Why does the C# compiler choose Dapper's extension method for IDbConnection instead of the instance method on my own type?

Question

I have a custom class LoggingDbConnection that implements IDbConnection. This LoggingDbConnection class has its own public instance method, for example, QueryAsync<T>(...), which includes custom logging logic.

In my service, I have a field:

private readonly IDbConnection _dbConnection;

This field is injected (e.g., via Autofac) with an instance of LoggingDbConnection.

When I call:

await _dbConnection.QueryAsync<T>(...);

I expect my LoggingDbConnection.QueryAsync<T>() instance method (with the logging) to be executed because the actual runtime object is a LoggingDbConnection.

However, what actually happens is that Dapper's QueryAsync<T>() extension method (for IDbConnection) gets called instead, and my custom logging is bypassed.

Why does the C# compiler choose Dapper's extension method for IDbConnection instead of the instance method on my concrete LoggingDbConnection class, even though the object instance at runtime is a LoggingDbConnection?

"When I call" - you are calling extension method yourself. To call instance method you need the instance of that type first, not some interface it implements. — Sinatr
– Sinatr, Commented May 12 at 16:01
As a side note: maybe MiniProfiler does what you want here. It was written in tandem with Dapper and works very well with it. — Marc Gravell
– Marc Gravell, Commented May 12 at 21:15
TLDR when you have a variable of type IDbConnection, you can only call methods of this interface or extension methods. The compiler doesn't know that an IDbConnection is actually a LoggingDbConnection with different functions. — Jeremy Lakeman
– Jeremy Lakeman, Commented May 13 at 1:48
@MarcGravell installing MiniProfiler when I just want to simple logs seems like an overkill, no? — Shad
– Shad, Commented May 13 at 7:33

Ivan Petrov · Accepted Answer · 2025-05-12 17:57:08Z

So you have:

public class LoggingDbConnection : IDbConnection
{
    public Task<T> QueryAsync<T>(...) { ... }
}

// In Dapper
public static class SqlMapper
{
    public static Task<T> QueryAsync<T>(this IDbConnection connection, ...) { ... }
}

And you do:

IDbConnection connection = new LoggingDbConnection(...);
connection.QueryAsync<T>(...);

That will not call LoggingDbConnection.QueryAsync<T>, no.

When you call connection.QueryAsync<T>(...) the compiler goes through a long process to resolve that to a method to call, but for our purposes here the process looks like:

I've got a variable connection of type IDbConnection. Is there a method QueryAsync<T> on IDbConnection? No.
Are there any extension methods on IDbConnection called QueryAsync<T>? Yes.

In other words, the compiler works out at compile-time whether you're calling an instance method or an extension method using the information available to it at compile-time, and that information is that you have a receiver of type IDbConnection.

Put another way, yes instance methods win out over extension methods if both are applicable, but if the QueryAsync<T> extension method didn't exist, then you wouldn't be able to call connection.QueryAsync<T> anyway. So it was never a candidate here. That's just not how interfaces work.

In other other words, extension methods are not virtual, and cannot participate in method overriding.

Sinus32 · Accepted Answer · 2025-05-13 08:44:18Z

@canton7's response answers the original question.

Dapper doesn't have interceptors, so to solve your real problem (add logging) you have two options:

1. Make own extension methods (bad option):

Make methods like .LoggingQueryAsync(...).
It looks simple at first, but have way too many downsides...

2. Implement IDbConnection method that Dapper calls (good option):

public class LoggingDbConnection : IDbConnection
{
    ...

    public IDbCommand CreateCommand()
    {
        return new LoggingDbCommand(this);
    }

    ...
}

Dapper have to call IDbConnection.CreateCommand() to do anything.

In LoggingDbCommand implement IDbCommand.ExecuteNonQuery(), IDbCommand.ExecuteReader(), IDbCommand.ExecuteReader(CommandBehavior) and IDbCommand.ExecuteScalar() to add logging.

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