Switch based on generic argument type

问题

In C# 7.1 the below is valid code:

object o = new object();
switch (o)
{
    case CustomerRequestBase c:
        //do something
        break;
}

However, I want to use the pattern switch statement in the following scenario:

public T Process<T>(object message, IMessageFormatter messageFormatter) 
    where T : class, IStandardMessageModel, new()
{
    switch (T)
    {
        case CustomerRequestBase c:
            //do something
            break;
    }
}

The IDE gives me the error "'T' is a type, which is not valid in the given context" Is there an elegant way to switch on the type of a generic parameter? I get that in my first example you are switching on the object and the second I'd want to switch on the type T. What would be the best approach to do this?

回答1:

Below are two different classes called Foo and Bar. You can use one instance of any of these classes as a parameter to a function named Process. After all, you can perform pattern matching as shown in the example function. There is a function named Test for the usage example..

public class Foo
{
    public string FooMsg { get; set; }
}

public class Bar
{
    public string BarMsg { get; set; }
}

public class Example
{
    public T Process<T>(T procClass) where T : class
    {
        switch (typeof(T))
        {
            case
                var cls when cls == typeof(Foo):
                {
                    var temp = (Foo)((object)procClass);
                    temp.FooMsg = "This is a Foo!";

                    break;
                }

            case
                var cls when cls == typeof(Bar):
                {
                    var temp = (Bar)((object)procClass);
                    temp.BarMsg = "This is a Bar!";

                    break;
                }
        }

        return
            procClass;
    }

    public void Test(string message)
    {
        Process(new Foo() { FooMsg = message });
        Process(new Bar() { BarMsg = message });
    }
}

来源：https://stackoverflow.com/questions/53597845/switch-based-on-generic-argument-type