Fixed: "CS0120: An object reference is required" for ClearAllPoolsAsync in MySql.Data

MySqlConnection.ClearAllPools() is static, so I wrote await MySqlConnection.ClearAllPoolsAsync(), and the compiler said CS0120. The async versions in MySql.Data aren't static, unlike the methods they mirror.

What I Was Doing

I was writing a small program to show how to clear the connection pool, on MySql.Data 26.10.0 and .NET 10:

clear-pool.cs

await MySqlConnection.ClearAllPoolsAsync();

Terminal

dotnet run clear-pool.cs

What I Expected

It to compile, as the method it mirrors does.

What I Got

Output

/Users/you/projects/mysql-connection-test/clear-pool.cs(5,7): error CS0120: An object reference is required for the non-static field, method, or property 'MySqlConnection.ClearAllPoolsAsync(CancellationToken)'

The build failed. Fix the build errors and run again.

What I Checked

The class's members, in MySql.Data 26.10.0:

MySqlConnection

public static void ClearPool(MySqlConnection connection)
public Task ClearPoolAsync(MySqlConnection connection, CancellationToken cancellationToken = default)
public static void ClearAllPools()
public Task ClearAllPoolsAsync(CancellationToken cancellationToken = default)

The sync ones are static. The async ones aren't.

Why It Happens

CS0120 means you've called an instance method as though it were static. Here, the two pairs of methods just differ.

The Fix

Use the static methods. They're simple calls, and there's no reason to wait on them in most code:

clear-pool.cs

MySqlConnection.ClearAllPools();

To clear one pool, pass a connection that has that connection string. It doesn't need to be open:

clear-pool.cs

await using (var connection = new MySqlConnection(connectionString))
{
    MySqlConnection.ClearPool(connection);
}

Or, if you do want the async versions, call them on an instance:

clear-pool.cs

await using var connection = new MySqlConnection(connectionString);
await connection.ClearAllPoolsAsync();

After the Fix

Terminal

dotnet run clear-pool.cs

Output

First open:          connection id 33
Second open:         connection id 33
After ClearAllPools: connection id 34
After ClearPool:     connection id 35

The id changes because each clear closed the pooled connection, so the next open needed a new one.

Common Mistakes

Mistake	What happens	Do this instead
Adding new MySqlConnection().ClearAllPoolsAsync() everywhere	It works, and reads oddly	Use the static method
Clearing pools routinely	Every request then pays for a new connection	Only clear after a real change, such as a failover

Troubleshooting

Symptom	Likely cause	Fix
CS0176 on the sync method through an instance	The reverse mistake	Call it on the class
Connections still there after clearing	Connections in use close when they're returned	Check again once they've finished

Conclusion

ClearPool and ClearAllPools are static, and their async versions aren't. Call the static ones on the class.

Additional Resources

• Compiler Error CS0120 — what it means.
• Managing a Connection Pool in Connector/NET — the pool.

Related

• How to Clear the MySql.Data Connection Pool
• How to See the MySql.Data Connection Pool Working
• How to Size the MySql.Data Connection Pool