不能隐式转换类型。存在明确的转换（您是否缺少演员？） [英] Cannot implicitly convert type. An explicit conversion exists (are you missing a cast?)

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

无法隐式转换类型 System.Collections.Generic.IEnumerable< WebApplication1.Models。 Product> to System.Collections.Generic.IEnumerable< WebApplication1.Product> 。存在明确的转换（您是否缺少转换？）

Controllers \ProductsController.cs 17 20 WebApplication1

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

Cannot implicitly convert type System.Collections.Generic.IEnumerable<WebApplication1.Models.Product> to System.Collections.Generic.IEnumerable<WebApplication1.Product>. An explicit conversion exists (are you missing a cast?)

Controllers\ProductsController.cs 17 20 WebApplication1

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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