CustomAttribute: how to set DateTime.Now

Question

I am loooking for an elegant way to set the value of DateTime.Now as value of an Attribute.

I have this code:

    [MyValue(DateTime.Now)]
    public DateTime? VertragDatum { get; set; }

But I am getting a syntax error and tells me that the Type DateTime is not a valid attribute parameter type. Do you have any idea how to can achieve that?

My implementation:

public class MyValueAttribute : Attribute
{
    public string MyValue { get; }

    public MyValueAttribute(DateTime myValue)
    {
        MyValue = myValue.ToString();
    }
}

when do you want that value set? on object creation, on type initialization? sounds like an xy problem here. — Daniel A. White, Aug 06 '19 at 14:21
Please note, that `DateTime.Now` will be set at *compile time* (e.g. `6 Aug 2019 17:21:05`) and be read at *runtime* (say somewhen at `14 Oct 2019 14:52:47`). Do you really want such behavior? — Dmitry Bychenko, Aug 06 '19 at 14:21
Following on with what @dymanoid says, the only thing you can use as a constructor parameter for an attribute is a constant expression. C# has no constant or literal `DateTime`s (unlike VB). If it's always `DateTime.Now`, you can set it in the constructor. But, not only will `DateTime.Now` not work where it is, no DateTime expression will. — Flydog57, Aug 06 '19 at 14:23
You can potentially create a "fody" weaver to inject the current date-time in as a string or some sort of roslyn based plugin. I suspect that's outside the scope of what users want for this question. — Glenn Watson, Aug 06 '19 at 14:30
Possible duplicate of [How to pass objects into an attribute constructor](https://stackoverflow.com/questions/1235617/how-to-pass-objects-into-an-attribute-constructor) — Glenn Watson, Aug 06 '19 at 14:33

score -1 · Answer 1 · answered Aug 06 '19 at 14:25

DateTime isn't a supported type.

As referenced here valid types are:

The types of positional and named parameters for an attribute class are limited to the attribute parameter types, which are:

One of the following types: bool, byte, char, double, float, int, long, sbyte, short, string, uint, ulong, ushort.

The type object.

The type System.Type.

An enum type, provided it has public accessibility and the types in which it is nested (if any) also have public accessibility (Attribute specification).

Single-dimensional arrays of the above types.

A constructor argument or public field which does not have one of these types, cannot be used as a positional or named parameter in an attribute specification.

I think due to the question in general being down voted – Glenn Watson Apr 01 '21 at 12:39 — Glenn Watson, Apr 01 '21 at 12:39

score -2 · Answer 2 · answered Aug 06 '19 at 14:21

-2

specs says:

The types of positional and named parameters for an attribute class are limited to the attribute parameter types, which are:

One of the following types: bool, byte, char, double, float, int, long, sbyte, short, string, uint, ulong, ushort.

answered Aug 06 '19 at 14:21

Antoine V

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This answer is missing valid parameter types. – Glenn Watson Aug 06 '19 at 14:25

CustomAttribute: how to set DateTime.Now

2 Answers2