nameof with casting

Question

I want to use nameof in order to get the name of a property.

The following work:

DerivedClass EC = baseClassObj as DerivedClass;
nameof(EC.propertyX)

but this doesn't work:

nameof((baseClassObj as DerivedClass).propertyX)

with the compile error of:

Sub-expression cannot be used in an argument to nameof

BTW, also this doesn't work:

nameof(((baseClassObj)DerivedClass).propertyX)

Can someone explain this casting + nameof problem?

Answer 1

As far as I know, the nameof() works with reflection based on the expression provided in nameof(). Therefore, the engine obviously has problems to "decompile" the expression if it's not "simple expression".

You might be interested in creating some extension method that will take baseClassObj as DerivedClass as parameter and return the result of nameof().

EDIT: nameof() is evaluated in compile time, that's the main reason :)

Answer 2

nameof works basically as a preprocessed macro. Before compile time it will be replaced by the actual name of the parameter, from there on it is like a "constant string" in the assembly.

The argument expression identifies a code definition, but it is never evaluated. see here: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/nameof

Answer 3

nameof is a compile time thing and therefore static. To get what you want, just use nameof(DerivedClass.PropertyX):

class BaseClass
{
}

class DerivedClass : BaseClass
{
    public string PropertyX { get; set; }
}

static class UsePropertyName
{
    public static string GetPropertyName(BaseClass classInstance)
    {
        //Instance not actually used.
        return nameof(DerivedClass.PropertyX);
    }
}