SqlCommand即使数据库中有数据也返回null

Question

I am looking at my code for a few hours now and I seriously can't seem to be able to find what is wrong.

The actual problem is that whenever I run my query it seems to be returning a null type also I am sure there is data in the database.

Error given:

An unhandled exception of type 'System.ArgumentNullException' occurred in System.Data.dll

Additional information: Value cannot be null.

I'll paste in some of my code that I use.

App.Config

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <configSections>
    </configSections>
    <connectionStrings>
        <add name="MarkProject.Properties.Settings.MoviesConnectionString"
            connectionString="Data Source=(LocalDB)\v11.0;AttachDbFilename=|DataDirectory|\MoviesUML.mdf;Integrated Security=True"
            providerName="System.Data.SqlClient" />
    </connectionStrings>
    <startup> 
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
    </startup>
</configuration>

Database.cs (My own class to fetch data from the db, only showing the Select method and my connection string as that's what's giving troubles)

Connection String:

private string connectionString = ConfigurationManager.ConnectionStrings["MarkProject.Properties.Settings.MoviesConnectionString"].ConnectionString;

Select Method:

public DataTable Select(string query, Dictionary<string, object> values = null)
{
    using(SqlConnection myConnection = new SqlConnection(connectionString))
    {
        using(SqlCommand cmd = new SqlCommand(query, myConnection))
        {
            myConnection.Open();

            if(values != null)
            {
                foreach(var item in values)
                {
                    cmd.Parameters.AddWithValue("@" + item.Key, item.Value);
                }
            }

            this._dataAdapter = new SqlDataAdapter(cmd);
            this._dataAdapter.Fill(this._dataTable);

            myConnection.Close();
        }
    }

    return this._dataTable;
}

And the query that I am running that gives the error.

DataTable movieData = this._db.Select("SELECT Title, Year FROM Movie");

And here is proof that the table is actually populated.

Answer 1

It is the reverse of a very common scenario. It is always a misunderstanding of the meaning of |DataDirectory| in a WinForms app. The substitution string |DataDirectory| points at the BIN\\DEBUG folder (or x86 variant) when you debug your app inside Visual Studio. So all the commands runs against a database located in that folder. Usually this creates problem with INSERT, UPDATE or DELETE operation but also the SELECT could be affected if the two databases contains different data.

To resolve it copy the MDF file from your project folder in your BIN\\DEBUG folder, then create a second connection in your Server Explorer. This one points to the database file in the BIN\\DEBUG folder and use it to check if your commands work.

Answer 2

public DataTable Select(string query, Dictionary<string, object> values = null)
{
    using(SqlConnection myConnection = new SqlConnection(connectionString))
    {
        using(SqlCommand cmd = new SqlCommand(query, myConnection))
        {
            myConnection.Open();

            if(values != null)
            {
                foreach(var item in values)
                {
                    cmd.Parameters.AddWithValue("@" + item.Key, item.Value);
                }
            }

            cmd.CommandType = CommandType.Text;
            sda = new SqlDataAdapter(cmd);
            dt = new DataTable("YourDataTable");
            new SqlDataAdapter(cmd).Fill(dt);            
        }
    }
    return dt
}