Why it is not possible to define generic indexers in .NET?

Question

Why can't you create a generic indexer in .NET?

the following code throws a compiler error:

public T this<T>[string key] {     get => /* Return generic type T. */ } 

Does this mean you can't create a generic indexer for a generic member collection?

Answer 1

Here's a place where this would be useful. Say you have a strongly-typed OptionKey<T> for declaring options.

public static class DefaultOptions {     public static OptionKey<bool> SomeBooleanOption { get; }     public static OptionKey<int> SomeIntegerOption { get; } } 

Where options are exposed through the IOptions interface:

public interface IOptions {     /* since options have a default value that can be returned if nothing's      * been set for the key, it'd be nice to use the property instead of the      * pair of methods.      */     T this<T>[OptionKey<T> key]     {         get;         set;     }      T GetOptionValue<T>(OptionKey<T> key);     void SetOptionValue<T>(OptionKey<T> key, T value); } 

Code could then use the generic indexer as a nice strongly-typed options store:

void Foo() {     IOptions o = ...;     o[DefaultOptions.SomeBooleanOption] = true;     int integerValue = o[DefaultOptions.SomeIntegerOption]; }