在Linq中使用多个where时如何跳过某些条件

Question

I'm using multiple where filter items in my list. How to properly skip some where blocks, if some filter fields are empty?

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

There are some options here:

• you can use an if and only conditionally add a Where clause; or
• you can use logic such that the filter is true if the filter field is empty.

The first one can be resolved like:

IEnumerable<MyNewClass> filterResult = itemList;

    Where(s => s.name == nameFilterTextBox.Text);

    Where(s => s.description == descriptionFilterTextBox.Text)

    MyNewClass.Colors col = (MyNewClass.Colors)comboBox1.SelectedIndex;
    Where(s => s.color == col);

// do something with filterResult.ToList()

The latter can be solved as follows: you change the filter:

s => some_condition

to:

s => filter_field_is_empty || some_condition

Such that if the filter_field_is_empty condition is true , it will thus let the element pass in the filter. Note that this will usually be less efficient because the test is done for every element :

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 2

You can use string.IsNullOrWhiteSpace in the Where clause. Here is the code:

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();