Solved Error : Microsoft.EntityFrameworkCore.Database.Connection[20004] An error occurred using the connection to database '' on server 'localhost'.

I created an ASP.NET Core API to work with an existing SQL Server database. I created the necessary models for my database tables and also generated the database context using Scaffold. Now, when attempting to access the database and fetch some data from the database table through my API, I encounter the following error: "Microsoft.EntityFrameworkCore.Database.Connection[20004] An error occurred using the connection to database 'QuickpickdealDb' on server 'localhost,4522'" , this error indicates a problem with connecting to the specified database.

To solve this issue, you need to check the possible reason and take appropriate actions to solve it.

Ye should ensure that the database server specified in the connection string ('localhost,4522' in this case) is running and accessible. We can do this by verifying the server's status and ensuring that there are no network connectivity issues.

Next, you need to verify that the database 'QuickpickdealDb' exists on the specified server. We can use database management tools such as SQL Server Management Studio or Azure Data Studio and Php my Admin in case mysql database to check for the existence of the database and its accessibility.

If the database also exists, the we should ensure that the credentials provided in the connection string are correct and have sufficient permissions to access the database. 

You can verify the credentials and permissions by logging in to the database server using the same credentials and checking the permissions assigned to the user.

Here's an example of a connection string in the appsettings.json file:

{
  "ConnectionStrings": {
    "DefaultConnection": "Server=localhost,4522;Database=QuickpickdealDb;User Id=sa;Password=pass@1234;"
  }
}
    

You should check for any firewall or security settings that might be blocking the connection to the database server. Ensure that the necessary ports are open and that the firewall allows traffic to and from the database server.

After troubleshooting, we managed to identify and fix the problem. Initially, it turned out to be a permission issue, which we addressed.

However, another error  up, indicating, "An error occurred using the connection to database '' on server 'localhost'". Upon further investigation, we discovered that the root cause was that our SQL Server instance was not running.

Once we started the SQL Server service, everything fell into place, and our application began functioning seamlessly.

First, we double-checked our connection string:

string connectionString = "Server=kdkmysql21plsk.secureserver.net; User ID=dealdb; Password=db@1997; Database=dealdb";

Upon confirming that the connection string was correct, and error still persist. We encountered a scenario similar to what my colleague experienced, where the issue occur because we were attempting to connect from the .NET Web Application running in a Docker Container to the MySQL installed on the machine where the Docker Container is running.

Initially, we assumed that everything would run smoothly using localhost. However, we soon realized that each container runs on a specific local IP, and the local machine itself has its own local IP in the Ethernet Adapter vEthernet (WSL).

To address this, we updated the connection string to use the specific IP address instead of localhost. This step resolved the issue, and our application successfully connected to the MySQL database.

By updating the connection string to use the correct IP address, we ensured communication between our .NET Web Application running in a Docker Container and the MySQL database, to resolving the connectivity issue.