未找到自定义 C# 属性的命名空间

Question

We have a Model that is passed from a web page as json and uses model building in our controller to resolve the properties within our .Net Core 3.1 server. We want to decorate some properties of that model with a custom attribute so that a "helper" method can perform a transformation on the specified property in our Controller.

When we have NO attributes on our model, everything compiles and works fine.

We have created a class for this model (LoginModel) and decorated a property with our custom attribute [Courier].

namespace THO.Demo.API.Models
{

    public class LoginModel
    {
        public string Email { get; set; }
        [Courier(foo: "secret")]
        public string Secret { get; set; }
        [Courier]
        public string PIN { get; set; }
    }
}

We have created a class for the Custom Attribute

namespace THO.Demo.API.Models
{
    [AttributeUsage(AttributeTargets.Property)]
    public class CourierAttribute : System.Attribute
    {
        private string foobar;
        public CourierAttribute(string foo)
        {
            foobar = foo;
        }

        public CourierAttribute() { }
    }
}

When I try to build our project, I get the following errors:

Error CS0246 The type or namespace name 'CourierAttribute' could not be found (are you missing a using directive or an assembly reference?)

and

Error CS0246 The type or namespace name 'Courier' could not be found (are you missing a using directive or an assembly reference?)

Both of these files are "side by side" in the same project and in the same namespace. I have tried naming the Attribute "MyCustomAttribute" with the same issue. When (In VS2019) I click on the attribute, I see a reference and can "jump" to it, so it appears that at least VS knows they are there, but the project will not compile.

Any thoughts? Thanks.

Answer 1

Our way of organizing projects is to keep all of our models ({projectRoot}/Models) in a folder and namespace. We also created a folder for "Attributes" ({projectRoot}/Attributes) where we put our attribute class. When I moved the "LoginModel" from our Models folder into the root folder of the project, the compiler was able to find the attribute (even when we leave the Attribute class in its own folder). This makes no sense, as all we did was move the file. We did not change the namespace. This seems to be a work around, but it does break our project organization. (BTW, when we do NOT have a property decorated with an attribute, the model is found and compiles correctly). Update - the "not found" issue was in a separate project (oops) with a link to the LoginModel. Once we looked carefully at the eaxact error message and added a link to the Attribute class, everything works.

Lesson learned - CAREFULLY examine the exact error message.