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。
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。
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($"Cannot convert {value ?? "(null)"} to {destinationType}", nameof(destinationType));
}
}
(请注意,为简洁起见,我只处理并转换string,在实际情况下,我们可能还希望支持转换int)
我们的ProductId使用TypeConverter特性将该转换器与记录相关联:
[TypeConverter(typeof(ProductIdConverter))]
public record ProductId(int Value);
现在,让我们尝试再次访问这个接口:
{
"id": {
"value": 1
},
"name": "Apple",
"unitPrice": 0.8
}
现在是返回了,但是还有点问题,id 在json中显示了一个对象,如何在json中处理,是我们下一篇文章给大家介绍的,现在还有一点是,我上面写了一个ProductId的转换器,但是如果我们的类型足够多,那也有很多工作量,所以需要一个公共的通用转换器。
首先,让我们创建一个Helper
•检查类型是否为强类型ID,并获取值的类型•获取值得类型,创建并缓存一个委托
public static class StronglyTypedIdHelper
{
private static readonly ConcurrentDictionary<Type, Delegate> StronglyTypedIdFactories = new();
public static Func<TValue, object> GetFactory<TValue>(Type stronglyTypedIdType)
where TValue : notnull
{
return (Func<TValue, object>)StronglyTypedIdFactories.GetOrAdd(
stronglyTypedIdType,
CreateFactory<TValue>);
}
private static Func<TValue, object> CreateFactory<TValue>(Type stronglyTypedIdType)
where TValue : notnull
{
if (!IsStronglyTypedId(stronglyTypedIdType))
throw new ArgumentException($"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/