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What is the yield keyword used for in C#?

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17 2007
盖世英雄少女心
盖世英雄少女心 2020-11-22 05:26

In the How Can I Expose Only a Fragment of IList<> question one of the answers had the following code snippet:

IEnumerable FilteredList()
{
         


        
      
      
      

        

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                 耶瑟儿～ 
              
              
    
                
                   耶瑟儿～
                                             
                
                
                (楼主)
            
              
                  
    
                2020-11-22 05:58
              
    

            
    
            
    
              
              

    The yield keyword actually does quite a lot here.
    


    The function returns an object that implements the IEnumerable interface. If a calling function starts foreaching over this object, the function is called again until it "yields". This is syntactic sugar introduced in C# 2.0. In earlier versions you had to create your own IEnumerable and IEnumerator objects to do stuff like this.
    


    The easiest way understand code like this is to type-in an example, set some breakpoints and see what happens. Try stepping through this example:
    


    public void Consumer()
{
    foreach(int i in Integers())
    {
        Console.WriteLine(i.ToString());
    }
}

public IEnumerable Integers()
{
    yield return 1;
    yield return 2;
    yield return 4;
    yield return 8;
    yield return 16;
    yield return 16777216;
}

    


    When you step through the example, you'll find the first call to Integers() returns 1. The second call returns 2 and the line yield return 1 is not executed again.
    


    Here is a real-life example:
    


    public IEnumerable Read(string sql, Func make, params object[] parms)
{
    using (var connection = CreateConnection())
    {
        using (var command = CreateCommand(CommandType.Text, sql, connection, parms))
        {
            command.CommandTimeout = dataBaseSettings.ReadCommandTimeout;
            using (var reader = command.ExecuteReader())
            {
                while (reader.Read())
                {
                    yield return make(reader);
                }
            }
        }
    }
}

    
    
    
             
    
                                                        
    
            
            
    
              
                
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