C＃不通过返回类型推断重载方法

Question

I'm writing a C# programlet to crawl a directory and give me a listing of files whose date in the last CSV line is less than the current date. Since this is a programlet, I'm not really spending too much time making the code very clean or anything -- but that's all a matter of opinion, I suppose.

The curious thing is the following set of code snippets. Three static methods all in the same class.

   public static DateTime dateStringConverter(string mmddyyyy, char delim='/')
    {
            string[] date = mmddyyyy.Split(delim);
            DateTime fileTime = new DateTime(Convert.ToInt32(date[2]), Convert.ToInt32(date[0]), Convert.ToInt32(date[1]));
            return fileTime;
    }

        public static string dateStringGetter()
        {
            string sYear = DateTime.Now.Year.ToString();
            string sMonth = DateTime.Now.Month.ToString().PadLeft(2, '0');
            string sDay = DateTime.Now.Day.ToString().PadLeft(2, '0');
            return sMonth + '/' + sDay + '/' + sYear;
        }

        public static DateTime dateStringGetter()
        {
            string datestring = dateStringGetter();
            return dateStringConverter(datestring);
        }

The error message says:

Error   1   Type 'Poller.Program' already defines a member called 
'dateStringGetter' with the same parameter types    

The problem method is the second overloaded copy of dateStringGetter(), which of course has the same parameter types as the second version (none) but has two completely different return types. One is DateTime and the other is string. The version with the DateTime return type -- in a string of bad coding -- calls the version of dateStringGetter() with a string type.

Isn't this curious? That C# will not overload methods based on return type alone? I think I've done overloading with libraries that will automatically detect the return type I want based on the call -- but I'm not sure. Something doesn't feel right about this.

So I suppose C# does not overload return types?

Answer 1

So I suppose C# does not overload return types?

No, indeed it doesn't. The return type isn't part of the signature.

From section 3.6 of the C# 5 specification (emphasis mine):

The signature of a method consists of the name of the method, the number of type parameters and the type and kind (value, reference, or output) of each of its formal parameters, considered in the order left to right. For these purposes, any type parameter of the method that occurs in the type of a formal parameter is identified not by its name, but by its ordinal position in the type argument list of the method. The signature of a method specifically does not include the return type , the params modifier that may be specified for the right-most parameter, nor the optional type parameter constraints.

and

Overloading of methods permits a class, struct, or interface to declare multiple methods with the same name, provided their signatures are unique within that class, struct, or interface.

And additionally (for completeness):

Although out and ref parameter modifiers are considered part of a signature, members declared in a single type cannot differ in signature solely by ref and out .

Aside from anything else, this restriction helps with readability - it can be hard enough to tell which overload is being called sometimes even when they vary by parameters - it would be even worse if methods could be overloaded by return type. In this case it doesn't even make sense for the methods to be overloaded, as they do opposite things . You should only overload a method if all the overloads perform the same basic task.

As a side note, your methods don't currently follow .NET naming conventions - and you should be using standard formatting/parsing methods instead of rolling your own.

Answer 2

No, C# (like C++ before it) does not allow an overload where the only difference is in the return type .

The big problem here is which one do you call if you don't assign the return value? Which one do you call if both types are castable to the return value assignment? Its highly ambiguous, and so not allowed.

Answer 3

Based on "programlet to crawl a directory and give me a listing of files whose date in the last CSV line is less than the current date. Since this is a programlet, I'm not really spending too much time making the code very clean or anything" I propose something similar to the following which I've used in the past. Take it or leave it.

What it does: Specify a root directory, function gets all files within directory of given type (for your time requirement, modify files.add(...) to match your criteria)

Only leaving this here as an alternative to your work since you said you don't want to spend much time on it.

    var data = GetAllFilesOfType('c:\rootpath', 'csv')
/// <summary>
    /// Gets files of specified type and appends them to the file list.
    /// </summary>
    /// <param name="basepath">Starting file path</param>
    /// <param name="type">File type - Do not include prefix ('txt' instead of '*.txt</param>
    /// <returns>Returns results of WalkDirectoryTree</returns>
    public static IEnumerable<FileInfo[]> GetAllFilesOfType(string basepath, string type)
    {
        var root = new DirectoryInfo(basepath);
        return WalkDirectoryTree(root, type);
    }


/// <summary>
    /// Recursively gets all files from a specified basepath (provided by GetAllFilesOfType)
    /// and appends them to a file list. This method reports all errors, and will break on
    /// things like security errors, non existant items, etc.
    /// </summary>
    /// <param name="root">Initially specified by calling function, set by recursive walk</param>
    /// <param name="type">File type that is desired. Do not include prefix ('txt' instead of '*.txt')</param>
    /// <returns></returns>
    private static List<FileInfo[]> WalkDirectoryTree(DirectoryInfo root, string type)
    {
        var files = new List<FileInfo[]>();

        //Traverse entire directory tree recursively - Will break on exception
        var subDirs = root.GetDirectories();
        foreach (var data in subDirs.Select(dirInfo => WalkDirectoryTree(dirInfo, type)).Where(data => data.Count > 0))
        {
            files.AddRange(data);
        }
        //If any file is found, add it to the file list
        if (root.GetFiles(string.Format("*.{0}", type)).Length > 0)
        {
            files.Add(root.GetFiles(string.Format("*.{0}", type)));
        }
        //Kicks the file list up a level until it reaches root, then returns to calling function
        return files;
    }

Answer 4

There is always a way to hammer it into place, unfortunately:

class Program
{
    static void Main(string[] args)
    {
        object stringDate = "";
        object dateTime = new DateTime();
        DateUtils.DateStringGetter(ref stringDate);
        DateUtils.DateStringGetter(ref dateTime);
    }
}

public static class DateUtils
{
    private static DateTime DateStringConverter(string mmddyyyy, char delim = '/')
    {
        string[] date = mmddyyyy.Split(delim);
        DateTime fileTime = new DateTime(Convert.ToInt32(date[2]), Convert.ToInt32(date[0]),
            Convert.ToInt32(date[1]));
        return fileTime;
    }

    public static void DateStringGetter(ref object date)
    {
        string sYear = DateTime.Now.Year.ToString();
        string sMonth = DateTime.Now.Month.ToString().PadLeft(2, '0');
        string sDay = DateTime.Now.Day.ToString().PadLeft(2, '0');

        if (date is String)
        {
            date = sMonth + '/' + sDay + '/' + sYear;
        }

        if (date is DateTime)
        {
            date = DateStringConverter(sMonth + '/' + sDay + '/' + sYear);
        }

    }
}

Answer 5

您不能只为返回类型提供两种不同的方法，因为代码无法推断哪些应该是返回类型。

Answer 6

Correct, C# does not let you overload methods on return types only.

Consider what happens if I just call dateStringGetter() without assignment of return value.

Which method have I called?