Question

我在列表中使用多个过滤器项目。如果某些过滤字段为空，如何正确跳过某些块？

IEnumerable<MyNewClass> filterResult = itemList.Where(s => s.name == nameFilterTextBox.Text)
                                                       .Where(s => s.description == descriptionFilterTextBox.Text)
                                                       .Where(s => s.color == (MyNewClass.Colors)comboBox1.SelectedIndex).ToList();

Answer 1

这里有一些选择：

您可以使用procedure TForm.btnCMDLaunchClick(Sender: TObject); var commandLine: string; si: TStartupInfo; pi: TProcessInformation; begin commandLine := Format('%s %s',['C:\Program Files (x86)\Adobe\Acrobat 11.0\Acrobat\Acrobat.exe', 'F:\Android-interview\Packt.Android.3.0.Application.Development.Cookbook.Jul.2011.ISBN.1849512949.pdf']); UniqueString(commandLine); si := Default(TStartupInfo); si.cb := sizeof(si); if CreateProcess( PChar(nil), //no module name (use command line) PChar(commandLine), //Command Line nil, //Process handle not inheritable nil, //Thread handle not inheritable False, //Don't inherit handles 0, //No creation flags nil, //Use parent's environment block PChar(nil), //Use parent's starting directory si, //Startup Info pi //Process Info ) then begin CloseHandle(pi.hProcess); CloseHandle(pi.hThread); end; end;并仅有条件地添加if子句;或
如果过滤器字段为空，则可以使用逻辑，以使过滤器为Where。

第一个可以解决如下：

true

后者可以解决如下：更改过滤器：

IEnumerable<MyNewClass> filterResult = itemList;
if(nameFilterTextBox.Text != "") {
    filterResult = filterResult.Where(s => s.name == nameFilterTextBox.Text);
} if(descriptionFilterTextBox.Text != "") {
    filterResult = filterResult.Where(s => s.description == descriptionFilterTextBox.Text)
} if(comboBox1.SelectedIndex != -1) {
    MyNewClass.Colors col = (MyNewClass.Colors)comboBox1.SelectedIndex;
    filterResult = filterResult.Where(s => s.color == col);
}
// do something with filterResult.ToList()

为：

s => some_condition

这样，如果s => filter_field_is_empty || some_condition条件为filter_field_is_empty，则会让元素在过滤器中传递。请注意，这通常会效率低下，因为测试是针对每个元素进行：

true

Answer 2

您可以在string.IsNullOrWhiteSpace子句中使用Where。这是代码：

IEnumerable<MyNewClass> filterResult = 
    itemList.Where(s =>
      (string.IsNullOrWhiteSpace(nameFilterTextBox.Text) || s.name == nameFilterTextBox.Text) &&
      (string.IsNullOrWhiteSpace(descriptionFilterTextBox.Text) || s.description == descriptionFilterTextBox.Text) &&
      s.color == (MyNewClass.Colors)comboBox1.SelectedIndex).ToList();

如何在linq中使用多个where时跳过某些条件

2 个答案: