如何设置索引属性

我正在尝试使用mock来验证是否设置了索引属性。下面是一个具有索引的moq-able对象:

public class Index
{
    IDictionary<object ,object> _backingField 
        = new Dictionary<object, object>();

    public virtual object this[object key]
    {
        get { return _backingField[key]; }
        set { _backingField[key] = value; }
    }
}

首先,尝试使用 Setup() :

[Test]
public void MoqUsingSetup()
{
    //arrange
    var index = new Mock<Index>();
    index.Setup(o => o["Key"]).Verifiable();
    // act
    index.Object["Key"] = "Value";
    //assert
    index.Verify();
}

…失败了-它肯定在验证 get{}

所以,我试着用 SetupSet() :

[Test]
public void MoqUsingSetupSet()
{
    //arrange
    var index = new Mock<Index>();
    index.SetupSet(o => o["Key"]).Verifiable();
}

…它给出一个运行时异常:

System.ArgumentException : Expression is not a property access: o => o["Key"]
at Moq.ExpressionExtensions.ToPropertyInfo(LambdaExpression expression)
at Moq.Mock.SetupSet(Mock mock, Expression`1 expression)
at Moq.MockExtensions.SetupSet(Mock`1 mock, Expression`1 expression)

正确的方法是什么?