Question

我尝试使用基本方法创建一个基类，该方法使用依赖于Type的函数对List进行排序。

我的编译器显示此错误

错误13 Impossibile转换'System.Linq.Expressions.Expression＆lt; System.Func＆lt; MLOLPlus.Business.Dealer，string＆gt;＆gt;'在'System.Linq.Expressions.Expression＆lt; System.Func＆lt; T，string＆gt;＆gt;'中。 D：\ Documenti \ Lavori \ timage \ MLOLPlus \ src \ MLOLPlus.Business \ DataAcess \ DataTablesClasses \ DataTableMultiSort.cs 197 20 MLOLPlus.Business

IdentityEntity是所有自定义类数据类型

的基本抽象类库

示例：

用户继承IdentityEntity
编辑也是

基类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;
    }

}

自定义用户多重排序

  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;
        }

    }

自定义编辑器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

您收到编译器错误的原因是您已将通用约束推送到各个方法，而不是将限制放在类上。您拥有它的方式，无法使用返回GetOrderingFunc<T>的方法覆盖内在泛型方法Expression<Func<User, string>>。

如果您使DataTableMultiSort具有通用性并从方法中删除通用参数，那么您发布的内容有效：

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;
        ...
     }
}

泛型C＃组织的方法取决于类型

1 个答案: