c# - 依赖于类型的方法的泛型 C# 组织

Question

我正在尝试使用基方法创建一个基类，该基方法对具有依赖于类型的函数的列表进行排序。

我的编译器显示此错误

错误 13 无法在“System.Linq.Expressions.Expression<System.Func<T,string>>”中转换“System.Linq.Expressions.Expression<System.Func<MLOLPlus.Business.Dealer,string>>”。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础抽象类基础

例子：

1. 用户继承 IdentityEntity
2. 编辑也是

基类多排序：

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;
    }

}

自定义用户多分类

  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

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，而不是将约束放在class上。按照你的方式，没有办法 GetOrderingFunc<T>用返回 a 的方法覆盖本质上的泛型方法Expression<Func<User, string>>。

如果您制作DataTableMultiSort通用并从方法中删除通用参数，则您发布的内容有效：

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;
        ...
     }
}