C＃图表轴标签未正确格式化

Question

Assume we have a method call that accesses a remote database, taking approximately 1 second to complete and returning a DataTable (simulated below):

DataTable GetData()
{
    Thread.Sleep(1000); // simulated delay

    var dt = new DataTable("DataTable");
    dt.Columns.Add("DateTime", typeof(DateTime));
    dt.Columns.Add("Value", typeof(double));

    var rand = new Random();
    var date = new DateTime(2016, 1, 1);
    for (int i = 0; i < 1000; i++)
    {
        dt.Rows.Add(date, rand.NextDouble() * 100);

        date = date.AddHours(12);
    }

    return dt;
}

The DataTable returned from this method is used to populate a Chart with a single line series. Why is it that this code shows correct, properly-formatted DateTime X-Axis labels:

async void MainForm_Load(object sender, EventArgs e)
{
    var data = GetData();
    chart.DataSource = data;
    chart.DataBind();
}

and this code shows incorrect, unformatted X-Axis labels?

async void MainForm_Load(object sender, EventArgs e)
{
    var data = await Task.Run(() => GetData());
    chart.DataSource = data;
    chart.DataBind();
}

Note that removing the Thread.Sleep() call also fixes this issue, even using the second version.

Answer 1

Answer lies in chart.Series[0].XValueType

First, couple of statements:

1. XValueType is Auto by default, and, if it is not specified explicitly, being resolved to specific type somewhere before Paint event.

   It is not something obvious, I found it through decompiling sources; if you do want to follow my research, here are some milestones on the way:

   • Chart constructor:

     this._dataManager.Initialize();

   • DataManager.Initialize :

     chartImage.BeforePaint += ChartPicture_BeforePaint

   • DataManager.ChartPicture_BeforePaint
   • DataManager.PrepareData
   • Series.PrepareData =>

     if (this._xValueType == ChartValueType.Auto) { this._xValueType = ChartValueType.Double;

2. DataBind will try to resolve Auto to specific type based on DataSource . If it is not Auto , it will not respect data type in DataSource .

Ok, so what's happening in your sample:

1. When you don't use task, whole body of Form_Load executes synchronously; when you call DataBind on chart, it has Auto for XValueType and it is resolved to DateTime based on provide DataSource .

2. When you do use task with sleep, Paint event occurs before you initialize DataSource , which resolves XValueType to Double . When after that you bind DataSource, it uses float representation of DateTime .

How to fix all that mess? A lot of ways, but all of them based on chart value type:

• Specify value type explicitly in Form_Load
• Remove series and add new one exactly before DataBind
• Specify value type explicitly before DataBind
• ...