Entity Framework Core遇到的问题汇总

1.add-migration : 无法将“add-migration”项识别为 cmdlet、函数、脚本文件或可运行程序的名称。请检查名称的拼写，如果包括路径，请确保路径正确，然后再试一次。

使用Entity Framework Core命令 add-migration manage 出现以下错误

PM> add-migration manage
add-migration : 无法将“add-migration”项识别为 cmdlet、函数、脚本文件或可运行程序的名称。请检查名称的拼写，如果包括路径，请确保路径正确，然后再试一次。
所在位置 行:1 字符: 1
+ add-migration manage
+ ~~~~~~~~~~~~~
    + CategoryInfo          : ObjectNotFound: (add-migration:String) [], CommandNotFoundException
    + FullyQualifiedErrorId : CommandNotFoundException
 

使用EF core命令需要安装迁移用的库 Microsoft.EntityFrameworkCore.Tools ，所以在nuget 添加 Microsoft.EntityFrameworkCore.Tools包 或者 输入 Install-Package Microsoft.EntityFrameworkCore.Tools

2.System.NullReferenceException: Object reference not set to an instance of an object.

删除model中的一个属性，然后提交 Add-Migration ，返回以下错误：

PM> Add-Migration ModifyCitys
Build started...
Build succeeded.
System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.Initialize(ColumnOperation columnOperation, IProperty property, CoreTypeMapping typeMapping, Boolean isNullable, IEnumerable`1 migrationsAnnotations, Boolean inline)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.Add(IProperty target, DiffContext diffContext, Boolean inline)+MoveNext()
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.DiffCollection[T](IEnumerable`1 sources, IEnumerable`1 targets, DiffContext diffContext, Func`4 diff, Func`3 add, Func`3 remove, Func`4[] predicates)+MoveNext()
   at System.Linq.Enumerable.ConcatIterator`1.MoveNext()
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.Diff(ITable source, ITable target, DiffContext diffContext)+MoveNext()
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.DiffCollection[T](IEnumerable`1 sources, IEnumerable`1 targets, DiffContext diffContext, Func`4 diff, Func`3 add, Func`3 remove, Func`4[] predicates)+MoveNext()
   at System.Linq.Enumerable.ConcatIterator`1.MoveNext()
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.Sort(IEnumerable`1 operations, DiffContext diffContext)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsModelDiffer.GetDifferences(IModel source, IModel target)
   at Microsoft.EntityFrameworkCore.Migrations.Design.MigrationsScaffolder.ScaffoldMigration(String migrationName, String rootNamespace, String subNamespace, String language)
   at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration(String name, String outputDir, String contextType)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigration.<>c__DisplayClass0_0.<.ctor>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.<Execute>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)
Object reference not set to an instance of an object.

用的EF版本

EF Core version:5.0.0-preview.2.20120.8
Database provider: Microsoft.EntityFrameworkCore.SqlServer 5.0.0-preview.2.20120.8

3.1.2版本没有问题，已提交issues

https://github.com/dotnet/efcore/issues/20348

3.More than one DbContext was found. Specify which one to use. Use the '-Context' parameter for PowerShell commands and the '--context' parameter for dotnet commands.

由于使用多个DbContext，在使用命令生成数据库文件没有指定DbContext

在原命令add-migration 后加上-c DbContext的类名

4.Unable to create an object of type 'MyDBContext'. For the different patterns supported at design time, see https://go.microsoft.com/fwlink/?linkid=851728

PM> Add-Migration Initial
Build started...
Build succeeded.
Unable to create an object of type 'MyDBContext'. For the different patterns supported at design time, see https://go.microsoft.com/fwlink/?linkid=851728

参考：

https://www.cnblogs.com/wyy1234/archive/2018/09/18/9670529.html

https://www.cnblogs.com/wyy1234/p/9647730.html