in-keyword

[IAmTheCShark]

C# really could use an in or alternatively is in keyword like other languages (e.g. TSQL) offer.

It allways feels wrong writing code like

if (new[] {1,2,3,4}.Contains(id)) {

}

if (id == 1 || id == 2 || id == 3 || id == 4) {

}

switch(id)
{
   case 1: case 2: case 3: case 4: {  ...  } break;
}


public static bool IsIn<T>(this T value, params T[] values) => values.Contains(value);

if(id.IsIn(1,2,3,4)) {

}

when this would feel / look so much better

if (id in { 1,2,3,4 }) {

}

The compiler should be able to generate something like the switch statement out of this so that there is no arraycreation involved like the extensionmethod does.

Making it work for Enumerables is a bonus

if (id in listOfIds) {
}

where the compiler should just translate it into Enumerable.Contains

I think this is connected to Discussion: Range operator #198

[DavidArno]

Active patterns (coupled with Permit IEnumerable as the type of a params parameter) would completely cover this use case (and many, many more);

public bool InPattern<T>(this IEnumerable<T> collection, T matchValue) =>
    collection.Contains(matchValue);

...
if (id is In(1,2,3,4)) 
{
}

switch(id)
{
   case In(1,2,3,4) : ...
}

[alrz]

Active patterns are not extremely useful here a regular extension method works just fine,

static bool In<T>(this T value, params T[] values) { .. }

if (value.In(1, 2, 3)) 

unless you want to use it in a switch statement.

OR patterns (#118) can also be helpful.

[IAmTheCShark]

@alrz I know it is nitpicking but the extensionmethod comes with a few "drawbacks"
Most important to mention is the array creation, while my idea of that keyword was to avoids this.

Apart from that you might need a reference to the assembly where that extensionmethod is located plus a using statement to make it avaible.