不能隐式转换类型。存在明确的转换（您是否缺少演员？）

问题描述

我正在尝试使用ASP.Net Web API开发一个Web应用程序。现在在创建产品模型和IProductRepository接口后，我很难显示所有产品。我收到错误消息

在这行代码中

return repo.GetAll（）;

我不明白要做什么。如果有人指出问题并解释我做错了什么，那将会非常有帮助。

ProductsController.cs

  using System; 
使用System.Collections.Generic; 
使用System.Linq; 
使用System.Net; 
使用System.Net.Http; 
使用System.Web.Http; 
使用WebApplication1.Models; 
 
 namespace WebApplication1.Controllers 
 {
 public class ProductsController：ApiController 
 {
 static readonly IProductRepository repo = new ProductRepository（）; 
 
 public IEnumerable< Product> GetAllProducts（）
 {
 return repo.GetAll（）; 
} 
} 
} 
  

ProductRepository.cs

  using System; 
使用System.Collections.Generic; 
使用System.Data.SqlClient; 
使用System.Linq; 
使用System.Web; 
 
 namespace WebApplication1.Models 
 {
 public class ProductRepository：IProductRepository 
 {
 public string cs = System.Configuration.ConfigurationManager.ConnectionStrings [MediaSoftAppEntities ] .ConnectionString; 
 
 public IEnumerable< Product> GetAll（）
 {
列表< Product> products = new List< Product>（）; 
 string sql = String.Format（Select [ProductID]，[ProductName]，[Price] FROM [Products]）; 
 
 using（SqlConnection con = new SqlConnection（cs））{
 using（SqlCommand cmd = new SqlCommand（sql，con））{
 con.Open（）; 
 SqlDataReader dr = cmd.ExecuteReader（）; 
 while（dr.Read（））{
 Product product = new Product（）; 
 product.ProductID = dr.GetInt32（0）; 
 product.ProductName = dr.GetString（1）; 
 product.Price = dr.GetInt32（2）; 
} 
} 
} 
 return products.ToArray（）; 
} 
} 
} 
  

IProductRepository.cs

  using System; 
使用System.Collections.Generic; 
使用System.Linq; 
使用System.Text; 
使用System.Threading.Tasks; 
 
 namespace WebApplication1.Models 
 {
 public interface IProductRepository 
 {
 IEnumerable< Product>得到所有（）; 
} 
} 
  

解决方案

错误是说编译器认为

ProductsController.GetAllProducts 想要返回一个 WebApplication1.Product

但 IProductRepository.GetAll 返回一个 WebApplication1.Models.Product 。

您在这两个不同的命名空间中有2 Product 类吗？

<或者，也许你之前在 WebApplication1 命名空间中有 Product 类，现在它已经被移入 WebApplication1.Models 命名空间，也许你只是有一个陈旧的参考和重建所有将修复它。

I am trying to develop a web application using ASP.Net Web API. Now after creating Product model and IProductRepository interface I'm having difficulty in showing all products. I'm getting error message

on this line of code

return repo.GetAll();

I don't understand what to do. If someone points out the problem and explain me what I'm doing wrong that would be very helpful.

ProductsController.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Web.Http;
using WebApplication1.Models;

namespace WebApplication1.Controllers
{
    public class ProductsController : ApiController
    {
        static readonly IProductRepository repo = new ProductRepository();

        public IEnumerable<Product> GetAllProducts()
        {
            return repo.GetAll();
        }
    }
}

ProductRepository.cs

using System;
using System.Collections.Generic;
using System.Data.SqlClient;
using System.Linq;
using System.Web;

namespace WebApplication1.Models
{
    public class ProductRepository : IProductRepository
    {
        public string cs = System.Configuration.ConfigurationManager.ConnectionStrings["MediaSoftAppEntities"].ConnectionString;

        public IEnumerable<Product> GetAll()
        {
            List<Product> products = new List<Product>();
            string sql = String.Format("Select [ProductID], [ProductName], [Price] FROM [Products]");

            using (SqlConnection con = new SqlConnection(cs)){
                using (SqlCommand cmd = new SqlCommand(sql, con)){
                    con.Open();
                    SqlDataReader dr = cmd.ExecuteReader();
                    while(dr.Read()){
                        Product product = new Product();
                        product.ProductID = dr.GetInt32(0);
                        product.ProductName = dr.GetString(1);
                        product.Price = dr.GetInt32(2);
                    }
                }
            }
            return products.ToArray();
        }
    }
}

IProductRepository.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace WebApplication1.Models
{
    public interface IProductRepository
    {
        IEnumerable<Product> GetAll();
    }
}

解决方案

The error is saying that the compiler thinks that
ProductsController.GetAllProducts wants to return a WebApplication1.Product
but IProductRepository.GetAll returns a WebApplication1.Models.Product instead.

Do you have 2 Product classes in these 2 different namespaces?

Alternatively, maybe you previously had the Product class in the WebApplication1 namespace, and it's now been moved into the WebApplication1.Models namespace, maybe you just have a stale reference and a "Rebuild All" will fix it.

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