即使参数没有null约束，也会收到有关可为null的类型参数的错误 [英] Receiving error about nullable type parameter even when parameter has notnull constraint

问题描述

我有一个通用接口IDataAdapter<T>;接口的实现者应该能够从数据源中读取具有Guid ID的POCO. IDataAdapter<T>有一个方法Read(Guid id)，我想返回一个T?，其中null表示在数据源中未找到匹配项.但是，即使在IDataAdapter<T>上具有约束T : notnull，尝试定义此方法也会出现错误CS8627: A nullable type parameter must be known to be a value type or non-nullable reference type. Consider adding a 'class', 'struct', or type constraint.，即使在T约束为notnull的情况下，为什么我仍然会收到此错误?

I have a generic interface IDataAdapter<T>; implementors of the interface should be able to read POCOs with Guid IDs from a data source. IDataAdapter<T> has a method Read(Guid id) which I want to return a T?, where null indicates that no matches were found in the data source. However, even with a constraint T : notnull on IDataAdapter<T>, attempting to define this method gives the error CS8627: A nullable type parameter must be known to be a value type or non-nullable reference type. Consider adding a 'class', 'struct', or type constraint. Why am I still getting this error, even with T constrained to notnull?

代码(应在带有<Nullable>enable</Nullable>的C#8环境中):

Code (should be in a C# 8 environment with <Nullable>enable</Nullable>):

interface IDataAdapter<T> where T : notnull
{
    T? Read (Guid id); // error CS8627
}

推荐答案

我认为此问题与请注意，在CLR中，T? where T : class和T? where T : struct的表示方式非常不同.前者只是CLR类型T. CLR中没有单独的类型来区分T和T?. T?在C#中只是添加了C#编译器检查额外的编译时间.另一方面，后者由CLR类型Nullable<T>表示.

Note that a T? where T : class and a T? where T : struct are represented very differently in the CLR. The former is just the CLR type T. There are not separate types in the CLR to differentiate between T and T?. T? in C# just adds extra compile time checking by the C# compiler. On the other hand, The latter is represented by the CLR type Nullable<T>.

因此，让我们考虑一下您的方法:

So let's consider your method:

T? Read (Guid id);

在CLR中应如何表示?什么是退货类型?编译器不知道T是引用类型还是值类型，因此编译器无法确定方法签名应为:

How should this be represented in the CLR? What is the return type? The compiler don't know whether T is a reference type or a value type, so the compiler cannot decide whether the method signature should be:

T Read (Guid id);

或:

Nullable<T> Read (Guid id);

这篇关于即使参数没有null约束，也会收到有关可为null的类型参数的错误的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！