使用 Dapper 和 SQLite，Query Method 抛出空引用异常

Question

Testing environment：

-- Framework version: .NET 6

-- Microsoft.Data.Sqlite, version: 6.0.6

-- Dapper, version: 2.0.123

Q: When only one query is executed, there is no problem. Problems occur when multiple concurrent queries are executed.

(Reference: When I write code in native ADO.NET instead of Dapper(conn.Query), there is no problem!)

Here is a simple demo that can reproduce the problem very easily.

using System.Data;
using Dapper;
using Microsoft.Data.Sqlite;

// SQLite db file
string connStr = $"Data Source={AppDomain.CurrentDomain.BaseDirectory}test.db";

// SQLite connection (Share the singleton connection) 
using SqliteConnection conn = new SqliteConnection(connStr);
conn.Open();
 
bool existError = false;
while (true)
{
    if (existError == true)
    {
        break;
    }

    // Perform concurrent reads
    Parallel.For(0, 100, index =>
    {
        try
        {
            // Test SQL
            string sql_test = " select * from T_Account where AccountId='ab001' ";

            // Perform query data (May throw a null reference exception )
            var t = conn.Query(sql_test);
        }
        catch (Exception ex)
        {
            existError = true;

            // Test output: Object reference not set to an instance of an object.
            Console.WriteLine($"Read error ({index}): {ex.Message}");
        }
    });

    Console.WriteLine($"{DateTime.Now}-------- split line --------");
}

The whole project has been using Dapper, I want to know where the problem is, how should I solve it?

Answer 1

You might think that it's a super good idea to reuse the connection, but it isn't. There already is a connection pool built-in to ADO, and it's far more efficient than anything we can come up with. I tried to run your code, obviously with another sqlite database and query, and on the 6th or 7th iteration I got the null reference exception. Then I changed your code like this:

// Perform concurrent reads
Parallel.For(0, 100, index =>
{
    try
    {
        // These two lines moved inside the loop
        using SqliteConnection conn = new SqliteConnection(connStr);
        conn.Open();

        // Test SQL
        string sql_test = " select * from T_Account where AccountId='ab001' ";

        // Perform query data (May throw a null reference exception )
        var t = conn.Query(sql_test);
    }
    catch (Exception ex)
    {
        existError = true;

        // Test output: Object reference not set to an instance of an object.
        Console.WriteLine($"Read error ({index}): {ex.Message}");
    }
});

And the code can go on and on as expected. Best practice is to create the database connection in a using statement and have it disposed as soon as you are done with it. Since I haven't seen your code with raw ADO.NET, I can't tell why that works.