Fixed: "Unable to connect to any of the specified MySQL hosts" in .NET C#

Unable to connect to any of the specified MySQL hosts is MySql.Data's message when it can't reach MySQL at all: it never got as far as checking a user name or password. The inner exception tells you more. I got it twice in a .NET C# program that connects to MySQL in Docker, for two different reasons, and four quick checks tell them apart.

What I Was Doing

I was connecting to MySQL from a small one-file C# program that opens a MySqlConnection with MySql.Data 26.10.0 and prints the server version. MySQL 9.7 ran in Docker, published on 127.0.0.1:3306. Two runs failed:

1. A connection string with Port=3307 instead of 3306.
2. The right connection string, while the MySQL container was stopped.

Terminal

dotnet run test-connection.cs

What I Expected

A connected session: Connected as inventory_app@% to MySQL 9.7.4.

What I Got

The same error both times:

Output (first lines)

Unhandled exception. MySql.Data.MySqlClient.MySqlException (0x80004005): Unable to connect to any of the specified MySQL hosts
 ---> System.Net.Sockets.SocketException (61): Connection refused

What I Checked

The inner exception first. SocketException (61): Connection refused means the computer at that address answered, and said no program was listening on that port. That rules out a wrong password, a missing database or a TLS problem: MySQL was never reached. Oracle's Connector/NET guide groups connection failures under error number 0, "Cannot connect to server", separate from 1045, "Invalid user name, user password, or both".

Then, from the outside in:

1. Is MySQL running?

Terminal

docker compose ps --format "table {{.Service}}\t{{.Status}}\t{{.Ports}}"

Output (healthy)

SERVICE   STATUS                    PORTS
mysql     Up 13 seconds (healthy)   127.0.0.1:3306->3306/tcp

During the second failure, this listed nothing: the container was stopped. The manual's connection problems page starts with exactly this: "Make sure that the server is running. If it is not, clients cannot connect to it."

2. Which address and port is it published on? The PORTS column says 127.0.0.1:3306->3306/tcp: reachable on this computer's address 127.0.0.1, port 3306. Compare that with Server= and Port= in your connection string. The first failure was this one: Port=3307.

3. Does anything answer on that port? Test the port itself, without .NET:

macOS / Linux

Terminal

nc -vz 127.0.0.1 3306
nc -vz 127.0.0.1 3307

Output

Connection to 127.0.0.1 port 3306 [tcp/mysql] succeeded!
nc: connectx to 127.0.0.1 port 3307 (tcp) failed: Connection refused

PowerShell 7

PowerShell

Test-Connection -TargetName 127.0.0.1 -TcpPort 3306
Test-Connection -TargetName 127.0.0.1 -TcpPort 3307

Output

True
False

Port 3306 answers; 3307 refuses, which is exactly what the .NET program saw.

4. Is the app looking at the same address? If your app runs in a container too, 127.0.0.1 means that container, not your computer. Use the Compose service name, such as Server=mysql, from another container in the same project.

Why It Happens

MySql.Data tries each host in Server= and gives up when none accepts a connection. Oracle's guide describes the loop: "If the attempted connection to a selected target is unsuccessful, Connector/NET selects a new target from the list until no more hosts are available." With one host, "any of the specified hosts" means that one. The message itself isn't in Oracle's documentation; the inner exception is the part to read.

The MySQL manual lists the usual causes, all network-level: the server isn't running, the client uses "a TCP/IP port, named pipe, or Unix socket file different from the one on which the server is listening", the server listens only locally, or a firewall blocks the port.

The Fix

Fix whichever check failed:

Check that failed	Fix
1. MySQL isn't running	docker compose start (or docker compose up -d), and wait for healthy
2. Port or address differs	Correct Server= and Port= in the connection string
3. Nothing answers on the right port	Check the ports: mapping in compose.yaml and that no other program holds the port
4. The app runs in a container	Use the service name, such as Server=mysql, instead of 127.0.0.1

In my two cases: Port=3307 became Port=3306, and the stopped container was started again.

After the Fix

Output (dotnet run test-connection.cs)

Connected as inventory_app@% to MySQL 9.7.4
TLS: TLSv1.2

Common Mistakes

Mistake	What happens	Do this instead
Changing the password first	Nothing changes: the server was never reached	Read the inner exception
Server=localhost inside a container	It points at the app's own container	Use the Compose service name
Assuming the container is up because Docker Desktop is	The service may have stopped or failed	docker compose ps
Publishing MySQL on 0.0.0.0 to "fix" access	Anyone on your network can reach the database	Keep 127.0.0.1 for local development

Troubleshooting

Inner exception	Meaning	Where to look
SocketException (61): Connection refused	Nothing listens at that address and port	Checks 1 to 3
A timeout after about 15 seconds	Packets are dropped, often by a firewall	The network path; Connection Timeout is 15 seconds by default
Access denied for user …	The server was reached; the account was rejected	User, password and host part of the account

The timeout row comes from Oracle's options reference, where Connection Timeout has "Default: 15", in seconds.

FAQ

Does EF Core show the same error?

Yes. Oracle's EF Core provider connects through MySql.Data, so the message is the same, usually inside an EF Core exception. A health check that uses the database simply reports "Unhealthy" without the reason, so check the logs.

Why does the message say "hosts", plural?

Server= can list several hosts separated by commas, and MySql.Data tries each one. The message covers both cases.

Conclusion

Unable to connect to any of the specified MySQL hosts means MySql.Data never reached MySQL, so credentials aren't the problem. Read the inner exception: Connection refused means nothing listens at that address and port. Check that MySQL is running (docker compose ps), compare the published port with Server= and Port=, test the port with nc or Test-Connection, and remember that 127.0.0.1 inside a container means the container itself.

Additional Resources

• Problems Connecting to MySQL — the usual causes, from the server's side.
• Connector/NET: Handling Connection Errors — the driver's error numbers.

Related

• How to Write a MySQL Connection String for .NET C#
• How to Run MySQL in Docker with Docker Compose
• How to Add Liveness and Readiness Health Checks to a .NET C# Web API