c# - 使用最小起订量抛出异常模拟 IMemoryCache

Question

我正在尝试模拟 IMemoryCache最小起订量。我收到此错误:

An exception of type 'System.NotSupportedException' occurred in Moq.dll but was not handled in user code

Additional information: Expression references a method that does not belong to the mocked object: x => x.Get<String>(It.IsAny<String>())

我的模拟代码:

namespace Iag.Services.SupplierApiTests.Mocks
{
    public static class MockMemoryCacheService
    {
        public static IMemoryCache GetMemoryCache()
        {
            Mock<IMemoryCache> mockMemoryCache = new Mock<IMemoryCache>();
            mockMemoryCache.Setup(x => x.Get<string>(It.IsAny<string>())).Returns("");<---------- **ERROR**
            return mockMemoryCache.Object;
        }
    }
}

为什么会出现该错误？

这是被测代码:

var cachedResponse = _memoryCache.Get<String>(url);

在哪里_memoryCache类型为 IMemoryCache

我如何模拟 _memoryCache.Get<String>(url)让它返回 null？

编辑:如果不是_memoryCache.Set<String>(url, response);，我会如何做同样的事情？ ？我不介意它返回什么，我只需要将方法添加到模拟中，这样它就不会在调用时抛出。

按照我尝试过的这个问题的答案:

mockMemoryCache
    .Setup(m => m.CreateEntry(It.IsAny<object>())).Returns(null as ICacheEntry);

因为在 memoryCache 扩展中它显示它使用 CreateEntry里面Set .但它因“对象引用未设置到对象的实例”而出错。

Answer 1

根据 MemoryCacheExtensions.cs 的源代码,

Get<TItem>扩展方法使用以下内容

public static TItem Get<TItem>(this IMemoryCache cache, object key) {
    TItem value;
    cache.TryGetValue<TItem>(key, out value);
    return value;
}

public static bool TryGetValue<TItem>(this IMemoryCache cache, object key, out TItem value) {
    object result;
    if (cache.TryGetValue(key, out result)) {
        value = (TItem)result;
        return true;
    }

    value = default(TItem);
    return false;
}

请注意，本质上它使用的是 TryGetValue(Object, out Object) 方法。

鉴于用 Moq 模拟扩展方法是不可行的，尝试模拟扩展方法访问的接口(interface)成员。

引用Moq's quickstart更新MockMemoryCacheService正确设置 TryGetValue测试方法。

public static class MockMemoryCacheService {
    public static IMemoryCache GetMemoryCache(object expectedValue) {
        var mockMemoryCache = new Mock<IMemoryCache>();
        mockMemoryCache
            .Setup(x => x.TryGetValue(It.IsAny<object>(), out expectedValue))
            .Returns(true);
        return mockMemoryCache.Object;
    }
}

来自评论

Note that when mocking TryGetValue (in lieu of Get), the out parameter must be declared as an object even if it isn't.

For example:

int expectedNumber = 1; 
object expectedValue = expectedNumber. 

If you don't do this then it will match a templated extension method of the same name.

这是一个使用修改后的服务来模拟 memoryCache.Get<String>(url) 的例子让它返回 null

[TestMethod]
public void _IMemoryCacheTestWithMoq() {
    var url = "fakeURL";
    object expected = null;

    var memoryCache = MockMemoryCacheService.GetMemoryCache(expected);

    var cachedResponse = memoryCache.Get<string>(url);

    Assert.IsNull(cachedResponse);
    Assert.AreEqual(expected, cachedResponse);
}

更新

可以对 Set<> 应用相同的过程看起来像这样的扩展方法。

public static TItem Set<TItem>(this IMemoryCache cache, object key, TItem value) {
    var entry = cache.CreateEntry(key);
    entry.Value = value;
    entry.Dispose();

    return value;
}

此方法利用了 CreateEntry返回 ICacheEntry 的方法这也被采取行动。因此，像下面的示例一样设置模拟以返回模拟条目

[TestMethod]
public void _IMemoryCache_Set_With_Moq() {
    var url = "fakeURL";
    var response = "json string";

    var memoryCache = Mock.Of<IMemoryCache>();
    var cachEntry = Mock.Of<ICacheEntry>();

    var mockMemoryCache = Mock.Get(memoryCache);
    mockMemoryCache
        .Setup(m => m.CreateEntry(It.IsAny<object>()))
        .Returns(cachEntry);

    var cachedResponse = memoryCache.Set<string>(url, response);

    Assert.IsNotNull(cachedResponse);
    Assert.AreEqual(response, cachedResponse);
}