C# Winform父窗体打开新的子窗体前，关闭其他子窗体

C:\Users\ufo19\Desktop\Winform\WindowsFormsApp1\WindowsFormsApp1\bin\Debug\WindowsFormsApp1.exe.config C:\Users\ufo19\Desktop\Winform\WindowsFormsApp1\WindowsFormsApp1\bin\Debug\WindowsFormsApp1.exe C:\Users\ufo19\Desktop\Winform\WindowsFormsApp1\WindowsFormsApp1\bin\Debug\WindowsFormsApp1.pdb C:\Users\ufo19\Desktop\Winform\WindowsFormsApp1\WindowsFormsApp1\obj\Debug\WindowsFormsApp1.csproj.AssemblyReference.cache C:\Users\ufo19\Desktop\Winform\WindowsFormsApp1\WindowsFormsApp1\obj\Debug\WindowsFormsApp1.Properties.Resources.resources C:\Users\ufo19\Desktop\Winform\WindowsFormsApp1\WindowsFormsApp1\obj\Debug\WindowsFormsApp1.csproj.GenerateResource.cache C:\Users\ufo19\Desktop\Winform\WindowsFormsApp1\WindowsFormsApp1\obj\Debug\WindowsFormsApp1.csproj.CoreCompileInputs.cache C:\Users\ufo19\Desktop\Winform\WindowsFormsApp1\WindowsFormsApp1\obj\Debug\WindowsFormsApp1.exe C:\Users\ufo19\Desktop\Winform\WindowsFormsApp1\WindowsFormsApp1\obj\Debug\WindowsFormsApp1.pdb C:\Users\ufo19\Desktop\Winform\WindowsFormsApp1\WindowsFormsApp1\obj\Debug\WindowsFormsApp1.MainForm.resources C:\Users\ufo19\Desktop\Winform\WindowsFormsApp1\WindowsFormsApp1\obj\Debug\WindowsFormsApp1.UserControl1.resources C:\Users\ufo19\Desktop\Winform\WindowsFormsApp1\WindowsFormsApp1\obj\Debug\WindowsFormsApp1.UserControl2.resources C:\Users\ufo19\Desktop\Winform\WindowsFormsApp1\WindowsFormsApp1\obj\Debug\WindowsFormsApp1.Form1.resources C:\Users\ufo19\Desktop\Winform\WindowsFormsApp1\WindowsFormsApp1\obj\Debug\WindowsFormsApp1.Form2.resources