Question

    public Form1()
    {
        InitializeComponent();
        comboBox1.DisplayMember = "Name";
        comboBox1.ValueMember = "ID";  
        LoadUsersToComboBox();
    }

    PersonRepository peopleRepo = new PersonRepository();

    private void LoadUsersToComboBox()
    {
        comboBox1.DataSource = peopleRepo.FindAllPeople().ToList();          
    }

    private void button2_Click(object sender, EventArgs e)
    {                        
        LoadUsersToComboBox();
    }

此方法将首次加载一个值仅的组合框，但不会在后续尝试中加载：

    private void LoadUsersToComboBox()
    {
        comboBox1.DataSource = peopleRepo.FindAllPeople(); /*Return IQueryable<Person>*/
    }

每次调用LoadUsersToComboBox（）时都会加载：

    private void LoadUsersToComboBox()
    {
        comboBox1.DataSource = peopleRepo.FindAllPeople().ToList();
    }

为什么第一次只加载第一次？

以下是PeopleRepository类的代码：

namespace SQLite_Testing_Grounds
{
    public class PersonRepository
    {
        private ScansEntities3 db = new ScansEntities3();

        public IQueryable<Person> FindAllPeople()
        {
            return db.People;
        }
    }
}

Answer 1

解决方案很简单：

// This method returns the same reference every time
public IQueryable<Person> FindAllPeople()
{
    return db.People;
}

结果：

// Nothing changes, DataSource old value is still the same (same reference,
// even is the content of the People list does change).
comboBox1.DataSource = peopleRepo.FindAllPeople();

// ToList() creates a new object each time, so DataSource is assigned to a 
// NEW object, and so calls a kind of invalidation of its visual.
comboBox1.DataSource = peopleRepo.FindAllPeople().ToList();

。
这是一个数据绑定的基础，实际上我根本不是一个WinForms人（我对WPF了解得更多），但我认为在内部，你有一些类似的东西：

private object dataSource;
public object DataSource {
    get {
        if (value != dataSource) {
            dataSource = value;
            RaisePropertyChanged("DataSource");
        }
    }
}

Answer 2

在此调用中，您要设置数据源，而不是数据绑定调用以实际绑定数据。你能发布调用.databind吗？

的方法

为什么这样做有效呢？

2 个答案: