This error generally occur when you change database (For example changing the primary key) and after it update project entity model.
You can see that all columns in model table were changed like primary key.
To solve the problem, you can select the columns that are not the primary key and change the field “Entity Key” to false.
Or in the properties
If you are creating a DLL using #EntityFramework (EF Designer from database) be careful of your Connection String.
#Entity creates the connection string in DLL and it will compile perfectly. but when you import the DLL in a project never forget to reference the same connection string in project’s “App.config” file too, because at system startup, the DLL’s “App.config” file will be replaced by project’s “App.config” file.