Question

考虑一个简单的Employee类。

Class Employee
{
  public String FirstName {get; set;}
  public int Id {get; set;}
  public int Marks {get; set;}
  public String LastName {get; set;}
}

因此，通过LINQ查询进行选择，我们可以编写。

var query = Employee.Where(i => i.Id > 2).OrderBy(i => i.Marks);

所以我们可以创建一个带2参数的函数，我们可以发送Id和Marks。这样我就可以进行函数调用并传递我需要的参数。

var query = Employee.Where(i => i.Marks > 2).OrderBy(i => i.FirstName);

示例查看功能，我们可以传递任何参数

Public String GetQuery(String para1,String para2,......)
{
  var query = Employee.Where(i => i.para1 > 2).OrderBy(i => i.para2); 
  return query;
}
   or

Public String GetQuery(String para1,String para2,......)
{
  String str1=para1...... // with some format included
  String str2=para2...... // with some format included

  var query = Employee.Where(str1).OrderBy(str2); 
  return query;
}

这个概念是我想创建一个公共（通用）查询，我可以在其中选择值传递的任何类型的参数。

Answer 1

类型安全的解决方案可以使用委托而不是字符串：

IEnumerable<Employee> employees = getExampleData();

IEnumerable<Employee> example1 = Query.Employees(employees, Query.UseID, Query.UseMarks);
IEnumerable<Employee> example2 = Query.Employees(employees, Query.UseMarks, Query.UseFirstName);

我创建了一个帮助类Query来包装功能：

static class Query
{
    public static int UseID(Employee employee, int i) { return employee.Id; }
    public static int UseMarks(Employee employee, int i) { return employee.Marks; }
    public static string UseFirstName(Employee employee, string s) { return employee.FirstName; }
    public static string UseLastName(Employee employee, string s) { return employee.LastName; }

    static public IEnumerable<Employee> Employees(IEnumerable<Employee> employees, func_returnInt where, func_returnInt orderby)
    {
        return employees.Where(i => where(i, 0) > 2).OrderBy(i => orderby(i, 0));
    }
    static public IEnumerable<Employee> Employees(IEnumerable<Employee> employees, func_returnInt where, func_returnString orderby)
    {
        return employees.Where(i => where(i, 0) > 2).OrderBy(i => orderby(i, ""));
    }
}

public delegate int func_returnInt(Employee employee, int i);
public delegate string func_returnString(Employee employee, string s);

Answer 2

您必须使用Expression：LambdaExpression。

http://msdn.microsoft.com/en-us/library/bb335710.aspx

在我的代码中，我使用了类似的东西：

private User GetUser(Expression<Func<User, bool>> query)
 {
    User user = context.Users.Where(query).FirstOrDefault();
    if (user == null)
    {
         throw new ProviderException("The supplied user name could not be found.");
    }
    return user;
 }

用户user = GetUser（u =＆gt; u.Marks == 123）;

用户user = GetUser（u =＆gt; u.FirstName ==“abc”）;

从参数构建LINQ查询

2 个答案: