SOLID 学习笔记 - 单一职责

　　六大设计原则

　单一责任原则：

　　　　一个类应该只做一件事，而且只做一件事。

　　　　错误示例：

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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();
    }
}

这个类做了三件事：

　　　　1. 读取 CSV 文件

　　　　2. 解析 CSV 文件

　　　　3. 存储数据

　　这样做如果数据验证和错误记录，该怎么办？该怎么对它进行单元测试。一般情况下我们使用代码重构修改此例子；

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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();
    }
}

我们在 ParseCsv（）方法中将 CSV 文件解析为行，但在 StoreCsvData（）方法中进行了额外的解析，以使每行成为列。

解决此问题的方法是使用存储每行数据的 ContactDTO。

下一步是添加 DTO，但我会跳过一个步骤，并将每个方法分解为自己的类。

但我也会在这里提前考虑。如果数据未以 CSV 格式提供，该怎么办？它的XML或JSON或其他东西是什么？

你可以用接口来解决这个问题。

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public interface IContactDataProvider
{
    string Read();
}

public interface IContactParser
{
    IList<ContactDTO> Parse(string contactList);
}

public interface IContactWriter
{
    void Write(IList<ContactDTO> contactData);
}

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();

    }
}

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