Cast T parameter in generic method to DateTime

Question

I have the following (simplified) method:

private static string GetStringFromValue<T>(T val)
{
    if (typeof(T) == typeof(DateTime))
    {
        return string.Format("{0}", ((DateTime)val).Year.ToString("0000"));
    }
    return string.Empty;
}

At the cast "(DateTime)val" I get the following error:

Cannot cast expression of Type 'T' to type 'DateTime'

What can I do to access the Year property of the DateTime parameter?

UPDATE: Thank you for all your very fast answers. This method (and method name) is really (!) simplified to show exactly my problem and to let everyone just copy&paste it into his own visual studio. It is just that I wanted to add some Type specific values if the type is a DateTime. Beside that 99% of the method is the same.

Why on earth are you writing `String,Format("{0}", something)`? — SLaks, Aug 24 '10 at 16:52
Year is not the only value I want to get. But I simplified the method. Or is it wrong that I use String.Format at all? It has a much better performance and readability than: x.Year + ":" + x.Month + ":" + ..... — Chris, Aug 24 '10 at 16:58
Actually, it has much _worse_ performance than `a + b + c` – it's got to parse the format string. It can also be less readable. Also, calling `String.Format("{0}", something)` (without any format string or concatenation is just dumb) — SLaks, Aug 24 '10 at 19:54
@ SLaks, thank you for your answer. But what is the alternative? Without using the Format string I need to go through boxing (like your seconds posted code). I need to build a complete date/time. In my tests string.Format is MUCH faster. In the other case there is a new string created for every concatenation! — Chris, Aug 24 '10 at 21:54
@SLaks the `string,Format("{0}", something)` will work if something is null, while `something.ToString()` will throw null reference exception — achekh, Feb 24 '12 at 16:50
As an aside you can use `if (val is DateTime)` to replace your condition — gav, May 15 '12 at 12:10

score 30 · Accepted Answer · answered Aug 24 '10 at 16:49

30

Change it to

    return string.Format("{0:yyyy}", val);

To answer the question, the compiler does not realize that T is DateTime.
To perform this cast, you need to cast through object, like this:

    return ((DateTime)(object)val).Year.ToString("0000");

answered Aug 24 '10 at 16:49

SLaks

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6

Of course, if you cast through object, there are boxing & unboxing step going on. – James Curran Aug 24 '10 at 17:03
@James: AFAIK, there is no way to avoid that without making a separate method. – SLaks Aug 24 '10 at 18:18
1

I agree, but it should be noted. – James Curran Aug 24 '10 at 18:43
For others about to walk into the same trap: I started making separate methods and going through all the trouble, before I realized that I was only doing this operation a few times and *the boxing overhead was negligible*. (Despite my itch at technically useless overhead...) – Timo Mar 07 '16 at 12:15

score 7 · Answer 2 · answered Aug 24 '10 at 16:58

7

I know you said the example was simplified, but consider handling it like this:

private static string GetStringFromValue(DateTime val) 
{ 
    return string.Format("{0}", val.Year.ToString("0000")); 
} 

private static string GetStringFromValue<T>(T val) 
{ 
    return string.Empty; 
}

The DateTime overload is the best match when an actual DateTime is passed, and the generic version will be used for everthing else. (You could even forgo generic for the second one, and just use an Object)

answered Aug 24 '10 at 16:58

James Curran

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upvote for the separate methods for each type. Would it be better to use {0:yyyy} instead though? – Greg Bogumil Aug 24 '10 at 17:04
or simply val.ToString("yyyy") – Greg Bogumil Aug 24 '10 at 17:05
1

@gbogumil: val.ToString("yyyy") - This will give me the error: Method 'ToString' has 0 parameter(s) but is invoked with 1 argument(s) – Chris Aug 24 '10 at 17:14
but val is a DateTime type. It has 4 overloads on ToString. – Greg Bogumil Aug 25 '10 at 12:16
Wow, nice one to avoid boxing unboxing. – Falaque Mar 06 '19 at 11:25
This will not work if you invoke GetStringFromValue with the T argument specified, like from another generic method. – Christian Lundheim Mar 22 '22 at 14:28
@ChristianLundheim Specifying the T argument is unnecessary if the compile can deduce it from the function argument passed in. – James Curran Mar 30 '22 at 19:18
@JamesCurran Yes, but it will not change its binding to "GetStringFromValue(DateTime val)", even if the compiler deduces it to "GetStringFromValue(DateTime val)". So you will get String.Empty even with a DateTime as T. The proof is here: https://dotnetfiddle.net/z6TKom – Christian Lundheim Apr 07 '22 at 13:28

score 1 · Answer 3 · answered Aug 24 '10 at 16:55

SLaks types faster than I do. :)

But let me add: you might want to re-think your implementation, here, depending on what you're trying to achieve. I assume the reason to have a generic GetStringFromValue method is to emit specific strings from various types. But that's going to end up a bit of a mess once you have, say, a dozen different types in there.

If they're System types, such as DateTime, string.Format() can probably handle all of them, with appropriate format strings. If they're your own types, consider overriding their ToString() methods.

Either way, a little more detail on the problem you're solving would make for interesting answers.

Cast T parameter in generic method to DateTime

3 Answers3