使用Moq的Raise触发事件时遭遇TargetParameterCountException求助

问题场景 #

我定义了一个带异步事件的类：

event Func<ApplicationMessageProcessedEventArgs, Task> ApplicationMessageProcessedAsync;

用Moq测试该事件处理逻辑时，写了这段代码：

mqttClient.Mock.Raise(client => client.ApplicationMessageReceivedAsync += null, eventArg);

结果抛出参数不匹配的异常，堆栈信息如下：

System.Reflection.TargetParameterCountException: Parameter count mismatch.
    at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at System.Delegate.DynamicInvokeImpl(Object[] args)
   at System.Delegate.DynamicInvoke(Object[] args)
   at Moq.Extensions.InvokePreserveStack(Delegate del, IReadOnlyList`1 args) in C:\projects\moq4\src\Moq\Extensions.cs:line 158
   at Moq.Mock.RaiseEvent(Mock mock, LambdaExpression expression, Stack`1 parts, Object[] arguments) in C:\projects\moq4\src\Moq\Mock.cs:line 745
   at Moq.Mock.RaiseEvent[T](Mock mock, Action`1 action, Object[] arguments) in C:\projects\moq4\src\Moq\Mock.cs:line 695
   at Moq.Mock`1.Raise(Action`1 eventExpression, EventArgs args) in C:\projects\moq4\src\Moq\Mock`1.cs:line 1366
   at MqttChannel.MqttChannelTest.MqttChannel_ProcessNewMessage() in C:\Development\Shared\OT.Test\MqttChannel\MqttChannelTest.cs:line 298
   at Microsoft.VisualStudio.TestPlatform.MSTestAdapter.PlatformServices.ThreadOperations.ExecuteWithAbortSafety(Action action)

问题根源 #

Moq的Raise方法是为标准事件委托设计的，比如EventHandler<T>，这类委托的签名是void (object sender, TEventArgs e)，需要两个参数。而你用的是异步委托Func<ApplicationMessageProcessedEventArgs, Task>，它只接受一个事件参数，两者参数数量不匹配，所以触发异常。

解决办法 #

放弃用Moq的Raise，手动触发异步事件：

异步测试方法中使用 #

var eventHandler = mqttClient.Mock.Object.ApplicationMessageReceivedAsync;
if (eventHandler != null)
{
    await eventHandler(eventArg);
}

同步测试方法中使用 #

var eventHandler = mqttClient.Mock.Object.ApplicationMessageReceivedAsync;
eventHandler?.Invoke(eventArg).Wait();

注意：检查事件名称是否一致——你定义的事件是ApplicationMessageProcessedAsync，但测试代码里写的是ApplicationMessageReceivedAsync，如果是笔误请修正。

内容的提问来源于stack exchange，提问作者Pavel Reichl