c# 7.0：打开 System.Type

Question

No existing question has an answer to this question.

In c# 7, can I switch directly on a System.Type ?

When I try:

    switch (Type)
    {
      case typeof(int):
        break;
    }

it tells me that typeof(int) needs to be a constant expression.

Is there some syntatic sugar that allows me to avoid case nameof(int): and directly compare the types for equality? nameof(T) in a case statement is not completely good because namespaces. So although name collision is probably not be applicable for int , it will be applicable for other comparisons.

In other words, I'm trying to be more type-safe than this:

    switch (Type.Name)
    {
      case nameof(Int32):
      case nameof(Decimal):
        this.value = Math.Max(Math.Min(0, Maximum), Minimum); // enforce minimum 
        break;
    }

Answer 1

The (already linked) new pattern matching feature allows this.

Ordinarily, you'd switch on a value:

switch (this.value) {
  case int intValue:
    this.value = Math.Max(Math.Min(intValue, Maximum), Minimum);
    break;
  case decimal decimalValue:
    this.value = Math.Max(Math.Min(decimalValue, Maximum), Minimum);
    break;
}

But you can use it to switch on a type, if all you have is a type:

switch (type) {
  case Type intType when intType == typeof(int):
  case Type decimalType when decimalType == typeof(decimal):
    this.value = Math.Max(Math.Min(this.value, Maximum), Minimum);
    break;
}

Note that this is not what the feature is intended for, it becomes less readable than a traditional if ... else if ... else if ... else chain, and the traditional chain is what it compiles to anyway. I do not recommend using pattern matching like this.

Answer 2

The issue raised here by the OP is that you can't use the new C# 7 type-based switch feature when you don't have an actual instance of the switched-upon type available, and you instead have only have its putative System.Type . The accepted answer , summarized as follows, works well for exact type matching (minor improvement shown here, but see my final example below for yet further streamlining)...

Type type = ...
switch (type)
{
    case Type _ when type == typeof(Int32):
    case Type _ when type == typeof(Decimal):
        this.value = Math.Max(Math.Min(this.value, Maximum), Minimum);
        break;
}

The important point to note is that for derived reference type hierarchies, this will not exhibit the same behavior as an if... else chain which uses the is keyword for matching. Consider:

class TBase { }
class TDerived1 : TBase { }
class TDerived2 : TBase { }
class TDerived3 : TDerived2 { }

TBase inst = ...

if (inst is TDerived1)
{
    // Handles case TDerived1
}
else if (inst is TDerived2)
{
    // Handles cases TDerived2 and TDerived3
}
else if (inst is TDerived3)
{
    // NOT EXECUTED                            <---  !
}

Since TDerived3 "is-a" TDerived2 , both cases are handled by the earlier condition when using is matching. This highlights the difference at runtime between 'strict' or 'exact' type equality versus the more nuanced notion of type subsumption (type "compatibility"). Because the types in the OP's question were ValueType primitives (which can't be derived-from), the difference couldn't matter. But if we adapt the 'exact type matching' of the accepted answer with the example classes shown above, we will get a different result:

Type type = ...

switch (type)
{
    case Type _ when type == typeof(TDerived1):
        // Handles case TDerived1
        break;

    case Type _ when type == typeof(TDerived2):
        // Handles case TDerived2
        break;

    case Type _ when type == typeof(TDerived3):
        // Handles case TDerived3              <---  !
        break;
}

In fact, C# 7 won't even compile a switch statement which corresponds to the if / else sequence shown earlier. ( nb It seems like the compiler should detect this as a warning , rather than an error , since the harmless result is just a branch of inaccessible code--a condition which the compiler deems a warning elsewhere--and also considering that the compiler doesn't even detect, at all, the seemingly identical situation in the if / else version). Here's that:

In any case, which one of the alternate behaviors is appropriate, or if it even matters, will depend on your application, so my point here is just to draw attention to the distinction. If you determine that you need the more savvy type-compatibility version of the switch approach, here is how to do it:

Type type = ...

switch (type)
{
    case Type _ when typeof(TDerived1).IsAssignableFrom(type):
        // Handles case TDerived1
        break;

    case Type _ when typeof(TDerived2).IsAssignableFrom(type):
        // Handles cases TDerived2 and TDerived3
        break;

    case Type _ when typeof(TDerived3).IsAssignableFrom(type):
        // NOT EXECUTED                       <-- !
        break;
}

Finally, as I mentioned in another answer on this page, you can simplify this usage of the switch statement even further. Since we're only using the when clause functionality, and since we presumably still have the original switched-upon Type instance available in a variable, there's no need to mention that variable in the switch statement, nor repeat its Type ( Type , in this case) in each case . Just do the following instead:

Type type = ...

switch (true)
{
    case true when typeof(TDerived1).IsAssignableFrom(type):
        break;

    case true when typeof(TDerived2).IsAssignableFrom(type):
        break;

    case true when typeof(TDerived3).IsAssignableFrom(type):
        break;
}

Notice the switch(true) and case(true) . I recommend this simpler technique whenever you are relying only on the when clause (that is, beyond just the situation of switching on System.Type as discussed here).

Answer 3

Starting with Paulustrious's idea of switching on a constant, but striving for the most readability:

  Type type = GetMyType();
  switch (true)
  {
    case bool _ when type == typeof(int):
      break;
    case bool _ when type == typeof(double):
      break;
    case bool _ when type == typeof(string):
      break;
    default:
      break;
  }

What's readable is subjective. I used to do something similar in VB a long time ago so I got used to this form (but in VB the bool _ was not needed so it wasn't there). Unfortunately in c# the bool _ required. I'm using c# 7.0 and I think switching on a constant may not be supported in earlier compilers but I am not sure about that, so try it if you want to. I think it's kindof amusing that the S/O code formatter doesn't know about when yet.

You wouldn't want to do this of course if you need the case variable, like for subclasses.

But for arbitrary boolean expressions it is more suited, for example:

  switch (true)
  {
    case bool _ when extruder.Temperature < 200:
      HeatUpExtruder();
      break;
    case bool _ when bed.Temperature < 60:
      HeatUpBed();
      break;
    case bool _ when bed.Y < 0 || bed.Y > 300:
      HomeYAxis();
      break;
    default:
      StartPrintJob();
      break;
  }

Some will argue this is worse than if..else. The only thing I can say is switch forces one path and it's impossible to break the switch statement itself, but it is possible to leave out an else and break an if..else into multiple statements unintentionally, possibly executing two "branches" accidentally.

Switching on a Type is really just an arbitrary switch because what we are really switching on is a property of the variable. Unless and until we can do case typeof(int) ( case on something that is not a constant expression), we are stuck with something akin to this if we don't want to use string constants, which in the case of type names, are not in the framework.

Answer 4

Although the question is on C# 7 but for those with a C# 8 version or higher, you can use the newer syntax which is shorter and I think is nicer as well.

type switch
{
    Type _ when type == typeof(int) || type == typeof(decimalType) =>
        this.value = Math.Max(Math.Min(this.value, Maximum), Minimum),
    _ => // default case
};

Another elegant option:

Type.GetTypeCode(type) switch
{
    TypeCode.Int32 or TypeCode.Decimal =>
        this.value = Math.Max(Math.Min(this.value, Maximum), Minimum),
    _ => // default case
};

For more info: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/switch-expression

Answer 5

@toddmo suggested the following:

switch (true)
{
    case bool _ when extruder.Temperature < 200:
        HeatUpExtruder();
        break;

    // etc..
    default:
        StartPrintJob();
        break;
}

...but why not go even further in his pursuit of simplicity. The following works just as well without needing the bool type qualification, nor the extraneous _ dummy variable:

switch (true)
{
    case true when extruder.Temperature < 200:
        HeatUpExtruder();
        break;

    // etc.
    default:
        StartPrintJob();
        break;
}

Answer 6

I found a simple and efficient way. It requires C# V7 to run. The switch("") means all cases will be satisfied up to the when clause. It uses the when clause to look at the type .

List<Object> parameters = new List<object>(); // needed for new Action
parameters = new List<object>
{
    new Action(()=>parameters.Count.ToString()),
    (double) 3.14159,
    (int) 42,
    "M-String theory",
    new System.Text.StringBuilder("This is a stringBuilder"),
    null,
};
string parmStrings = string.Empty;
int index = -1;
foreach (object param in parameters)
{
    index++;
    Type type = param?.GetType() ?? typeof(ArgumentNullException);
    switch ("")
    {
        case string anyName when type == typeof(Action):
            parmStrings = $"{parmStrings} {(param as Action).ToString()} ";
            break;
        case string egStringBuilder when type == typeof(System.Text.StringBuilder):
            parmStrings = $"{parmStrings} {(param as System.Text.StringBuilder)},";
            break;
        case string egInt when type == typeof(int):
            parmStrings = $"{parmStrings} {param.ToString()},";
            break;
        case string egDouble when type == typeof(double):
            parmStrings = $"{parmStrings} {param.ToString()},";
            break;
        case string egString when type == typeof(string):
            parmStrings = $"{parmStrings} {param},";
            break;
        case string egNull when type == typeof(ArgumentNullException):
            parmStrings  = $"{parmStrings} parameter[{index}] is null";
            break;
        default: throw new System.InvalidOperationException();
    };
} 

This leaves parmStrings containing...

System.Action 3.14159, 42, M-String theory, This is a stringBuilder, parameter[5] is null

Answer 7

Adding to the above answer, if you do not need the value to be used inside the case statement, you can use _ . This syntax can be used to remove an unused variable warning message.

switch (this.value) {
  case int _:
    //Do something else without value.
    break;
  case decimal decimalValue:
    this.value = Math.Max(Math.Min(decimalValue, Maximum), Minimum);
    break;
}

Answer 8

I know it's not applicable in all cases, but maybe my example will help someone. In ASP.NET Core I implemented my custom model binder provider and I had to resolve binder type depending on model type.

The initial idea (apart from the if/else block ofc but I kept thinking it could be shorter) was the switch:

    public IModelBinder GetBinder(ModelBinderProviderContext context)
    {
        switch (context.Metadata.ModelType)
        {
            case Type _ when context.Metadata.ModelType == typeof(Model1):
                return new BinderTypeModelBinder(typeof(Binder1));
            case Type _ when context.Metadata.ModelType == typeof(Model2):
                return new BinderTypeModelBinder(typeof(Binder2));
            case Type _ when context.Metadata.ModelType == typeof(Model3):
                return new BinderTypeModelBinder(typeof(Binder3));
        }

        return null;
    }

This is the same with a dictionary:

    public IModelBinder GetBinder(ModelBinderProviderContext context)
    {
        var bindersByType = new Dictionary<Type, Type>
        {
            {typeof(Model1),  typeof(Binder1)},
            {typeof(Model2),  typeof(Binder2)},
            {typeof(Model3),  typeof(Binder3)}
        };

        return bindersByType.TryGetValue(context.Metadata.ModelType, out Type binderType) ? new BinderTypeModelBinder(binderType) : null;
    }

Credits for the idea go to @xxbbcc who posted this in a comment to the first question

Answer 9

Well, it probably break your code convention, but I use this approach;

var result = this.Value is int intValue
? Math.Max(Math.Min(intValue, Maximum), Minimum)
: this.Value is decimal decimalValue
? Math.Max(Math.Min(decimalValue, Maximum), Minimum)
: this.Value is double doubleValue
? Math.Max(Math.Min(decimalValue, Maximum), Minimum)
: throw new Exception($"Cannot handle '{this.Value.GetType().Name}' value.");

You can use Type.IsAssignableFrom() in condition statement when it is more appropriate.

Answer 10

Here is an alternative which won't compile as the classes are missing:

bool? runOnUI = queuedAction.RunOnUI;  // N=null, T=true F=False
bool isOnUI = Statics.CoreDispatcher.HasThreadAccess;
bool isFF = queuedAction.IsFireAndForget;   // just makes it easier to read the switch statement
if (false == queuedAction.IsFireAndForget)
    IsOtherTaskRunning = true;

/* In the case statements below the string name is something like noFF_TN
 * The compiler ignores the string name. I have used them here to represent
 * the logic in the case statement:   ( FF = FireAnd Forget, T=true, F=false, N = null, * means 'whatever')
 * 
 *      isFF_** = FireAndForget QueuedAction
 *      noFF_** = Not FireAndForget so Wait for Task to Finish (as opposed to Complete)
 *      ****_T* = We are already on the UI thread 
 *      ****_F* = We are not on the UI thread 
 *      ****_*T = Run on the UI thread.
 *      ****_*F = Do not run on UI thread
 *      ****_*N = Don't care so run on current thread
 *      
 *  The last character is a "bool?" representing RunOnUI. It has
 *  three values each of which has a different meaning */

bool isTask;
switch ("ignore")
{
    /* Run it as an Action (not Task) on current Thread   */
    case string noFF_TT when !isFF && isOnUI && runOnUI == true:
    case string isFF_TN when isFF && isOnUI && !runOnUI == null:
    case string isFF_FN when isFF && !isOnUI && runOnUI == null:
    case string isFF_TT when isFF && isOnUI && runOnUI == true:
    case string isFF_FF when isFF && !isOnUI && runOnUI == false:
        isTask = false;
        queuedAction.ActionQA(queuedAction); // run as an action, not as a Task
        break;

    /* Create a Task, Start it */

    case string isFF_TF when isFF && isOnUI == true && runOnUI == false:
    case string noFF_TN when !isFF && isOnUI == true && runOnUI == null:     // <== not sure if I should run it on UI instead
    case string noFF_TF when !isFF && isOnUI && runOnUI == false:
    case string noFF_FN when !isFF && !isOnUI && runOnUI == null:
    case string noFF_FF when !isFF && !isOnUI && runOnUI == false:
        var cancellationTokenSource = new CancellationTokenSource();
queuedAction.Canceller?.SetCancellationTokenSource(cancellationTokenSource);
        isTask = true;
        new Task
            (
                (action) => queuedAction.ActionQA(queuedAction),
                queuedAction,
                cancellationTokenSource.Token
            )
            .Start();
        break;

    /* Run on UI and don't wait */

    case string isFF_FT when isFF && !isOnUI && runOnUI == true:
        isTask = true;
        Statics.RunOnUI(() => queuedAction.ActionQA(queuedAction), asTaskAlways: true);
        break;

    /* Run on the UI as a Task and Wait */

    case string noFF_FT when !isFF && !isOnUI && runOnUI == true:
        isTask = true;
        Statics.RunOnUI(() => queuedAction.ActionQA(queuedAction), asTaskAlways: true);
        break;

    default:
        throw new LogicException("unknown case");
}