表达式转换错误 - 类型之间未定义强制运算符

Question

在我的数据存储库中，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“Cast”功能时，出现以下错误：

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

如何转换此表达式值？

Answer 1

我认为你想要的无法完成......
查看this question了解更多信息。
如果你很幸运并且你的表达方式很简单，Marc Gravell 的 Convert 方法可能对你有用

还有一个更简单的例子来说明你的问题

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; }
    }
}