    }

    public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
    {
        if (value is null)
            throw new ArgumentNullException(nameof(value));

        var stronglyTypedId = (StronglyTypedId<TValue>)value;
        TValue idValue = stronglyTypedId.Value;
        if (destinationType == typeof(string))
            return idValue.ToString()!;
        if (destinationType == typeof(TValue))
            return idValue;
        return base.ConvertTo(context, culture, value, destinationType);
    }
}

然后再创建一个非泛型的 Converter

public class StronglyTypedIdConverter : TypeConverter
{
    private static readonly ConcurrentDictionary<Type, TypeConverter> ActualConverters = new();

    private readonly TypeConverter _innerConverter;

    public StronglyTypedIdConverter(Type stronglyTypedIdType)
    {
        _innerConverter = ActualConverters.GetOrAdd(stronglyTypedIdType, CreateActualConverter);
    }

    public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType) =>
        _innerConverter.CanConvertFrom(context, sourceType);
    public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType) =>
        _innerConverter.CanConvertTo(context, destinationType);
    public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value) =>
        _innerConverter.ConvertFrom(context, culture, value);
    public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType) =>
        _innerConverter.ConvertTo(context, culture, value, destinationType);


    private static TypeConverter CreateActualConverter(Type stronglyTypedIdType)
    {
        if (!StronglyTypedIdHelper.IsStronglyTypedId(stronglyTypedIdType, out var idType))
            throw new InvalidOperationException($"The type '{stronglyTypedIdType}' is not a strongly typed id");

        var actualConverterType = typeof(StronglyTypedIdConverter<>).MakeGenericType(idType);
        return (TypeConverter)Activator.CreateInstance(actualConverterType, stronglyTypedIdType)!;
    }
}

到这里,我们可以直接删除之前的 ProductIdConvert, 现在有一个通用的可以使用,现在.NET Core 的路由匹配已经没有问题了,接下来的文章,我会介绍如何处理在JSON中出现的问题。

[TypeConverter(typeof(StronglyTypedIdConverter))]
public abstract record StronglyTypedId<TValue>(TValue Value)
    where TValue : notnull
{
    public override string ToString() => Value.ToString();
}

原文作者: thomas levesque
原文链接:https://thomaslevesque.com/2020/11/23/csharp-9-records-as-strongly-typed-ids-part-2-aspnet-core-route-and-query-parameters/

最后

欢迎扫码关注我们的公众号 【全球技术精选】,专注国外优秀博客的翻译和开源项目分享,也可以添加QQ群 897216102

使用 C# 9 的records作为强类型ID - 路由和查询参数

发表评论

评论已关闭。

相关文章