属性不变不正确地应用于支持字段。 [英] property invariant incorrectly applied to backing field.

问题描述

代码合同（1.4.5109.0）似乎没有注意到以下代码中的空引用异常。我的简短调查似乎表明，如果从getter设置了一个字段，那么应用于该属性的任何不变量也适用于该字段。

Code contracts (1.4.5109.0) doesn't seem to notice the null reference exception in the following code. My brief investigations seem to indicate that if a field is set from the getter, then any invariant applied to the property also applies to the field.

	public class Functastic
	{
		public int Get()
		{
			return foo.GetHashCode();
		}

		object Foo
		{
			get { return foo ?? (foo = new object()); }
		}

		object foo;

		[ContractInvariantMethod]
		void ObjectInvariant()
		{
			Contract.Invariant(Foo != null);
		}
	}

推荐答案

不变量旨在提供针对私有州的全局内部后置条件，因此它们通常应用于字段，而不是属性。 将不变量视为在每种方法中定义相同的后置条件。

Invariants are intended to provide global internal post-conditions against private state, thus they are typically applied to fields, not properties.  Think of an invariant as defining the same post-condition in every method.

如果您打算在 Foo 上定义后置条件，那么只需定义一个
的后置条件 Foo 。

If your intention was to define a post-condition on Foo, then simply define a post-condition on Foo.

object Foo
{
	get
	{
		Contract.Ensures(Contract.Result<object>() != null);

		return foo ?? (foo = new object());
	}
}

请注意，您可以为自动属性定义不变量，因为没有其他方法可以为它们定义公共后置条件，但这不是一回事。 在以下示例中，不变量实际上定义了 Foo 上的公共后置条件
，但仅仅因为 Foo 是自动属性并且是
public 。

Note that you can define invariants for automatic properties simply because there's no other way to define public post-conditions for them, but it's not the same thing.  In the following example, the invariant actually defines a public post-condition on Foo, but only because Foo is an automatic property and is public.

public object Foo { get; private set; }

[ContractInvariantMethod]
void ObjectInvariant()
{
	Contract.Invariant(Foo != null);
}

- 戴夫

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