在 switch 语句中为 LINQ 创建 where 条件

Question

所以基本上我必须为 4 个不同的角色获得 4 个不同的员工列表。 List of Roles 是 List of Enum 如下：

public enum Roles {
       [Description("Level 1")]
       L1,

       [Description("Level 2")]
       L2,

       [Description("Level 3")]
       L3,

       [Description("Level 4")]
       L4
};

• L1可以查看L1,L2,L3和L4
• L2可以查看L2,L3和L4
• L3 可以查看L3 和L4。
• L4 无权查看任何员工。

下面是我为获取员工编写的查询。

if (!Equals(Role, Enum.GetName(typeof(GlobalMethods.Roles), 3)))
{
      var _role = (GlobalMethods.Roles)Enum.Parse(typeof(GlobalMethods.Roles), Role,true);
      List<EmployeeViewModel> employees = new List<EmployeeViewModel>();
      switch (_role)
      {
            case GlobalMethods.Roles.L1:
                 employees = context.tblEmployees.Where(x => x.EmpID != homeViewModel.UserViewModel.EmpID).ToList().Select(x => new EmployeeViewModel
                 {
                      EmpActive = x.EmpActive,
                      EmpDOB = x.EmpDOB,
                      EmpName = x.EmpName
                 }).ToList();
                 break;
            case GlobalMethods.Roles.L2:
                 employees= context.tblEmployees.Where(x => x.EmpID != homeViewModel.UserViewModel.EmpID).ToList().Where(x=>x.EmpRole != Enum.GetName(typeof(GlobalMethods.Roles), 0)).Select(x => new EmployeeViewModel
                 {
                      EmpActive = x.EmpActive,
                      EmpDOB = x.EmpDOB,
                      EmpName = x.EmpName,
                 }).ToList();
                 break;
            case GlobalMethods.Roles.L3:
                 employees = context.tblEmployees.Where(x => x.EmpID != homeViewModel.UserViewModel.EmpID).ToList().Where(x.EmpRole != Enum.GetName(typeof(GlobalMethods.Roles), 0) && x.EmpRole != Enum.GetName(typeof(GlobalMethods.Roles), 1)).Select(x => new EmployeeViewModel
                 {
                      EmpActive = x.EmpActive,
                      EmpDOB = x.EmpDOB,
                      EmpName = x.EmpName,
                 }).ToList();
                 break;
            default: break;
      }
}

所以在上面的代码中，我有相同的 LINQ 查询，但只有 where 条件不同。有什么方法可以准备一个动态的where 条件并一次性获取列表？

Answer 1

这应该可行。它假设每个 GlobalMethods.Roles 可以查看具有更大 int 值的所有角色。角色就是你所说的规则。

if (!Equals(Role, Enum.GetName(typeof(GlobalMethods.Roles), 3)))
{
     var _role = (GlobalMethods.Roles)Enum.Parse(typeof(GlobalMethods.Roles), Role, true);
     List<EmployeeViewModel> employees = new List<EmployeeViewModel>();
     string[] viewableRoles = GetViewableRoles(_role);
     employees = context.tblEmployees
                .Where(x => x.EmpID != homeViewModel.UserViewModel.EmpID && viewableRoles.Contains(x.EmpRole))
                .Select(x => new EmployeeViewModel
                {
                    EmpActive = x.EmpActive,
                    EmpDOB = x.EmpDOB,
                    EmpName = x.EmpName
                }).ToList();
}

private string[] GetViewableRoles(GlobalMethods.Roles userRole)
{
    //Uncomment if L4 can actually view no roles, including itself.
    // /if (userRole == GlobalMethods.Roles.L4)
    // {
    //  return new string[0];
    // }

    IEnumerable<GlobalMethods.Roles> allRoles = Enum.GetValues(typeof(GlobalMethods.Roles)).Cast<GlobalMethods.Roles>();
    return (from role in allRoles 
                where (int)role >= (int)userRole 
                select role.ToString()).ToArray();
}

Answer 2

这是简化的，但你可以这样做：

class Program
{
    static void Main(string[] args)
    {
        var list = new List<Person>
        {
            new Person{ Name = "John" },
            new Person{ Name = "Amy" }
        };

        Func<Person, bool> pred = null;
        Roles role = Roles.RoleA;
        switch (role)
        {
            case Roles.RoleA:
                pred = p => p.Name.StartsWith("J");
                break;
            case Roles.RoleB:
                pred = p => p.Name.StartsWith("A") && p.Name.Length >= 3;
                break;
            default:
                break;
        }

        var result = list.Where(pred);
    }
}

class Person
{
    public string Name { get; set; }
}

enum Roles
{
    RoleA,
    RoleB
}

Answer 3

以下是通用表达式树扩展方法，以照顾以下要求（您当然需要修改以适应您的具体要求）

• T 是 IEnumerable 的最终类型

• T1是where子句中过滤的列类型

• 在编译Expression Tree to Func时，为给定类型T声明ParameterExpression以供重用，尽管这也可以在Extension method和Func生成期间单独完成

  ParameterExpression parameterType = Expression.Parameter(typeof(T), "object");

  public static class CustomExpression
  {
   // Create Initial Expression Tree
   public static BinaryExpression InitialExpression<T,T1>(
                                                          ParameterExpression parameterType
                                                          string columnName,
                                                          T1 value)
  {
      // Optional can be taken outside the Extension method to create a Func<T,bool>
  
      //ParameterExpression parameterType = Expression.Parameter(typeof(T), "object");
  
      MemberExpression typeColumn = Expression.Property(parameterType, columnName);
  
      ConstantExpression constant = Expression.Constant(value, typeof(T1));
  
      return Expression.NotEqual(typeColumn, constant);
  }
  
  // Create Combined Expression Tree
  public static BinaryExpression CombinedExpression<T,T1>(this BinaryExpression mainExpression,
                                                          ParameterExpression parameterType
                                                          string columnName,
                                                          T1 value)
  {
      // Optional can be taken outside the Extension method to create a Func<T,bool>
  
      //ParameterExpression parameterType = Expression.Parameter(typeof(T), "object");
  
      MemberExpression typeColumn = Expression.Property(parameterType, columnName);
  
      ConstantExpression constant = Expression.Constant(value, typeof(T1));
  
      return Expression.And(mainExpression,Expression.NotEqual(typeColumn, constant));
  }
  }
  

以下是您案例中的调用层次结构：

• 声明Linq 查询的最终二进制表达式：

  BinaryExpression finalBinaryExpression = null;

   switch (_role)
    {
          case GlobalMethods.Roles.L1:
            finalBinaryExpression =          
            CustomExpression.InitialExpression<EmployeeViewModel,int>     
            (parameterType,"EmpID",homeViewModel.UserViewModel.EmpID);
           break;
          case GlobalMethods.Roles.L2:
            finalBinaryExpression =     
            CustomExpression.InitialExpression<EmployeeViewModel,int>               
            (parameterType,"EmpID",homeViewModel.UserViewModel.EmpID)
            .CombinedExpression<EmployeeViewModel,Roles>                    
        (parameterType,"EmpRole",Enum.GetName(typeof(GlobalMethods.Roles), 0));
           break;
          case GlobalMethods.Roles.L3:
              finalBinaryExpression =       
              CustomExpression.InitialExpression<EmployeeViewModel,int>
              (parameterType,"EmpID",homeViewModel.UserViewModel.EmpID)
              .CombinedExpression<EmployeeViewModel,Roles>
  
        (parameterType,"EmpRole",Enum.GetName(typeof(GlobalMethods.Roles), 0))
              .CombinedExpression<EmployeeViewModel,Roles>  
  
        (parameterType,"EmpRole",Enum.GetName(typeof(GlobalMethods.Roles), 1));
           break;
      }
  

• 通过如下编译二进制表达式创建genericFunc

  Func<T, bool> filterFunc = Expression.Lambda<Func<T, bool>>
                             (finalBinaryExpression, parameterType).Compile();
  

• 最终结果应用上面创建的Func

    var finalResult = context.tblEmployees.Where(o => filterFunc(o));