Can a C# Property ever have no GetMethod and no SetMethod

Question

Browsing the .NET core source code for System.Linq.Expressions, I found the following code located here:

MethodInfo mi = property.GetGetMethod(true);
if (mi == null)
{
    mi = property.GetSetMethod(true);
    if (mi == null)
    {
        throw Error.PropertyDoesNotHaveAccessor(property, nameof(property));
    }
}

Is there any way that GetGetMethod and GetSetMethod can both return null, as seems to be accounted for here? Is this dead code? The C# compiler doesn't allow for a property to have no getter and no setter, so how is this possible for PropertyInfo.

My motivation is to contribute to the OSS code by adding test coverage, so I'm trying to see what test cases would cover this

Answer 1

You can create a property with no accessors in IL:

.class public C
{
  .property int32 Count() { } 
}

Which can then trigger the code path you mentioned:

var prop = typeof(C).GetProperty("Count", BindingFlags.NonPublic | BindingFlags.Instance);

Expression.Property(null, prop);

This code throws:

ArgumentException: The property 'Int32 Count' has no 'get' or 'set' accessors

Answer 2

According to the CLI Specification,

CLS Rule 28: Properties shall adhere to a specific naming pattern. See §I.10.4. The SpecialName attribute referred to in CLS rule 24 shall be ignored in appropriate name comparisons and shall adhere to identifier rules. A property shall have a getter method, a setter method, or both.

That's on page 52 of the linked PDF.

That seems to say that one or the other must be there.

It appears that the developer decided to interpret the specification that way, and acted accordingly in the face of invalid data. Seems like quite a reasonable thing to do.

It might be possible, by writing IL or by editing the metadata afterwards, to create a property that has no getter or setter. And the .NET loader might go ahead and load it anyway, even if it is considered an invalid property. I've seen that kind of thing happen in other fields.

With that in mind, code that handles the "impossible" situation of a property with neither getter nor setter doesn't seem like dead code to me.

Answer 3

After some research, there is a way to achieve this, via System.Reflection.Emit

AssemblyBuilder assembly = AssemblyBuilder.DefineDynamicAssembly(new AssemblyName("Name"), AssemblyBuilderAccess.Run);
ModuleBuilder module = assembly.DefineDynamicModule("Module");

TypeBuilder type = module.DefineType("Type");
PropertyBuilder property = type.DefineProperty("Property", PropertyAttributes.None, typeof(void), new Type[0]);

Type createdType = type.CreateType();
PropertyInfo createdProperty = createdType.GetTypeInfo().DeclaredProperties.First();

Console.WriteLine(createdProperty.GetGetMethod(true) == null);
Console.WriteLine(createdProperty.GetSetMethod(true) == null);

This is obviously a strange way to have no setter or getter without IL