调用在C＃中的参数中使用表达式和委托的扩展方法

Question

Consider the below statement:

recorder.AddActivity(new Activity { ActivityName = "DeepSeaDiving", DayOfWeek = DayOfWeek.Monday });

Instead of this, there was a post here , using Expression Trees for fancy-calling like this:

WeeklyActivityRecorder weeklyActivities = new WeeklyActivityRecorder () .WithActivities( Monday => "Lawn Moving",Tuesday => "Cooking");

I saw that extension method here , and which is given below.

public static WeeklyActivityRecorder WithActivities(this WeeklyActivityRecorder recorder, params Expression<Func<DayOfWeek, string>>[] activityList) 
    {
    foreach (var activity in activityList)
                {
                    LambdaExpression expression = activity;
                    ConstantExpression enteredActivity = expression.Body as ConstantExpression;
                    DayOfWeek day = expression.Parameters[0];
                    recorder.AddActivity(new Activity{DayOfWeek = day, ActivityName = activity});
                }

                return recorder;
    }

But, when I compile this, the compiler is unhappy about the extension method and complains that `Cannot convert sourceType ParameterExpression to DayOfWeek .

Any ideas what I am missing here ?

Answer 1

你需要传递一个有效的lambda ..你在实际的DayOfWeek传递：

recorder.WithActivities(lambda_variable_here => "LawnMoving");

Answer 2

You need to pass parameter, but you try to create lambda, where parameter name is static property, which is roughly equivalent to:

public string SomeMethodName(DayOfWeek.Monday) //incorrect method declaration
{

}

You need to call WithActivities method like

recorder.WithActivities(dayOfWeek => "LawnMoving"); //dayOfWeek is a parameter name

If you need to pass conversion method from DayOfWeek to String , you can use Dictionary, like this:

var recorder = new WeeklyActivityRecorder();

var daysTranslation = new Dictionary<DayOfWeek, string>()
{
    {DayOfWeek.Monday, "LawnMoving"}
    //other pairs
};

recorder.WithActivities(dayOfWeek => daysTranslation[dayOfWeek]);