一个类应该只有一个改变的理由。

定义 - 在这种情况下，责任被认为是改变的一个原因。

该原则指出，如果我们有两个原因要更改某个类，则必须将功能拆分为两个类。每个类仅处理一项职责，如果将来我们需要进行一项更改，我们将在处理它的类中进行更改。当我们需要对具有更多职责的类进行更改时，该更改可能会影响与该类的其他职责相关的其他功能。

示例

代码之前单一职责原则

using System;
using System.Net.Mail;
namespace SolidPrinciples.Single.Responsibility.Principle.Before {
   class Program{
      public static void SendInvite(string email,string firstName,string lastname){
         if(String.IsNullOrWhiteSpace(firstName)|| String.IsNullOrWhiteSpace(lastname)){
            throw new Exception("Name is not valid");
         }
         if (!email.Contains("@") || !email.Contains(".")){
            throw new Exception("Email is not Valid!");
         }
         SmtpClient client = new SmtpClient();
         client.Send(new MailMessage("Test@gmail.com", email) { Subject="Please Join the Party!"})
      }
   }
}

遵循单一职责原则编写代码

using System;
using System.Net.Mail;
namespace SolidPrinciples.Single.Responsibility.Principle.After{
   internal class Program{
      public static void SendInvite(string email, string firstName, string lastname){
         UserNameService.Validate(firstName, lastname);
         EmailService.validate(email);
         SmtpClient client = new SmtpClient();
         client.Send(new MailMessage("Test@gmail.com", email) { Subject = "Please Join the Party!" });
      }
   }
   public static class UserNameService{
      public static void Validate(string firstname, string lastName){
         if (string.IsNullOrWhiteSpace(firstname) || string.IsNullOrWhiteSpace(lastName)){
            throw new Exception("Name is not valid");
         }
      }
   }
   public static class EmailService{
      public static void validate(string email){
         if (!email.Contains("@") || !email.Contains(".")){
            throw new Exception("Email is not Valid!");
         }
      }
   }
}