防止重复的 MDI 子表单

Question

如果 MDI 容器中的某个表单已经打开，是否有办法阻止打开该表单？

Answer 1

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();

Answer 2

据我所知，没有标准方法。您必须自己实施它。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);
    }
}

如果担心线程安全，请添加适当的

lock

。

Answer 3

此代码有效

    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();
    }

Answer 4

虽然这篇文章很旧，但我认为这会增加帮助。

如果表单也最小化，则需要处理。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();

Answer 5

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()

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

Answer 6

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

C#

如下使用（在

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();
}

中注明

false

以防止它们）

multipleInstances

---

VB.NET

OpenMDI<Form2>(false);

按如下方式使用（为

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

标出

False

以防止它们）

bMultipleInstances

Answer 7

这段代码在 C# 中对我有用

Open_MDI(Of Form2)(False)

Answer 8

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

{ 静态类程序 { [DllImport("user32.dll")] [返回：MarshalAs(UnmanagedType.Bool)] 私有静态 extern bool SetForegroundWindow(IntPtr hWnd);

namespace YourWinFormsAppNamespace

}