Question

在我的Data Repository中，我有一个基类和派生类，如下所示。

 public abstract class RepositoryBase<T> : IRepository<T> where T : EntityBase
{
      public async Task<T> FindOneAsync(Expression<Func<T, bool>> predicate)
    {
        List<T> list = await SearchForAsync(predicate);
        return list.FirstOrDefault();
    }
}

 public class CommentUrlRepository : RepositoryBase<CommentUrl>, ICommentUrlRepository
{
     public async Task<CommentUrlCommon> FindOneAsync(Expression<Func<CommentUrlCommon, bool>> predicate)
    {
        Expression<Func<CommentUrl, bool>> lambda = Cast(predicate);
        CommentUrl commentUrl = await FindOneAsync(lambda);
        return MappingManager.Map(commentUrl);
    }

    private Expression<Func<CommentUrl, bool>> Cast(Expression<Func<CommentUrlCommon, bool>> predicate)
    {
        Expression converted=   Expression.Convert(predicate, typeof (Expression<Func<CommentUrl, bool>>));
        // throws exception
        // No coercion operator is defined between types
        return Expression.Lambda<Func<CommentUrl, bool>>
     (converted, predicate.Parameters);
    }
}

当我点击“投射”功能时，我收到以下错误：

在类型System.Func 2[CommentUrlCommon,System.Boolean]' and 'System.Linq.Expressions.Expression 1 [System.Func`2 [CommentUrl，System.Boolean]]'之间没有定义强制运算符。

如何转换此Expression值？

Answer 1

我认为你想做什么是不可能的...... 请查看this question了解更多信息如果你很幸运并且你的表达很简单，Marc Gravell的转换方法可能适合你

一个更简单的例子来证明你的问题

using System;
using System.Linq.Expressions;

namespace Program
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            Expression<Func<CommentUrlCommon, bool>> predicate = f => f.Id == 1;

            //As you know this doesn't work
            //Expression converted = Expression.Convert(predicate, typeof(Expression<Func<CommentUrl, bool>>));

            //this doesn't work either...
            Expression converted2 = Expression.Convert(predicate, typeof(Expression<Func<CommentUrlCommon, bool>>));

            Console.ReadLine();
        }
    }

    public class CommentUrlCommon
    {
        public int Id { get; set; }
    }

    public class CommentUrl
    {
        public int Id { get; set; }
    }
}

表达式转换错误 - 类型之间没有定义强制运算符

1 个答案: