Python CLR WinForms从现有Form打开新窗体报错问题问询

错误原因 #

InvalidOperationException : Starting a second message loop on a single thread is not a valid operation. Use Form.ShowDialog instead.报错的核心逻辑是WinForms的UI线程同一时间仅允许运行一个消息循环，程序启动时调用的Application.Run(MyForm())已经初始化了全局消息循环，按钮点击事件中再次调用Application.Run()属于非法操作。

解决方案 #

自定义窗体类调用ShowDialog不生效通常是用法错误或实例被GC回收导致，按照实际需求选择对应方案修改即可：

场景1：弹出模态窗体（弹出后无法操作原窗体，需关闭新窗体后才可操作原窗体） #

直接修改按钮点击事件逻辑，替换Application.Run调用为ShowDialog()：

def Add_Control_Limits_Form_Click(self, sender, args):
    # 先实例化自定义的第二个窗体
    second_form = MySecondForm()
    # 模态弹出，不需要额外持有引用也能正常运行
    second_form.ShowDialog()

场景2：弹出非模态窗体（两个窗体可同时操作互不阻塞） #

使用Show()方法，同时要将新窗体实例存为原窗体的属性，避免局部变量在方法执行完成后被Python GC回收导致窗体闪退：

def Add_Control_Limits_Form_Click(self, sender, args):
    # 将新窗体实例存为当前窗体的属性，延长生命周期
    self.second_form = MySecondForm()
    # 非模态弹出
    self.second_form.Show()

修改后的完整可运行代码 #

import clr

clr.AddReference("System.Windows.Forms")
clr.AddReference("System.Drawing")
clr.AddReference("System.ComponentModel")

from System.Windows.Forms import *
from System.Drawing import *

class MyForm(Form):
    def __init__(self):
        self.StartPosition = FormStartPosition.CenterScreen

        self.btnTest = Button()
        self.btnTest.Name = 'btnTest'
        self.btnTest.Text = "Test"
        self.btnTest.Size = Size(80, 30)

        self.Controls.Add(self.btnTest)
        self.btnTest.Click += self.Add_Control_Limits_Form_Click

    def Add_Control_Limits_Form_Click(self, sender, args):
        # 模态弹窗写法二选一
        # second_form = MySecondForm()
        # second_form.ShowDialog()
        
        # 非模态弹窗写法二选一
        self.second_form = MySecondForm()
        self.second_form.Show()

class MySecondForm(Form):   
    def __init__(self2):
        self2.StartPosition = FormStartPosition.CenterScreen
        self2.AutoScaleMode = AutoScaleMode.Font
        self2.ClientSize = Size(800, 450)
        self2.Text = "Form2"

Application.EnableVisualStyles()
Application.SetCompatibleTextRenderingDefault(False)

Application.Run(MyForm())

内容的提问来源于stack exchange，提问作者EricALionsFan