C# 防止重复的 MDI 子表格

Prevent duplicate MDI children forms

提问by user

Is there a way to prevent the opening of a certain form within an MDI container if that said form is already opened?

如果该表单已经打开，是否有办法防止在 MDI 容器中打开某个表单？

采纳答案by Fredrik M?rk

You can interate over the OpenForms collection to check if there is already a form of the given type:

您可以对 OpenForms 集合进行交互以检查是否已经存在给定类型的表单：

foreach (Form form in Application.OpenForms)
{
    if (form.GetType() == typeof(MyFormType))
    {
        form.Activate();
        return;
    }
}

Form newForm = new MyFormType();
newForm.MdiParent = this;
newForm.Show();

回答by Vilx-

AFAIK there is no standard way. You'll have to implement it yourself. I'd do it this way:

AFAIK 没有标准的方法。你必须自己实现它。我会这样做：

class TheForm: Form
{
    private static TheForm Instance;

    private TheForm() // Constructor is private
    {
    }

    public static Show(Form mdiParent)
    {
        if ( Instance == null )
        {
            // Create new form, assign it to Instance
        }
        else
            Instance.Activate(); // Not sure about this line, find the appropriate equivalent yourself.
    }

    protected override OnFormClose(EventArgs e)
    {
        Instance = null;
        base.OnFormClose(e);
    }
}

If thread safety is of concern, add the appropriate locks.

如果关注线程安全，请添加适当的locks。

回答by user3163424

this code working

此代码工作

    private void openToolStripMenuItem_Click(object sender, EventArgs e)
    {
        foreach (Form form in Application.OpenForms)
        {


            if (form.GetType() == typeof(Form2))
            {
                form.Activate();
                return;
            }
        }

        Form2 newForm = new Form2();
        newForm.MdiParent = this;
        newForm.Show();
    }

回答by Avijit

Though this post is very old, I thought this will add a help.

虽然这篇文章很旧，但我认为这会增加帮助。

Need to handle if form is Minimized too. Here is the complete example:

如果表单也被最小化，也需要处理。这是完整的示例：

foreach (Form form in this.MdiChildren)
{
    if (form.GetType() == typeof(frmMain))
    {
        if (form.WindowState == FormWindowState.Minimized)
        {
            form.WindowState = FormWindowState.Normal;
        }
        form.Activate();
        return;
    }
}
Form frm = new frmMain();
frm.MdiParent = this;
frm.Show();

回答by destinydz

This code work for me in vb.net

这段代码在 vb.net 中对我有用

 


For Each f As Form In Application.OpenForms
            If TypeOf f Is form_name Then
                f.Activate()
                f.WindowState = FormWindowState.Normal
                f.StartPosition = FormStartPosition.WindowsDefaultLocation
                f.WindowState = FormWindowState.Maximized

            Return
        End If    
    Next 
    form_name .MdiParent = Me
    form_name .Show()

回答by Dr Yunke

A method can be implemented using Generics (below C# and VB.net options), which can be useful if different MDI Forms need to be opened.

可以使用泛型（低于 C# 和 VB.net 选项）实现方法，如果需要打开不同的 MDI 表单，这会很有用。

C#

C＃

private void OpenMDI<T>(bool multipleInstances)
    where T : Form, new()
{
    if (multipleInstances == false)
    {
        // Look if the form is open
        foreach (Form f in this.MdiChildren)
        {
            if (f.GetType() == typeof(T))
            {
                // Found an open instance. If minimized, maximize and activate
                if (f.WindowState == FormWindowState.Minimized)
                {
                    f.WindowState = FormWindowState.Maximized;
                }

                f.Activate();
                return;
            }
        }
    }

    T newForm = new T();
    newForm.MdiParent = this;
    newForm.Show();
}

Use it as follows (indicate falsein multipleInstancesto prevent them)

如下使用它（指出false的multipleInstances，以防止它们）

OpenMDI<Form2>(false);

---

---

VB.NET

网络

Public Sub Open_MDI(Of T As {New, Form})(bMultipleInstances As Boolean)
    If bMultipleInstances = False Then
        For Each f As Form In Me.MdiChildren
            If TypeOf f Is T Then
                If (f.WindowState = FormWindowState.Minimized) Then
                    f.WindowState = FormWindowState.Maximized;
                End If

                f.Activate()
                Exit Sub
            End If
        Next
    End If

    Dim myChild As New T()
    myChild.MdiParent = Me
    myChild.Show()
End Sub

Use it as follows (indicate Falsefor bMultipleInstancesto prevent them)

按如下方式使用（指示False为bMultipleInstances防止它们）

Open_MDI(Of Form2)(False)

回答by mamal

This code work for me in C#

这段代码在 C# 中对我有用

      private void btn1_Click(object sender, EventArgs e)
            {
               Form2 new_form = new Form2();
                if (new_form.visible)
                    new_form.Show();
                else
                    new_form.ShowDialog();
            }