.NET Web服务中的DateTime转换行为不一致

Question

Problem

I've a Windows app syncs with the Server using SharePoint hosted Web Services. When the app syncs to the server using LAN (goes through an internal Proxy server) all the DateTime formats are in dd/MM/yyyy format (which is how it is intended to be)
However, when the app syncs over 4G,all DateTime formats are in MM/dd/yyyy format.
This happens for all data inbound and outbound.

Server

Windows Server 2012 with SharePoint 2013 hosting SOAP services
Region: Singapore
Format: English (Singapore)

Client

Windows 10 tablet app
Region: Singapore
Format: English (Singapore)

Other information:
1. It is the same tablet being used on both WiFi and 4G, so we can rule out 2 tablets having different regional settings.
2. I've verified that the Windows 10 app passes the formats correctly and it is the server that behaves differently over WiFi and 4G.
3. I beleive that the issue is caused by .NET itself and not because of SharePoint. However, I don't want to rule it out as I'm not sure of the actual cause. Please comment if you require any further information if you feel that it is caused because of SharePoint

Snippets:

I've skipped the using statements and SPWeb statements in the snippet to keep it simple. And the LastModifiedTime field in the SPList of type DateTime and not single line text.

Model

public class Record
{
    public string ID {get; set}
    public string ModifiedDateTime {get; set;} //Don't ask why it is not a DateTime object. It was too late by the time I took over
}

Web Service

public class WebService : IWebService
{
    public List<Record> GetUpdates(string lastModifiedTime)
    {
        SPQuery query= QueryBuilder.GetUpdateQuery(lastModifiedDateTime);
        SPList spRecordList = spWeb.Lists["Record"];
        SPListItemCollection results = spRecordList.GetItems(query);
        List<Record> records = new List<Record>();
        foreach(SPListItem spRecord in results)
        {
            Record record = new Record();
            record.ID = spRecord.ID.ToString();
            record.ModifiedDateTime = Convert.ToString(spRecord["LastModifiedTime"]);
            //1 June 2015 would return as 01/06/2015 in WiFi but 06/01/2015 on 4G

            records.Add(record);
        } 
        return records;
    }

    public Record CreateOrUpdateRecord(Record record)
    {
        SPListItem spRecord = null;
        SPList spRecordList = spWeb.Lists["Record"];
        if(string.IsNullOrEmpty(record.ID))
        {
            spRecord = spRecordList.AddItem();
            record.ID = spRecord.ID.ToString();
        }
        else
        {
            spRecord = spRecordList.GetItemByID(record.ID);
        }

        DateTime modified = Convert.ToDateTime(record.Modified);
        spRecord["LastModifiedTime"] = modified;

        /*
            Say ModifiedDateTime is 1 June 2015.
            Then on WiFi, modified = 01/06/2015
            On 4G, modified = 06/01/2015
        */

        return record;

    }
}

Now, I've fixed the problem by using format strings when converting between string and DateTime and vice-versa. So more or less, I've got it working for now.

So my question here is, what is the reason behind this behaviour? If possible, please cite links to documentation or references to any other sources that explain this behaviour

Is it possible that the server infers the culture info from the request header? I've always thought that the DateTime.Parse()/Convert.ToDateTime() always got the defaults from the regional settings of the machine it runs on.

Answer 1

First of all, a DateTime does not have any implicit format. It just have date and time values. Format concept only applies when you get it textual (string) representation. I strongly suggest to change this data type from string to DateTime if you can that returns by web service.

I've verified that the Windows 10 app passes the formats correctly and it is the server that behaves differently over WiFi and 4G

There is no such a thing. Parsing string to DateTime or vice versa does not depends on how you connected to internet. It is all about culture settings.

Since you use it as;

DateTime modified = Convert.ToDateTime(record.Modified);

This code will use CurrentCulture settings by default where it's located. Since you said;

It is the same tablet being used on both WiFi and 4G, so we can rule out 2 tablets having different regional settings

One regional settings parse your string as a 6 January and the other settings parse your string as 1 June. That's too normal. Looks like one setting uses dd/MM/yyyy format and the other one uses MM/dd/yyyy .

As a solution, you can use DateTime.ParseExact method to specify exact culture that matches with your string. Or you can equalize regional settings on both tablet.

For example;

DateTime dt = DateTime.ParseExact("01/06/2015", "dd/MM/yyyy", CultureInfo.InvariantCulture);

will parse as 1 June 2015 but

DateTime dt = DateTime.ParseExact("01/06/2015", "MM/dd/yyyy", CultureInfo.InvariantCulture);

will parse as 6 January 2015.