c# - 在没有 setter 的情况下设置属性。这怎么不是编译错误？

Question

我有课

 public class Settings : ProviderSettings {
        internal Settings(MyProvider provider) {
            this.Provider = provider;
            LoadFromConfig();
        }

        protected override IProvider Provider {
            get;
        }
}

ProviderSettings 类是:

 public abstract class ProviderSettings {
          protected abstract IProvider Provider { get; }
 }

在 Visual Studio 2015 中，当我以 .NET 4.0 为目标时，我没有收到编译错误。我想我应该收到一个编译错误，说“Provider 是只读的，无法设置”。为什么编译器允许这样做？

Answer 1

如果您不指定 setter，则 getter-only 自动属性的支持字段将隐式声明为 readonly。您可以从构造函数或使用属性初始化程序对其进行初始化。这是 C# 6 的新特性。

所以实际上你的代码将被编译为

public abstract class ProviderSettings
{
    protected abstract IProvider get_Provider();
    // there is no property setter
}

public class Settings : ProviderSettings
{
    private readonly IProvider _provider;

    internal Settings(MyProvider provider) {
        _provider = provider; // assignment directly to backing field
        LoadFromConfig();
    }

    protected override IProvider get_Provider()
    {
        return _provider;
    }
    // there is no property setter
}

C# 6 Language Specification的相关部分(草稿):

If the auto-property has no set accessor, the backing field is considered readonly. Just like a readonly field, a getter-only auto-property can also be assigned to in the body of a constructor of the enclosing class. Such an assignment assigns directly to the readonly backing field of the property.