What Is the Purpose of Nameof

What is the purpose of nameof?

What about cases where you want to reuse the name of a property, for example when throwing exception based on a property name, or handling a PropertyChanged event. There are numerous cases where you would want to have the name of the property.

Take this example:

switch (e.PropertyName)
{
    case nameof(SomeProperty):
    { break; }

    // opposed to
    case "SomeOtherProperty":
    { break; }
}

In the first case, renaming SomeProperty will cause a compilation error if you don't change both the property definition and the nameof(SomeProperty) expression. In the second case, renaming SomeOtherProperty or altering the "SomeOtherProperty" string will result in silently broken runtime behavior, with no error or warning at build time.

This is a very useful way to keep your code compiling and bug free (sort-of).

(A very nice article from Eric Lippert why infoof didn't make it, while nameof did)

How does nameof work?

It's not "accessing" the property - that operator is purely a compiler mechanism to inject the "name" of the argument into the code. In this case it will replace nameof(TestClass.Name) with "Name". The fact that it's non-static is irrelevant.

nameof() operator for static string

nameof is an operator that is evaluated at compile time, so once your application is compiled there is actually no difference between those two solutions.

However, using nameof in this case has a few benefits:

• It makes the string value less “magic”. So instead of being some disconnected magic string, the semantic reasoning behind that value is very clear: It’s the name of the variable itself.
• The name is an actual reference to the name, so they are both connected. This allows you to refactor either of them and automatically affect the other one. It also makes that “string” appear as a reference when looking up references to that variable. So you know exactly where it has been used.

nameof equivalent in Java

Sadly, there is nothing like this. I had been looking for this functionality a while back and the answer seemed to be that generally speaking, this stuff does not exist.

See Get name of a field

You could, of course, annotate your field with a "Named" annotation to essentially accomplish this goal for your own classes. There's a large variety of frameworks that depend upon similar concepts, actually. Even so, this isn't automatic.

Difference between nameof and typeof

Two reasons:

nameof turns into a compile-time constant. typeof(...).Name requires a bit of reflection. It's not overly expensive, but it can hurt in some cases.

Second, it's used for other things than type names. For example, arguments:

void SomeMethod(int myArgument)
{
  Debug.WriteLine(nameof(myArgument));
}

You can also get the name of class members and even locals. Needless to say, this is quite useful for debugging information. It's also one of the ways to implement less fragile reflection when e.g. parsing expression trees (sadly, on the project where I'd use this, we're still stuck on .NET 4.0 with C# 5 - it'd save me a few hacks here and there).

And to clear up some confusion, nameof is not a function, and neither is typeof. It's a compile-time operator, and it's always evaluated at compile-time (though obviously, generics move the "compile-time" a bit further in time).

Is nameof() evaluated at compile-time?

Yes. nameof() is evaluated at compile-time. Looking at the latest version of the specs:

The nameof expression is a constant. In all cases, nameof(...) is evaluated at compile-time to produce a string. Its argument is not evaluated at runtime, and is considered unreachable code (however it does not emit an "unreachable code" warning).

From nameof operator - v5

You can see that with this TryRoslyn example where this:

public class Foo
{
    public void Bar()
    {
        Console.WriteLine(nameof(Foo));
    }
}

Is compiled and decompiled into this:

public class Foo
{
    public void Bar()
    {
        Console.WriteLine("Foo");
    }
}

Its run-time equivalent is:

public class Foo
{
    public void Bar()
    {
        Console.WriteLine(typeof(Foo).Name);
    }
}

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As was mentioned in the comments, that means that when you use nameof on type parameters in a generic type, don't expect to get the name of the actual dynamic type used as a type parameter instead of just the type parameter's name. So this:

public class Foo
{
    public void Bar<T>()
    {
        Console.WriteLine(nameof(T));
    }
}

Will become this:

public class Foo
{
    public void Bar<T>()
    {
        Console.WriteLine("T");
    }
}

Implicit and explicit typing with C# 6 nameof

The language team felt that this wasn't worth the spec complexity.

You can see the discussion here.

The underlying reason for this behavior is that the spec says (§8.5.1) names declared with var aren't visible in the declaring statement, since before nameof, there was no way in which that could be valid.

Implicitly typed local variable declarations are subject to the following restrictions:

• ...
• The initializer expression cannot refer to the declared variable itself

Without var, statements like int x = x = 1; or int x = 1, y = x; are legal; with var, nothing in that form is valid.

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