TreeView的CheckBoxs的第三种状态（有部分子结点被选中）

C:\MyProject\TreeView\TreeView\bin\Debug\TreeView.exe C:\MyProject\TreeView\TreeView\bin\Debug\TreeView.pdb C:\MyProject\TreeView\TreeView\obj\Debug\ResolveAssemblyReference.cache C:\MyProject\TreeView\TreeView\obj\Debug\TreeView.Form1.resources C:\MyProject\TreeView\TreeView\obj\Debug\TreeView.Properties.Resources.resources C:\MyProject\TreeView\TreeView\obj\Debug\TreeView.csproj.GenerateResource.Cache C:\MyProject\TreeView\TreeView\obj\Debug\TreeView.exe C:\MyProject\TreeView\TreeView\obj\Debug\TreeView.pdb