泛型C＃依賴於類型的方法組織

Question

我試着用一個基本的方法來做出一個基類，該方法用一個依賴於類型的函數來排序列表。

我的編譯器顯示此錯誤

錯誤13 Impossibile轉換 'System.Linq.Expressions.Expression < System.Func < MLOLPlus.Business.Dealer，串> >' 在「System.Linq.Expressions.Expression < System.Func < T，字符串> >'。 d：\ Documenti \ Lavori \的TImage \ MLOLPlus \ SRC \ MLOLPlus.Business \ DataAcess \ DataTablesClasses \ DataTableMultiSort.cs 197個20 MLOLPlus.Business

IdentityEntity是所有自定義類數據類型

的基抽象類基例如：

1. 用戶繼承IdentityEntity
2. 編輯器太

基類MultiSort：

public class DataTableMultiSort 
{ 

    public DataTableParameterModel DataTable { get; set; } 

    public IQueryable<T> MultiSort<T>(IQueryable<T> basequery) where T : IdentityEntity{ 
     return CreateSortedQuery<T>(basequery, DataTable); 
    } 

    private IOrderedQueryable<T> CreateSortedQuery<T>(IQueryable<T> baseQuery, DataTableParameterModel parameterModel) where T : IdentityEntity 
    { 
     var orderedQuery = (IOrderedQueryable<T>)baseQuery; 

     for (int i = 0; i < parameterModel.iSortingCols; ++i) 
     { 
      var ascending = string.Equals("asc", parameterModel.sSortDir[i], StringComparison.OrdinalIgnoreCase); 
      int sortCol = parameterModel.iSortCol[i]; 

      Expression<Func<T, string>> orderByExpression = GetOrderingFunc<T>(sortCol); 
      if (orderByExpression != null) 
      { 
       ...do things 
      } 
      else 
      { 
       if (ascending) 
       { 
        orderedQuery = (i == 0) 
         ? orderedQuery.OrderBy(c => c.Id) 
         : orderedQuery.ThenBy(c => c.Id); 
       } 
       else 
       { 
        ... 
       } 
      } 

     } 
     return orderedQuery; 
    } 

    protected virtual Expression<Func<T, string>> GetOrderingFunc<T>(int ColumnIndex) where T : IdentityEntity 
    { 
     return null; 
    } 

} 

自定義用戶Multisort

public class UserMultiSort : DataTableMultiSort 
    { 
     protected override Expression<Func<T, string>> GetOrderingFunc<T>(int ColumnIndex) 
     { 
      Expression<Func<User, string>> InitialorderingFunction; 
      switch (ColumnIndex) 
      { 
       case 1: 
        InitialorderingFunction = c => c.FirstName; 
        break; 
       case 2: 
        InitialorderingFunction = c => c.LastName; 
        break; 
       default: 
        InitialorderingFunction = null; 
        break; 
      } 

      return InitialorderingFunction; 
     } 

    } 

自定義編輯器Multisort

public class EditorMultiSort : DataTableMultiSort 
{ 
    protected override Expression<Func<T, string>> GetOrderingFunc<T>(int ColumnIndex) 
    { 
     Expression<Func<Editor, string>> InitialorderingFunction; 
     switch (ColumnIndex) 
     { 
      case 1: 
       InitialorderingFunction = c => c.BusinessName; 
       break; 
      case 2: 
       InitialorderingFunction = c => c.Address; 
       break; 
      default: 
       InitialorderingFunction = null; 
       break; 
     } 

     return InitialorderingFunction; 
    } 

} 

Answer 1

你得到編譯器錯誤的原因是你已經推動了通用約束到單獨的方法，而不是將約束放在類上。您擁有它的方式無法使用返回Expression<Func<User, string>>的方法覆蓋內在通用方法GetOrderingFunc<T>。

你張貼什麼作品，如果你讓DataTableMultiSort通用，並採取通用paramteres關閉的方法：

public class DataTableMultiSort<T> where T : IdentityEntity 
{ 
... 
    protected virtual Expression<Func<T, string>> GetOrderingFunc(int ColumnIndex) 
    { 
     return null; 
    } 
} 

public class UserMultiSort : DataTableMultiSort<User> 
{ 
    protected override Expression<Func<User, string>> GetOrderingFunc(int ColumnIndex) 
    { 
     Expression<Func<User, string>> InitialorderingFunction; 
     ... 
    } 
}