FormStartPosition.CenterParent does not work

问题

In the following code, only the second method works for me (.NET 4.0). FormStartPosition.CenterParent does not center the child form over its parent. Why?

Source: this SO question

using System;
using System.Drawing;
using System.Windows.Forms;

class Program
{
  private static Form f1;

  public static void Main()
  {
    f1 = new Form() { Width = 640, Height = 480 };
    f1.MouseClick += f1_MouseClick; 
    Application.Run(f1);
  }

  static void f1_MouseClick(object sender, MouseEventArgs e)
  {
    Form f2 = new Form() { Width = 400, Height = 300 };
    switch (e.Button)
    {
      case MouseButtons.Left:
      {
        // 1st method
        f2.StartPosition = FormStartPosition.CenterParent;
        break;
      }
      case MouseButtons.Right:
      {
        // 2nd method
        f2.StartPosition = FormStartPosition.Manual;
        f2.Location = new Point(
          f1.Location.X + (f1.Width - f2.Width) / 2, 
          f1.Location.Y + (f1.Height - f2.Height) / 2
        );
        break;
      }
    }
    f2.Show(f1); 
  }
}

回答1:

This is because you are not telling f2 who its Parent is.

If this is an MDI application, then f2 should have its MdiParent set to f1.

Form f2 = new Form() { Width = 400, Height = 300 };
f2.StartPosition = FormStartPosition.CenterParent;
f2.MdiParent = f1;
f2.Show();

If this is not an MDI application, then you need to call the ShowDialog method using f1 as the parameter.

Form f2 = new Form() { Width = 400, Height = 300 };
f2.StartPosition = FormStartPosition.CenterParent;
f2.ShowDialog(f1);

Note that CenterParent does not work correctly with Show since there is no way to set the Parent, so if ShowDialog is not appropriate, the manual approach is the only viable one.

回答2:

If you set the owner of the child form like so:

Form2 f = new Form2();
f.Show(this);

You can then center it easily like this:

Form2_Load(object sender, EventArgs e)
{
    if (Owner != null)
        Location = new Point(Owner.Location.X + Owner.Width / 2 - Width / 2,
            Owner.Location.Y + Owner.Height / 2 - Height / 2);
}

回答3:

I'm using this code inside my main form, hope it helps:

var form = new MyForm();
form.Show();
if (form.StartPosition == FormStartPosition.CenterParent)
{
    var x = Location.X + (Width - form.Width) / 2;
    var y = Location.Y + (Height - form.Height) / 2;
    form.Location = new Point(Math.Max(x, 0), Math.Max(y, 0));
}

回答4:

I found setting the location manually is the only reliable option in some more complex cases when form is auto-sized and dynamically modified.

However rather than computing the coordinates manually, I'd suggest using existing method:

this.CenterToParent();

回答5:

I found a solution that will center modeless window position to parent's position, and the child window can be still covered by parent window. You just have to call

f2.Show(f1);

which will set f2 owner to f1, f2 will show over the f1 at it's center position.

Next you set

f2.Owner = null;

and there you go, f2 is a separate window, with correct startup position.

回答6:

I had the same problem, I eventually went with this:

protected override void OnActivated(EventArgs e) {
    if(this.Modal == false && this.StartPosition == FormStartPosition.CenterParent) {
        if(!(this.Owner is Form)) {
            // Center to the last form opened before this one
            int numforms = Application.OpenForms.Count;
            this.Owner = Application.OpenForms[numforms - 2];
        }
        this.CenterToParent();
        Application.DoEvents();
    }
    base.OnActivated(e);
}

Used as:

MyForm form = new MyForm();
form.Show(this); // with or without

The main advantage is that it does what your colleagues expect it to do, without requiring any hack in the calling form.

回答7:

I realize this is an old question, but I was recently having the same problem and for reasons I won't get in to, I did not want to use the form.ShowDialog() method and my application was not an MDI application, therefore the CenterParent method was not having any effect. This is how I solved the problem, by computing the coordinates for the form that I wanted centered and triggering the new location in the main form's LocationChanged event. Hopefully this will help someone else having this problem.

In the example below, the parent form is called MainForm and the form I want centered in MainForm is called pleaseWaitForm.

private void MainForm_LocationChanged(object sender, EventArgs e)
    {
        Point mainFormCoords = this.Location;
        int mainFormWidth = this.Size.Width;
        int mainFormHeight = this.Size.Height;
        Point mainFormCenter = new Point();
        mainFormCenter.X = mainFormCoords.X + (mainFormWidth / 2);
        mainFormCenter.Y = mainFormCoords.Y + (mainFormHeight / 2);
        Point waitFormLocation = new Point();
        waitFormLocation.X = mainFormCenter.X - (pleaseWaitForm.Width / 2);
        waitFormLocation.Y = mainFormCenter.Y - (pleaseWaitForm.Height / 2);
        pleaseWaitForm.StartPosition = FormStartPosition.Manual;
        pleaseWaitForm.Location = waitFormLocation;           
    }

If you have a resizable parent form and you wanted your sub form to also be centered whenever the main form is resized, you should, in theory, be able to place this code in a method and then call the method on both the LocationChanged and SizeChanged events.

回答8:

JYelton's answer worked for me, but the form is only centered the first time Show() is called. If you want to Hide() the form, and then have it re-centered on the parent every time Show() is called you need use the following in your form:

public new void Show(IWin32Window owner)
{
    base.Show(owner);

    if (Owner != null)
        Location = new Point(Owner.Location.X + Owner.Width / 2 - Width / 2,
            Owner.Location.Y + Owner.Height / 2 - Height / 2);
}

回答9:

Maybe this can help somebody.

Form frmMessage = new Form();

From experience, although they look similar, they behave different:

This variant doesn't work:

if (frmMessage.Parent != null)
    frmMessage.CenterToParent();
else
    frmMessage.CenterToScreen();

And this variant works

if (frmMessage.Parent != null)
    frmMessage.StartPosition = FormStartPosition.CenterParent;
else
    frmMessage.StartPosition = FormStartPosition.CenterScreen;

回答10:

Using

form.Show(this);

throws an exception if you call it a second time. Manually setting the location seems to be the only reliable option :/ (wasn't it fairly recently that CenterParent used to work?)

回答11:

Small Change to JYelton's answer

Form2_Load(object sender, EventArgs e)
{
    if (Owner != null && Parent == null && StartPosition == FormStartPosition.CenterParent)
    Location = new Point(Owner.Location.X + Owner.Width / 2 - Width / 2,
        Owner.Location.Y + Owner.Height / 2 - Height / 2);
}

回答12:

An old question, I know, but I had the same issue but for a different reason.

The Form I was opening had an overridden OnLoad method:

protected override void OnLoad(EventArgs e)
{
   //... etc.
}

but was not calling the base implementation as it must do:

protected override void OnLoad(EventArgs e)
{
   //... etc.
   base.OnLoad(e);
}

When overriding OnLoad(EventArgs) in a derived class, be sure to call the base class's OnLoad(EventArgs) method so that registered delegates receive the event.

回答13:

just put the code in the constructor of your form.

    public FrmSample()
    {
        InitializeComponent();

        // must be after the InitializeComponent()
        this.StartPosition = FormStartPosition.CenterParent;
    }

来源：https://stackoverflow.com/questions/8567058/formstartposition-centerparent-does-not-work

标签

winforms

forms

parent-child

centering