单一职责原则 Single Responsibility Principle（SRP）

最理想的状态（或者目的）是一个类应该做一件事或者只做一件事。

1.典型代码（Csv文件处理程序）

public class CsvFileProcessor
{
    public void Process(string filename)
    {
        TextReader tr = new StreamReader(filename);
        tr.ReadToEnd();
        tr.Close();
 
        var conn = new SqlConnection("server=(local);integrated security=sspi;database=SRP");
        conn.Open();
 
        string[] lines = tr.ToString().Split(new string[] {@"\r\l"}, StringSplitOptions.RemoveEmptyEntries);
        foreach( string line in lines)
        {
            string[] columns = line.Split(new string[] {","}, StringSplitOptions.RemoveEmptyEntries);
            var command = conn.CreateCommand();
            command.CommandText = "INSERT INTO People (FirstName, LastName, Email) VALUES (@FirstName, @LastName, @Email)";
            command.Parameters.AddWithValue("@FirstName", columns[0]);
            command.Parameters.AddWithValue("@LastName", columns[1]);
            command.Parameters.AddWithValue("@Email", columns[2]);
            command.ExecuteNonQuery();
        }
        conn.Close();
    }
}

函数 void Process(string filename)使用中用到了太多的类，但是分析其核心功能仅有读取，转换以及存储。
2.由此进行下一步改造

public class CsvFileProcessor
{
    public void Process(string filename)
    {
        var csvData = ReadCsv(filename);
        var parsedData = ParseCsv(csvData);
        StoreCsvData(parsedData);
    }
 
    public string ReadCsv(string filename)
    {
        TextReader tr = new StreamReader(filename);
        tr.ReadToEnd();
        tr.Close();
        return tr.ToString();
    }
 
    public string[] ParseCsv(string csvData)
    {
        return csvData.ToString().Split(new string[] { @"\r\l" }, StringSplitOptions.RemoveEmptyEntries);
    }
 
    public void StoreCsvData(string[] csvData)
    {
        var conn = new SqlConnection("server=(local);integrated security=sspi;database=SRP");
        conn.Open();
        foreach (string line in csvData)
        {
            string[] columns = line.Split(new string[] { "," }, StringSplitOptions.RemoveEmptyEntries);
            var command = conn.CreateCommand();
            command.CommandText = "INSERT INTO People (FirstName, LastName, Email) VALUES (@FirstName, @LastName, @Email)";
            command.Parameters.AddWithValue("@FirstName", columns[0]);
            command.Parameters.AddWithValue("@LastName", columns[1]);
            command.Parameters.AddWithValue("@Email", columns[2]);
            command.ExecuteNonQuery();
        }
        conn.Close();
    }
}

现在看上去好多了，但是依旧没有能达到目标（比如：如果文件不是Csv格式，亦或Xml？Json？等等），设计不够牢靠。

3.再次改进

public interface IContactDataProvider
{
    string Read();
}
public interface IContactParser
{
    IList<ContactDTO> Parse(string contactList);
}
public interface IContactWriter
{
    void Write(IList<ContactDTO> contactData);
}

public class ContactDTO
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }
}

public class ContactProcessor
{
    public void Process(IContactDataProvider cdp, IContactParser cp, IContactWriter cw)
    {
        var providedData = cdp.Read();
        var parsedData = cp.Parse(providedData);
        cw.Write(parsedData);
    }
}
public class CSVContactDataProvider : IContactDataProvider
{
    private readonly string _filename;
 
    public CSVContactDataProvider(string filename)
    {
        _filename = filename;
    }
     
    public string Read()
    {
        TextReader tr = new StreamReader(_filename);
        tr.ReadToEnd();
        tr.Close();
        return tr.ToString();
    }
}
 
public class CSVContactParser : IContactParser
{
    public IList<ContactDTO> Parse(string csvData)
    {
        IList<ContactDTO> contacts = new List<ContactDTO>();
        string[] lines = csvData.Split(new string[] { @"\r\l" }, StringSplitOptions.RemoveEmptyEntries);
        foreach (string line in lines)
        {
            string[] columns = line.Split(new string[] { "," }, StringSplitOptions.RemoveEmptyEntries);
            var contact = new ContactDTO
            {
                FirstName = columns[0],
                LastName = columns[1],
                Email = columns[2]
            };
            contacts.Add(contact);
        }
 
        return contacts;
    }
}
 
public class ADOContactWriter : IContactWriter
{
    public void Write(IList<ContactDTO> contacts)
    {
        var conn = new SqlConnection("server=(local);integrated security=sspi;database=SRP");
        conn.Open();
        foreach (var contact in contacts)
        {
            var command = conn.CreateCommand();
            command.CommandText = "INSERT INTO People (FirstName, LastName, Email) VALUES (@FirstName, @LastName, @Email)";
            command.Parameters.AddWithValue("@FirstName", contact.FirstName);
            command.Parameters.AddWithValue("@LastName", contact.LastName);
            command.Parameters.AddWithValue("@Email", contact.Email);
            command.ExecuteNonQuery();
        }
        conn.Close();
 
    }
}

至少现在看上去完美了，以后呢？？？
参考：https://www.dotnetcurry.com/software-gardening/1148/solid-single-responsibility-principle