For a while I’ve used the DateTime ToString method and noticed the “hint” for showing the possible formats, but I’ve not really thought about how this happens, until now.

Note: This attribute came in for projects targeting .NET 7 or later.

The title of this post gives away the answer to how this all works, but let’s take a look anyway…

If you type

DateTime.Now.ToString("

Visual Studio kindly shows a list of different formatting such as Long Date, Short Date etc.

We can use this same technique in our own code (most likely libraries etc.) by simply adding the StringSyntax attribute to our method parameter(s).

For example

static void Write(
   [StringSyntax(StringSyntaxAttribute.DateOnlyFormat)] string input)
{
    Console.WriteLine(input);
}

This attribute does not enforce the format (in the example above), i.e. yo can enter whatever you like as a string. It just gives you some help (or hint) as to possible values. In the case of the DateOnlyFormat these are possible date formatters. StringSyntax actually supports other syntax hints such as DateTimeFormat, GuidFormat and more.

Sadly (at least at the time of writing) I don’t see any options for custom formats.