generic-constraints

Given the following code... type Indexable<TKey, TValue> = { [index: TKey]: TValue } This produces the following error: An index signature parameter type must be 'string' or 'number'. Is there a way to constrain TKey to be 'string' or 'number'? As @TitianCernicova-Dragomir indicates, you can't use TKey as the type in an index signature, even if it is equivalent to string or number . If you know that TKey is exactly string or number , you can just use it directly and not specify TKey in your type: type StringIndexable<TValue> = { [index: string]: TValue } type NumberIndexable<TValue> = { [index

After failing to get something like the following to compile: public class Gen<T> where T : System.Array { } with the error A constraint cannot be special class `System.Array' I started wondering, what exactly is a "special class"? People often seem to get the same kind of error when they specify System.Enum in a generic constraint. I got the same results with System.Object , System.Delegate , System.MulticastDelegate and System.ValueType too. Are there more of them? I cannot find any info on "special classes" in C#. Also, what is so special about those classes that we can't use them as a

Item class public class Item { public bool Check(int value) { ... } } Base abstract class with generic type constraint public abstract class ClassBase<TItem> where TItem : Item { protected IList<TItem> items; public ClassBase(IEnumerable<TItem> items) { this.items = items.ToList(); } public abstract bool CheckAll(int value); } Inherited class without constraints public class MyClass<TItem> : ClassBase<TItem> { public override bool CheckAll(int value) { bool result = true; foreach(TItem item in this.items) { if (!item.Check(value)) // this doesn't work { result = false; break; } } return result

In C# you can put a constraint on a generic method like: public class A { public static void Method<T> (T a) where T : new() { //...do something... } } Where you specify that T should have a constructor that requires no parameters. I'm wondering whether there is a way to add a constraint like " there exists a constructor with a float[,] parameter? " The following code doesn't compile: public class A { public static void Method<T> (T a) where T : new(float[,] u) { //...do something... } } A workaround is also useful? Tim Robinson As you've found, you can't do this. As a workaround I normally