Question

我有下面的Expression Func，它接收两个输入参数，第一个是Person对象，第二个是bool并返回另一种类型的对象PersonProfile

private Exression<Func<Person, bool, PersonProfile>> PersonProfileProjection => (person, isValid) =>
        new PersonProfile
        {
             FirstName = person.FirstName,
             HasAddress = isValid ? person.Address1 : null
        };

并且我试图在从dbContext中获取Person表时调用它。

_dbContext.Persons.Select(PersonProfileProjection);

我很困惑如何在PersonProfileProjection内部发送布尔参数。当我只这样输入一个输入和一个输出参数时，它就可以工作。但是我也想要额外的布尔输入。

任何帮助将不胜感激。

Answer 1

您可以按照Microsoft文档的说明进行操作：Expression Class

为SQLite创建的一个示例，显示了上述功能的用法。

public void GetData()
    {
        var connection = new SQLiteConnection(@"Data Source=database.sqlite;Version=3;");
        var context = new DataContext(connection);

        connection.Open();

        var createtableQuery = @"
                                drop table Company;
                                CREATE TABLE[Company]
                                (
                                    [ID] INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT UNIQUE,
                                    [Seats] INTEGER NOT NULL
                                );
                                ";

        var command = new SQLiteCommand(createtableQuery, connection);
        command.ExecuteNonQuery();
        Company com = new Company()
        {
            Id = 6,
            Seats = 7
        };

        context.GetTable<Company>().InsertOnSubmit(com);
        context.SubmitChanges();

        var companies = context.GetTable<Company>();

        foreach (var company in companies)
        {
            Console.WriteLine("Company: {0} {1}",
                company.Id, company.Seats);
        }

        //compile Expression using Compile method to invoke it as Delegate
        Func<int,int, Company> PersonProfileProjectionComp = PersonProfileProjection.Compile();

        var dd = companies.Select(p => PersonProfileProjectionComp(p.Id,p.Seats));

      //// Below line inline use. Both works.
      //var dd = companies.Select(p => PersonProfileProjection.Compile().Invoke(p.Id,p.Seats));
    }

    private System.Linq.Expressions.Expression<Func<int, int, Company>> PersonProfileProjection => (person, seats) =>
    new Company
    {
        Id = person,
        Seats = seats
    };

或一行使用： PersonProfileProjection.Compile（）。Invoke（人，isValid）

Answer 2

您可以将其声明为Func而不是表达式：

private Func<Person, bool, PersonProfile> PersonProfileProjection => (person, isValid) =>
        new PersonProfile
        {
            FirstName = person.FirstName,
            HasAddress = isValid // do what you meant to do
        };

...，并将其称为：

_dbContext.Persons.Select(p => PersonProfileProjection(p, true));

您还可以编写一个普通方法：

private PersonProfile PersonProfileProjection(Person person, bool isValid)
{
    return new PersonProfile
    {
        FirstName = person.FirstName,
        HasAddress = isValid // do what you want to do
    };
}

...并以相同的方式调用它：

_dbContext.Persons.Select(p => PersonProfileProjection(p, true));

如何使用两个输入参数调用Expression Func

2 个答案: