Switch by type of generic parameter

Question

I would like to switch the type of generic parameter and I have found one way to do it that works and pass the test (*1) (Edited: this approach doesn't work for string, check @dbc's comment):

public static string HelpWithThis<T>(this DummyClass<T> _)
    =>
    (T)default! switch // <- Code smell (in my opinion)
    {
        DateOnly => "This day",
        TimeOnly => "This hour",
        int or float => "This number",
        _ => "This",
    };    

However, I am not fond of using type casting for default!.

Could someone kindly guide me on how to rewrite this function in a more elegant syntax?

Test

(*1) I paste here the whole test and instructions to reproduce:

dotnet new xunit -o gentyp
cd gentyp
dotnet add package FluentAssertions

// UnitTest1.cs
using FluentAssertions;
namespace gentyp;

public class DummyClass<T>{}
public static class DummyExtensions
{
    public static string HelpWithThis<T>(this DummyClass<T> _)
        =>
        (T)default! switch
        {
            DateOnly => "This day",
            TimeOnly => "This hour",
            int or float => "This number",
            _ => "This",
        };    
}

public class UnitTest1
{
    [Fact]
    public void Test1()
        =>
        new DummyClass<int>()
        .HelpWithThis()
        .Should()
        .Be("This number");
}

Answer 1

As an alternative to overloaded methods you could go old-school and use a static lookup table to convert a type into a string:

static readonly Dictionary<Type, string> _lookup = new ()
{
    [typeof(DateOnly)]  = "This day",
    [typeof(TimeOnly)]  = "This time",
    [typeof(int)]       = "This number",
    [typeof(float)]     = "This number"
};

public static string HelpWithThis<T>(this DummyClass<T> _)
{
    return _lookup.TryGetValue(typeof(T), out var result)
        ? result
        : "This";
}

I personally prefer your original solution (as long as it's not being used with large structs!) but you might prefer this.