C# nameof generic type without specifying type

Question

Assume I have the type

public class A<T> { }

and somewhere in the code I want to throw an exception related to incorrect usage of that type:

throw new InvalidOperationException("Cannot use A<T> like that.");

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So far, so good, but I don't want to hardcode the classes name, so I thought I could maybe use

throw new InvalidOperationException($"Cannot use {nameof(A<T>)} like that.");

instead, but in this context I don't know the exact type T. So I thought maybe I could do it with template specialization like in C++:

throw new InvalidOperationException($"Cannot use {nameof(A)} like that.");

or

throw new InvalidOperationException($"Cannot use {nameof(A<>)} like that.");

but those yield

Incorrect number of type parameters.

and

Type argument is missing.

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I absolutely don't want to hardcode the classes name for it might change later. How can I get the name of the class, preferably via nameof?

Optimally, what I want to achieve is "Cannot use A<T> like that." or "Cannot use A like that.".

Delwin

Just Curious. Is there a reason you don't just add a where constraint on the type of T so that you can't call the method if T is not of some Type? – Punchball

@Punchball sure, constraints would be the way to go, but this is not about the type argument, but rather it's like a warning for the developer that they shouldn't use A in that particular case at all. – Delwin

nameof(type) is compiled into the assembly, so it is hardcoded, but in the assembly. Runtime it can be considered a constant. – Citizen

@Citizen you're right, but when I change A's name in the IDE, it will change all the ``nameof(type)` with it, which would not be a case for a "develop-time" constant. – Delwin

@ThomasFlinkow I see, so A<T> is being passed to some method, and that method is throwing the exception because it shouldn't be passed A<T> objects? Just asking since if you want this as a warning to developers, throwing an exception only warns them when they try to run it. It seems like you could resolve this be restricting the Type of T or the types of the parameters used by methods where A<T> is being erroneously passed. Anyway design questions probably off topic to the question. – Punchball

@Punchball yeah, you got that quite right. Thanks for the suggestion of overthinking the design :) – Delwin

Answer 1

If you don't care about displaying T, you can just use e.g. nameof(A<object>), assuming object complies with the generic type constraints.

This results in "Cannot use A like that."

If you want to print exactly A<T>, you could use:

$"{nameof(A<T>)}<{nameof(T)}>"

But only from within the class, as T does not exist elsewhere.

Alfeus

thank you, this is sufficient and perfectly fine. I will accept this as soon as SO will let me. – Delwin

This will give you A<T> though, not A<sometype> – Soche

@Soche From what I understand that is what OP is asking for: "Optimally, what I want to achieve is "Cannot use A<T> like that."" – Alfeus

Yep, @Alfeus is correct. Sorry to all if that wasn't clear in the question. – Delwin

Because almost everything is treated as an object (in an OOP language) you can do it that way. – Taxi

@quetzalcoatl: nameof only takes what the specification calls "simple names". It then devotes more than a page to specifying exactly what's simple, but bottom line, open generics aren't simple names. What it calls "unbound generic types" are only allowed in certain contexts, and are treated specially. – Teston

Hmm.. interesting. Intellisense fooled me. Before I wrote it, I checked it in VS, I wrote var foo = nameof(List<>) and it didn't get highlighted. It got highlighted only now when I tried building it. Sorry for the confusion. It's not allowed obviously. – Sedlik

Answer 2

Have you tried:

typeof(T).FullName;

or

t.GetType().FullName;

Hope it works for you.

Woolgrower

I'm afraid that won't help, because I don't care about T, but rather about A only. Thank you anyways. – Delwin

Sorry in thi case.. will becan use.. 'This': this.GetType().FullName. But glad you already get an answer – Woolgrower

nameof() evaluates to a const String so it can be used in other const strings and concatenated in string literals - you can't do that with Type.FullName, unfortunately. – Tutorial

Answer 3

Depending on the place you want to raise that exception, you can use the type.

On the instance, call this.GetType() and then get the Name or FullName property:

throw new InvalidOperationException($"Cannot use {this.GetType().Name} like that.");

Medievalism

Thank you, I didn't think of this at all. That would be a good way as well, but AFAIK it resolves the name at runtime rather than at compile time. – Delwin

You can do that, but most people don't find A`1 (the real, internal name of the class) very readable. – Teston

That doesn't matter too much since type metadata is cache. – Medievalism

@ThomasFlinkow Well you don't know what T is until runtime. – Soche

@JeroenMostert When derived (B : A<string>), it can be clearer than the other option. So the answer is: it depends. – Medievalism

@Soche you're right, but I don't really care about T too much as well. Sorry if that wasn't very clear in the question. – Delwin

Answer 4

typeof(A<>).Name should work.

Monolingual