接口实现中使用更具体属性类型为何非法？有无合法实现方式？

Great question! The problem with your original code is that C# requires exact return type matching for implicit interface implementations. Your MyClass defines a Things property with IList<string>, which the compiler treats as a brand-new class property—not the implementation of MyInterface.Things (which expects ICollection<string>). That's why you get a compiler error.

Luckily, there are two solid ways to fix this:

1. Explicit Interface Implementation + Class-Specific Property #

Use explicit interface implementation to satisfy the interface contract, then expose a public IList<string> property for direct use with MyClass instances:

interface MyInterface { ICollection<string> Things { get; } }

class MyClass : MyInterface
{
    // Explicitly implement the interface member (only accessible via MyInterface type)
    ICollection<string> MyInterface.Things => this.Things;

    // Public property with the more specific IList<string> type
    public IList<string> Things { get; } = new List<string>();
}

How this works: #

• The explicit implementation (MyInterface.Things) maps directly to the interface's required return type. When you cast a MyClass instance to MyInterface, this is the property that gets used.
• The public Things property lets consumers work with the more capable IList<string> type when using MyClass directly.

2. Use C# 9.0+ Covariant Return Types #

If you're using C# 9 or later, you can take advantage of covariant return types, which allow implementing interface members with a more derived return type:

interface MyInterface { ICollection<string> Things { get; } }

class MyClass : MyInterface
{
    // Directly implement the interface with IList<string> (C# 9+)
    public IList<string> Things { get; } = new List<string>();
}

How this works: #

• Covariant return types let you return a subtype (like IList<string>, which inherits from ICollection<string>) when implementing an interface member that expects a base type. The compiler recognizes this as a valid implementation of the interface contract.
• This is the cleaner option if your project supports C# 9 or newer.

内容的提问来源于stack exchange，提问作者Hiram Katz