c# - ASP.NET Core - 执行命令时未找到 key

Question

我正在尝试使用 ASP.NET Core 连接到我的本地数据库 - 我可以使用 localhost/phpmyadmin 访问该数据库。我是这门语言的新手，并且正在努力解决与 MySQL 相关的异常。

string constr = Configuration.GetSection("ConnectionStrings:con").Value;
using (MySqlConnection con = new MySqlConnection(constr))
{
    string query = @"SELECT id
                    FROM `test`";
    using (MySqlCommand cmd = new MySqlCommand(query, con))
    {
        con.Open();
        using (var sqlDataReader = cmd.ExecuteReader())
        {
            while (sqlDataReader.Read())
            {
                var id = sqlDataReader.GetInt32(0);
            }
        }
        con.Close();
    }
}

异常发生在 using (var sqlDataReader = cmd.ExecuteReader()) 行。

这是我的连接字符串:

"ConnectionStrings": {
    "con": "Server=localhost;Database=musense;User=root;Password=********;Charset=utf8"
 }

我正在 appsettings.json 中设置连接字符串。如果没有字符集，我会得到一个异常:“‘windows-1252’不是受支持的编码名称。”

运行上述代码后，我在 using (var sqlDataReader = cmd.ExecuteReader()) 行出现以下异常:

/home/memonick/Documents/Projects/musense/musense.csproj : warning NU1701: Package 'MySql.Data 6.9.9' was restored using '.NETFramework,Version=v4.6.1' instead of the project target framework '.NETCoreApp,Version=v2.0'. This package may not be fully compatible with your project.
/home/memonick/Documents/Projects/musense/musense.csproj : warning NU1701: Package 'MySql.Data 6.9.9' was restored using '.NETFramework,Version=v4.6.1' instead of the project target framework '.NETCoreApp,Version=v2.0'. This package may not be fully compatible with your project.

Unhandled Exception: System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
   at System.ThrowHelper.ThrowKeyNotFoundException()
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at MySql.Data.MySqlClient.MySqlField.SetFieldEncoding()
   at MySql.Data.MySqlClient.NativeDriver.GetColumnData(MySqlField field)
   at MySql.Data.MySqlClient.NativeDriver.GetColumnsData(MySqlField[] columns)
   at MySql.Data.MySqlClient.Driver.GetColumns(Int32 count)
   at MySql.Data.MySqlClient.ResultSet.LoadColumns(Int32 numCols)
   at MySql.Data.MySqlClient.ResultSet..ctor(Driver d, Int32 statementId, Int32 numCols)
   at MySql.Data.MySqlClient.Driver.NextResult(Int32 statementId, Boolean force)
   at MySql.Data.MySqlClient.MySqlDataReader.NextResult()
   at MySql.Data.MySqlClient.MySqlDataReader.Close()
   at MySql.Data.MySqlClient.MySqlCommand.ResetReader()
   at MySql.Data.MySqlClient.MySqlCommand.ExecuteReader(CommandBehavior behavior)
   at musense.Startup.ConfigureServices(IServiceCollection services) in /home/memonick/Documents/Projects/musense/Startup.cs:line 46
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Microsoft.AspNetCore.Hosting.ConventionBasedStartup.ConfigureServices(IServiceCollection services)
   at Microsoft.AspNetCore.Hosting.Internal.WebHost.EnsureApplicationServices()
   at Microsoft.AspNetCore.Hosting.Internal.WebHost.BuildApplication()
   at Microsoft.AspNetCore.Hosting.WebHostBuilder.Build()
   at musense.Program.BuildWebHost(String[] args) in /home/memonick/Documents/Projects/musense/Program.cs:line 21
   at musense.Program.Main(String[] args) in /home/memonick/Documents/Projects/musense/Program.cs:line 17

值得注意的是，连接正在建立。因此，例如，我可以更新一行，或执行 SELECT VERSION()。仅当我尝试在 ASP.NET Core 的表上执行 SELECT 语句时，才会出现此问题。该语句本身似乎是有效的，因为它在 phpmyadmin 中正常执行。

测试表具有以下架构:

CREATE TABLE IF NOT EXISTS `test` (
  `id` int(11) NOT NULL,
  `value` int(11) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_swedish_ci;

我尝试了各种排序规则，但异常仍然存在。欢迎任何帮助!

Answer 1

由于您使用的 NuGet 包与 .NETCoreApp2.0 不兼容，因此出现了问题:

/home/memonick/Documents/Projects/musense/musense.csproj : warning NU1701: Package 'MySql.Data 6.9.9' was restored using '.NETFramework,Version=v4.6.1' instead of the project target framework '.NETCoreApp,Version=v2.0'. This package may not be fully compatible with your project.

您需要升级MySql.Data 6.9.9并至少使用6.10.3-rc版本(记得勾选show prerelease packages 在 nuget 浏览器中)，兼容 .netstandard1.3 及以上版本(包括 .NETCoreApp2.0)。