Obsolete attribute - Avoid magic string

Question

I want to mark an old class as Obsolete and redirect to the new class in the Obsolete attribute comment. However, if I write the new class name as a magic string, I won't be able to use the might of IDE.

For example, if I rename my class, I will loose the "link". And the reader won't be able to Ctrl + click on my comment to go to the new class.

The official documentation does seems to say that it is not possible, but I find strange that nobody has this need.

Instead of

// Mark OldProperty As Obsolete.
[ObsoleteAttribute("This property is obsolete. Use NewProperty instead.", false)]
public static string OldProperty
{ get { return "The old property value."; } }

public static string NewProperty
{ get { return "The new property value."; } }

I would like to be able to write

[ObsoleteAttribute($"This property is obsolete. Use {nameof(NewProperty)} instead.", false)]

As a workaround, I can use the XML doc, but it's not the best way to do it.

/// <remarks>Replaced by <see cref="LoadBrainIndMeasMessTable"/></remarks>

"I would like to be able to write ..." -- as of C# 10 (.NET 6), you can. sharplab.io/… — canton7
– canton7, Commented Oct 17, 2024 at 7:59
Before C#10, you can write "This property is obsolete. Use " + nameof(NewProperty) + " instead." — SomeBody
– SomeBody, Commented Oct 17, 2024 at 8:01
Thanks! I forgot I was on an old project. I got An attribute argument must be a constant expression, 'typeof()' expression or array creation expression of an attribute parameter type when trying to do so, I did not think to check in later language version. Would you like to write it as an answer so I can accept it as resolved? — Damien R
– Damien R, Commented Oct 17, 2024 at 8:43
As an aside, classes that define attributes by convention always end with "Attribute", but when you use them as attributes, you drop that part of the name: [Obsolete] not [ObsoleteAttribute] — iakobski
– iakobski, Commented Oct 17, 2024 at 10:42

canton7 · Accepted Answer · 2024-10-17 09:00:40Z

6

This is already supported in C# 10 (.NET 6):

public class C
{
    [Obsolete($"This property is obsolete. Use {nameof(NewProperty)} instead")]
    public static string OldProperty => "The old property value.";

    public static string NewProperty => "The new property value.";
}

Before then, string concatenation should work for any C# version:

public class C
{
    [Obsolete("This property is obsolete. Use " + nameof(NewProperty) + " instead")]
    public static string OldProperty => "The old property value.";

    public static string NewProperty => "The new property value.";
}

answered Oct 17, 2024 at 9:00

canton7

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